Re: RFC v3: Another proposed hash function transition plan

[Jeff King]

On Mon, Oct 02, 2017 at 10:18:02AM -0700, Linus Torvalds wrote:

> On Mon, Oct 2, 2017 at 7:00 AM, Jason Cooper  wrote:
> >
> > Ahhh, so if I understand you correctly, you'd prefer SHA-256 over
> > SHA3-256 because it's more performant for your usecase?  Well, that's a
> > completely different animal that cryptographic suitability.
> 
> In almost all loads I've seen, zlib inflate() cost is a bigger deal
> than the crypto load. The crypto people talk about cycles per byte,
> but the deflate code is what usually takes the page faults and cache
> misses etc, and has bad branch prediction. That ends up easily being
> tens or thousands of cycles, even for small data.

If anyone is interested in the user-visible effects of slower crypto, I
think, there are some numbers in 8325e43b82 (Makefile: add DC_SHA1 knob,
2017-03-16). I don't know how SHA-256 compares to sha1dc exactly, but
certainly the latter is a lot slower than normal sha1.

The only real-world case I found with a noticeable slowdown was
index-pack.  Which in the worst case is roughly the same operation as
"git fsck" (inflate and compute the sha1 on every byte), but people tend
to actually do it a lot more often.

And it really _is_ slower for real-world operations; the CPU for
computing the sha1 of an incoming clone of linux.git jumped from ~3
minutes to ~6 minutes.  But I don't think we've seen a lot of
complaints, probably because that time is lumped in with "time to
transfer a gigabyte of data", so unless you're on a slow machine on fast
connection, you don't even really notice.

For day-to-day operations in a repository, I never came up with a good
example where the speed difference mattered. I think Dscho's giant-index
example is an outlier and the right answer there is not "pick a fast
crypto algorithm" but "stop using a slow crypto algorithm as a
checksum" (and also, stop routinely reading and writing 400MB for
day-to-day operations).

-Peff

Re: RFC v3: Another proposed hash function transition plan

[Linus Torvalds]

On Mon, Oct 2, 2017 at 7:00 AM, Jason Cooper  wrote:
>
> Ahhh, so if I understand you correctly, you'd prefer SHA-256 over
> SHA3-256 because it's more performant for your usecase?  Well, that's a
> completely different animal that cryptographic suitability.

In almost all loads I've seen, zlib inflate() cost is a bigger deal
than the crypto load. The crypto people talk about cycles per byte,
but the deflate code is what usually takes the page faults and cache
misses etc, and has bad branch prediction. That ends up easily being
tens or thousands of cycles, even for small data.

But it does obviously depend on exactly what you do. The Windows
people saw SHA1 as costly mainly due to the index file (which is just
a "fancy crc", and not even cryptographically important, and where the
cache misses actually happen when doing crypto, not decompressing the
data).

And fsck and big initial checkins can have a very different profile
than most "regular use" profiles. Again, there the crypto happens
first, and takes the cache misses. And the crypto is almost certainly
_much_ cheaper than just the act of loading the index file contents in
the first place. It may show up on profiles fairly clearly, but that's
mostly because crypto is *intensive*, not because crypto takes up most
of the cycles.

End result: honestly, the real cost on almost any load is not crypto
or necessarily even (de)compression, even if those are the things that
show up. It's the cache misses and the "get data into user space"
(whether using "read()" or page faulting). Worrying about cycles per
byte of compression speed is almost certainly missing the real issue.

The people who benchmark cryptography tend to intentionally avoid the
actual real work, because they just want to know the crypto costs. So
when you see numbers like "9 cycles per byte" vs "12 cycles per byte"
and think that it's a big deal - 30% performance difference! -  it's
almost certainly complete garbage. It may be 30%, but it is likely 30%
out of 10% total, meaning that it's almost in the noise for any but
some very special case.

 Linus

Re: RFC v3: Another proposed hash function transition plan

[Brandon Williams]

On 10/02, Jason Cooper wrote:
> Hi Jonathan,
> 
> On Tue, Sep 26, 2017 at 04:51:58PM -0700, Jonathan Nieder wrote:
> > Johannes Schindelin wrote:
> > > On Tue, 26 Sep 2017, Jason Cooper wrote:
> > >> For my use cases, as a user of git, I have a plan to maintain provable
> > >> integrity of existing objects stored in git under sha1 while migrating
> > >> away from sha1.  The same plan works for migrating away from SHA2 or
> > >> SHA3 when the time comes.
> > >
> > > Please do not make the mistake of taking your use case to be a template
> > > for everybody's use case.
> > 
> > That said, I'm curious at what plan you are alluding to.  Is it
> > something that could benefit others on the list?
> 
> Well, it's just a plan at this point.  As there's a lot of other work to
> do in the mean-time, and there's no possibility of transitioning until
> the dust has settled on NEWHASH.  :-)
> 
> Given an existing repository that needs to migrate from SHA1 to NEWHASH,
> and maintain backwards compatibility with clients that haven't migrated
> yet, how do we
> 
>   a) perform that migration,
>   b) allow non-updated clients to use the data prior to the switch, and
>   c) maintain provable integrity of the old objects as well as the new.
> 
> The primary method is counter-hashing, which re-uses the blobs, and
> creates parallel, deterministic tree, commit, and tag objects using
> NEWHASH for everything up to flag day.  post-flag-day only uses NEWHASH.
> A PGP "transition" key is used to counter-sign the NEWHASH version of
> the old signed tags.  The transition key is not required to be different
> than the existing maintainers key.
> 
> A critical feature is the ability of entities other than the maintainer
> to migrate to NEWHASH.  For example, let's say that git has fully
> implemented and tested NEWHASH.  linux.git intends to migrate, but it's
> going to take several months (get all the developers herded up).
> 
> In the interim, a security company, relying on Linux for it's products
> can counter-hash Linus' repo, and continue to do so every time he
> updates his tree.  This shrinks the attack window for an entity (with an
> undisclosed break of SHA1) down to a few minutes to an hour.  Otherwise,
> a check of the counter hashes in the future would reveal the
> substitution.
> 
> The deterministic feature is critical here because there is valuable
> integrity and trust built by counter-hashing quickly after publication.
> So once Linux migrates to NEWHASH, the hashes calculated by the security
> company should be identical.  IOW, use the timestamps that are in the
> SHA1 commit objects for the NEWHASH objects.  Which should be obvious,
> but it's worth explicitly mentioning that determinism provides great
> value.
> 
> We're in the process of writing this up formally, which will provide a
> lot more detail and rationale that this quick stream of thought.  :-)
> 
> I'm sure a lot of this has already been discussed on the list.  If so, I
> apologize for being repetitive.  Unfortunately, I'm not able to keep up
> with the MLs like I used to.
> 
> thx,
> 
> Jason.

Given the interests that you've expressed here I'd recommend taking a
look at
https://public-inbox.org/git/20170928044320.ga84...@aiede.mtv.corp.google.com/
which is the current version of the transition plan that the community
has settled on
(https://public-inbox.org/git/xmqqlgkyxgvq@gitster.mtv.corp.google.com/
shows that it should be merged to 'next' soon).  Once neat aspect of
this transition plan is that it doesn't require a flag day but rather
anyone can migrate to the new hash function and still interact with
repositories (via the wire) which are still running SHA1.

-- 
Brandon Williams

Re: RFC v3: Another proposed hash function transition plan

[Jason Cooper]

Hi Jonathan,

On Tue, Sep 26, 2017 at 04:51:58PM -0700, Jonathan Nieder wrote:
> Johannes Schindelin wrote:
> > On Tue, 26 Sep 2017, Jason Cooper wrote:
> >> For my use cases, as a user of git, I have a plan to maintain provable
> >> integrity of existing objects stored in git under sha1 while migrating
> >> away from sha1.  The same plan works for migrating away from SHA2 or
> >> SHA3 when the time comes.
> >
> > Please do not make the mistake of taking your use case to be a template
> > for everybody's use case.
> 
> That said, I'm curious at what plan you are alluding to.  Is it
> something that could benefit others on the list?

Well, it's just a plan at this point.  As there's a lot of other work to
do in the mean-time, and there's no possibility of transitioning until
the dust has settled on NEWHASH.  :-)

Given an existing repository that needs to migrate from SHA1 to NEWHASH,
and maintain backwards compatibility with clients that haven't migrated
yet, how do we

  a) perform that migration,
  b) allow non-updated clients to use the data prior to the switch, and
  c) maintain provable integrity of the old objects as well as the new.

The primary method is counter-hashing, which re-uses the blobs, and
creates parallel, deterministic tree, commit, and tag objects using
NEWHASH for everything up to flag day.  post-flag-day only uses NEWHASH.
A PGP "transition" key is used to counter-sign the NEWHASH version of
the old signed tags.  The transition key is not required to be different
than the existing maintainers key.

A critical feature is the ability of entities other than the maintainer
to migrate to NEWHASH.  For example, let's say that git has fully
implemented and tested NEWHASH.  linux.git intends to migrate, but it's
going to take several months (get all the developers herded up).

In the interim, a security company, relying on Linux for it's products
can counter-hash Linus' repo, and continue to do so every time he
updates his tree.  This shrinks the attack window for an entity (with an
undisclosed break of SHA1) down to a few minutes to an hour.  Otherwise,
a check of the counter hashes in the future would reveal the
substitution.

The deterministic feature is critical here because there is valuable
integrity and trust built by counter-hashing quickly after publication.
So once Linux migrates to NEWHASH, the hashes calculated by the security
company should be identical.  IOW, use the timestamps that are in the
SHA1 commit objects for the NEWHASH objects.  Which should be obvious,
but it's worth explicitly mentioning that determinism provides great
value.

We're in the process of writing this up formally, which will provide a
lot more detail and rationale that this quick stream of thought.  :-)

I'm sure a lot of this has already been discussed on the list.  If so, I
apologize for being repetitive.  Unfortunately, I'm not able to keep up
with the MLs like I used to.

thx,

Jason.

Re: RFC v3: Another proposed hash function transition plan

[Johannes Schindelin]

Hi Joan,

On Sun, 1 Oct 2017, Joan Daemen wrote:

> On 30/09/17 00:33, Johannes Schindelin wrote:
> 
> > As far as Git is concerned, we not only care about the source code of
> > the hash algorithm we use, we need to care even more about what you
> > call "executable": ready-to-use, high quality, well-tested
> > implementations.
> > 
> > We carry source code for SHA-1 as part of Git's source code, which was
> > hand-tuned to be as fast as Linus could get it, which was tricky given
> > that the tuning should be general enough to apply to all common intel
> > CPUs.
> > 
> > This hand-crafted code was blown out of the water by OpenSSL's SHA-1
> > in our tests here at Microsoft, thanks to the fact that OpenSSL does
> > vectorized SHA-1 computation now.
> > 
> > To me, this illustrates why it is not good enough to have only a
> > reference implementation available at our finger tips. Of course,
> > above-mentioned OpenSSL supports SHA-256 and SHA3-256, too, and at
> > least recent versions vectorize those, too.
> 
> There is a lot of high-quality optimized code for all SHA-3 functions
> and many CPUs in the Keccak code package
> https://github.com/gvanas/KeccakCodePackage but also OpenSSL contains
> some good SHA-3 code and then there are all those related to Ethereum.
> 
> By the way, you speak about SHA3-256, but the right choice would be to
> use SHAKE128. Well, what is exactly the right choice depends on what you
> want. If you want to have a function in the SHA3 standard (FIPS 202), it
> is SHAKE128.  You can boost performance on high-end CPUs by adopting
> Parallelhash from NIST SP 800-185, still a NIST standard. You can
> multiply that performance again by a factor of 2 by adopting
> KangarooTwelve. This is our (Keccak team) proposal for a parallelizable
> Keccak-based hash function that has a safety margin comparable to that
> of the SHA-2 functions. See https://keccak.team/kangarootwelve.html May
> I also suggest you read https://keccak.team/2017/is_sha3_slow.html

Thanks.

I have to admit that all those names that do not start with SHA and do not
end in 256 make me a bit dizzy.

> > Back to Intel processors: I read some vague hints about extensions
> > accelerating SHA-256 computation on future Intel processors, but not
> > SHA3-256.
> > 
> > It would make sense, of course, that more crypto libraries and more
> > hardware support would be available for SHA-256 than for SHA3-256
> > given the time since publication: 16 vs 5 years (I am playing it loose
> > here, taking just the year into account, not the exact date, so please
> > treat that merely as a ballpark figure).
> > 
> > So from a practical point of view, I wonder what your take is on, say,
> > hardware support for SHA3-256. Do you think this will become a focus
> > soon?
> 
> I think this is a chicken-and-egg problem. In any case, hardware support
> for one SHA3-256 will also work for the other SHA3 and SHAKE functions
> as they all use the same underlying primitive: the Keccak-f permutation.
> This is not the case for SHA2 because SHA224 and SHA256 use a different
> compression function than SHA384, SHA512, SHA512/224 and SHA512/256.

Okay.

So given that Git does not exactly have a big sway on hardware vendors, we
would have to hope that some other chicken lays that egg.

> > Also, what is your take on the question whether SHA-256 is good
> > enough?  SHA-1 was broken theoretically already 10 years after it was
> > published (which unfortunately did not prevent us from baking it into
> > Git), after all, while SHA-256 is 16 years old and the only known
> > weakness does not apply to Git's usage?
> 
> SHA-256 is more conservative than SHA-1 and I don't expect it to be
> broken in the coming decades (unless NSA inserted a backdoor but I don't
> think that is likely). But looking at the existing cryptanalysis, I
> think it is even less likely that I SHAKE128, ParallelHash or
> KangarooTwelve will be broken anytime.

That's reassuring! ;-)

> > Also, while I have the attention of somebody who knows a heck more
> > about cryptography than Git's top 10 committers combined: how soon do
> > you expect practical SHA-1 attacks that are much worse than what we
> > already have seen? I am concerned that if we do not move fast enough
> > to a new hash algorithm, and somebody finds a way in the meantime to
> > craft arbitrary messages given a prefix and an SHA-1, then we have a
> > huge problem on our hands.
> 
> This is hard to say. To be honest, when witnessing the first MD5
> collisions I did not expect them to lead to some real world attacks and
> just a few years later we saw real-world forged certificates based on
> MD5 collisions. And SHA-1 has a lot in common with MD5...

Oh, okay. I did not realize that MD5 and SHA-1 are so similar in design,
thank you for educating me!

> But let me end with a philosophical note. Independent of all the
> arguments for and against, I think this is ultimately about doing the
> right thing. The choice is here 

Re: RFC v3: Another proposed hash function transition plan

[Jason Cooper]

Hi Johannes,

Thanks for the response.  Sorry for the delay.  Had a large deadline for
$dayjob.

On Wed, Sep 27, 2017 at 12:11:14AM +0200, Johannes Schindelin wrote:
> On Tue, 26 Sep 2017, Jason Cooper wrote:
> > On Thu, Sep 14, 2017 at 08:45:35PM +0200, Johannes Schindelin wrote:
> > > On Wed, 13 Sep 2017, Linus Torvalds wrote:
> > > > On Wed, Sep 13, 2017 at 6:43 AM, demerphq  wrote:
> > > > > SHA3 however uses a completely different design where it mixes a 1088
> > > > > bit block into a 1600 bit state, for a leverage of 2:3, and the excess
> > > > > is *preserved between each block*.
> > > > 
> > > > Yes. And considering that the SHA1 attack was actually predicated on
> > > > the fact that each block was independent (no extra state between), I
> > > > do think SHA3 is a better model.
> > > > 
> > > > So I'd rather see SHA3-256 than SHA256.
> > 
> > Well, for what it's worth, we need to be aware that SHA3 is *different*.
> > In crypto, "different" = "bugs haven't been found yet".  :-P
> > 
> > And SHA2 is *known*.  So we have a pretty good handle on how it'll
> > weaken over time.
> 
> Here, you seem to agree with me.

Yep.

> > > SHA-256 got much more cryptanalysis than SHA3-256, and apart from the
> > > length-extension problem that does not affect Git's usage, there are no
> > > known weaknesses so far.
> > 
> > While I think that statement is true on it's face (particularly when
> > including post-competition analysis), I don't think it's sufficient
> > justification to chose one over the other.
> 
> And here you don't.
> 
> I find that very confusing.

What I'm saying is that there is more to selecting a hash function for
git than just the cryptographic assessment.  In fact I would argue that
the primary cryptographic concern for git is "What is the likelihood
that we'll wake up one day to full collisions with no warning?"

To that, I'd argue that SHA-256's time in the field and SHA3-256's
competition give them both passing marks in that regard.  fwiw, I'd also
put Blake and Skein in there as well.

The chance that any of those will suffer sudden, catastrophic failure is
minimal.  IOW, we'll have warnings, and time to migrate to the next
function.

None of us can predict the future, but having a significant amount of
vetting reduces the chances of catastrophic failure.

> > > It would seem that the experts I talked to were much more concerned about
> > > that amount of attention than the particulars of the algorithm. My
> > > impression was that the new features of SHA3 were less studied than the
> > > well-known features of SHA2, and that the new-ness of SHA3 is not
> > > necessarily a good thing.
> > 
> > The only thing I really object to here is the abstract "experts".  We're
> > talking about cryptography and integrity here.  It's no longer
> > sufficient to cite anonymous experts.  Either they can put their
> > thoughts, opinions and analysis on record here, or it shouldn't be
> > considered.  Sorry.
> 
> Sorry, you are asking cryptography experts to spend their time on the Git
> mailing list. I tried to get them to speak out on the Git mailing list.
> They respectfully declined.

Ok, fair enough.  Just please understand that it's difficult to place
much weight on statements that we can't discuss with the person who made
them.

> > However, whether we chose SHA2 or SHA3 doesn't matter.
> 
> To you, it does not matter.

Well, I'd say it does not matter for *most* users.

> To me, it matters. To the several thousand developers working on Windows,
> probably the largest Git repository in active use, it matters. It matters
> because the speed difference that has little impact on you has a lot more
> impact on us.

Ahhh, so if I understand you correctly, you'd prefer SHA-256 over
SHA3-256 because it's more performant for your usecase?  Well, that's a
completely different animal that cryptographic suitability.

Have you been able to crunch numbers yet?  Will you be able to share
some empirical data?  I'd love to see some comparisons between SHA1,
SHA-256, SHA512-256, and SHA3-256 for different git operations under
your work load.

> > If SHA3 is chosen as the successor, it's going to get a *lot* more
> > adoption, and thus, a lot more analysis.  If cracks start to show, the
> > hard work of making git flexible is already done.  We can migrate to
> > SHA4/5/whatever in an orderly fashion with far less effort than the
> > transition away from SHA1.
> 
> Sure. And if XYZ789 is chosen, it's going to get a *lot* more adoption,
> too.
> 
> We think.
> 
> Let's be realistic. Git is pretty important to us, but it is not important
> enough to sway, say, Intel into announcing hardware support for SHA3.
> And if you try to force through *any* hash function only so that it gets
> more adoption and hence more support,

That's quite a jump from what I was saying.  I would never advise using
code in a production setting just to increase adoption.

What I /was/ saying: Let's say you don't get 

Re: RFC v3: Another proposed hash function transition plan

[Joan Daemen]

time.


Also, while I have the attention of somebody who knows a heck more 
about
cryptography than Git's top 10 committers combined: how soon do you 
expect

practical SHA-1 attacks that are much worse than what we already have
seen? I am concerned that if we do not move fast enough to a new hash
algorithm, and somebody finds a way in the meantime to craft arbitrary
messages given a prefix and an SHA-1, then we have a huge problem on
our hands.


This is hard to say. To be honest, when witnessing the first MD5 
collisions I did not expect them to lead to some real world attacks and 
just a few years later we saw real-world forged certificates based on 
MD5 collisions. And SHA-1 has a lot in common with MD5...


But let me end with a philosophical note. Independent of all the 
arguments for and against, I think this is ultimately about doing the 
right thing. The choice is here between SHA1/SHA2 on the one hand and 
SHA3/Keccak on the other. The former standards are imposed on us by NSA 
and the latter are the best that came out of an open competition 
involving all experts in the field worldwide. What would be closest to 
the philosophy of Git (and by extension Linux or open-source in 
general)?


Kind regards,

Joan


On 30/09/17 00:33, Johannes Schindelin wrote:

Hi Joan,

On Fri, 29 Sep 2017, Joan Daemen wrote:

if ever there was a SHA-2 competition, it must have been held inside 
NSA:-)

Oops. My bad, I indeed got confused about that, as you suggest below (I
actually thought of the AES competition, but that was obviously not 
about

SHA-2). Sorry.


But maybe you are confusing with the SHA-3 competition. In any case,
when considering SHA-2 vs SHA-3 for usage in git, you may have a look 
at

arguments we give in the following blogpost:

https://keccak.team/2017/open_source_crypto.html

Thanks for the pointer!

Small nit: the post uses "its" in place of "it's", twice.

It does have a good point, of course: the scientific exchange (which 
you

call "open-source" in spirit) makes tons of sense.

As far as Git is concerned, we not only care about the source code of 
the

hash algorithm we use, we need to care even more about what you call
"executable": ready-to-use, high quality, well-tested implementations.

We carry source code for SHA-1 as part of Git's source code, which was
hand-tuned to be as fast as Linus could get it, which was tricky given
that the tuning should be general enough to apply to all common intel
CPUs.

This hand-crafted code was blown out of the water by OpenSSL's SHA-1 in
our tests here at Microsoft, thanks to the fact that OpenSSL does
vectorized SHA-1 computation now.

To me, this illustrates why it is not good enough to have only a 
reference

implementation available at our finger tips. Of course, above-mentioned
OpenSSL supports SHA-256 and SHA3-256, too, and at least recent 
versions

vectorize those, too.

Also, ARM processors have become a lot more popular, so we'll want to 
have

high-quality implementations of the hash algorithm also for those
processors.

Likewise, in contrast to 2005, nowadays implementations of Git in
languages as obscure as Javascript are not only theoretical but do 
exist
in practice (https://github.com/creationix/js-git). I had a *very* 
quick

look for libraries providing crypto in Javascript and immediately found
the Standford Javascript Crypto library
(https://github.com/bitwiseshiftleft/sjcl/) which seems to offer 
SHA-256

but not SHA3-256 computation.

Back to Intel processors: I read some vague hints about extensions
accelerating SHA-256 computation on future Intel processors, but not
SHA3-256.

It would make sense, of course, that more crypto libraries and more
hardware support would be available for SHA-256 than for SHA3-256 given
the time since publication: 16 vs 5 years (I am playing it loose here,
taking just the year into account, not the exact date, so please treat
that merely as a ballpark figure).

So from a practical point of view, I wonder what your take is on, say,
hardware support for SHA3-256. Do you think this will become a focus 
soon?


Also, what is your take on the question whether SHA-256 is good enough?
SHA-1 was broken theoretically already 10 years after it was published
(which unfortunately did not prevent us from baking it into Git), after
all, while SHA-256 is 16 years old and the only known weakness does not
apply to Git's usage?

Also, while I have the attention of somebody who knows a heck more 
about
cryptography than Git's top 10 committers combined: how soon do you 
expect

practical SHA-1 attacks that are much worse than what we already have
seen? I am concerned that if we do not move fast enough to a new hash
algorithm, and somebody finds a way in the meantime to craft arbitrary
messages given a prefix and an SHA-1, then we have a huge problem on
our hands.

Ciao,
Johannes




Begin forwarded message:

From: Gilles Van Assche <gilles.van.ass...@noekeon.org>
Subject:

Re: RFC v3: Another proposed hash function transition plan

[Johannes Schindelin]

Hi Joan,

On Fri, 29 Sep 2017, Joan Daemen wrote:

> if ever there was a SHA-2 competition, it must have been held inside NSA:-)

Oops. My bad, I indeed got confused about that, as you suggest below (I
actually thought of the AES competition, but that was obviously not about
SHA-2). Sorry.

> But maybe you are confusing with the SHA-3 competition. In any case,
> when considering SHA-2 vs SHA-3 for usage in git, you may have a look at
> arguments we give in the following blogpost:
> 
> https://keccak.team/2017/open_source_crypto.html

Thanks for the pointer!

Small nit: the post uses "its" in place of "it's", twice.

It does have a good point, of course: the scientific exchange (which you
call "open-source" in spirit) makes tons of sense.

As far as Git is concerned, we not only care about the source code of the
hash algorithm we use, we need to care even more about what you call
"executable": ready-to-use, high quality, well-tested implementations.

We carry source code for SHA-1 as part of Git's source code, which was
hand-tuned to be as fast as Linus could get it, which was tricky given
that the tuning should be general enough to apply to all common intel
CPUs.

This hand-crafted code was blown out of the water by OpenSSL's SHA-1 in
our tests here at Microsoft, thanks to the fact that OpenSSL does
vectorized SHA-1 computation now.

To me, this illustrates why it is not good enough to have only a reference
implementation available at our finger tips. Of course, above-mentioned
OpenSSL supports SHA-256 and SHA3-256, too, and at least recent versions
vectorize those, too.

Also, ARM processors have become a lot more popular, so we'll want to have
high-quality implementations of the hash algorithm also for those
processors.

Likewise, in contrast to 2005, nowadays implementations of Git in
languages as obscure as Javascript are not only theoretical but do exist
in practice (https://github.com/creationix/js-git). I had a *very* quick
look for libraries providing crypto in Javascript and immediately found
the Standford Javascript Crypto library
(https://github.com/bitwiseshiftleft/sjcl/) which seems to offer SHA-256
but not SHA3-256 computation.

Back to Intel processors: I read some vague hints about extensions
accelerating SHA-256 computation on future Intel processors, but not
SHA3-256.

It would make sense, of course, that more crypto libraries and more
hardware support would be available for SHA-256 than for SHA3-256 given
the time since publication: 16 vs 5 years (I am playing it loose here,
taking just the year into account, not the exact date, so please treat
that merely as a ballpark figure).

So from a practical point of view, I wonder what your take is on, say,
hardware support for SHA3-256. Do you think this will become a focus soon?

Also, what is your take on the question whether SHA-256 is good enough?
SHA-1 was broken theoretically already 10 years after it was published
(which unfortunately did not prevent us from baking it into Git), after
all, while SHA-256 is 16 years old and the only known weakness does not
apply to Git's usage?

Also, while I have the attention of somebody who knows a heck more about
cryptography than Git's top 10 committers combined: how soon do you expect
practical SHA-1 attacks that are much worse than what we already have
seen? I am concerned that if we do not move fast enough to a new hash
algorithm, and somebody finds a way in the meantime to craft arbitrary
messages given a prefix and an SHA-1, then we have a huge problem on
our hands.

Ciao,
Johannes

Re: RFC v3: Another proposed hash function transition plan

[Joan Daemen]

Dear Johannes,

if ever there was a SHA-2 competition, it must have been held inside 
NSA:-) But maybe you are confusing with the SHA-3 competition. In any 
case, when considering SHA-2 vs SHA-3 for usage in git, you may have a 
look at arguments we give in the following blogpost:


https://keccak.team/2017/open_source_crypto.html

Kind regards,

Joan Daemen

On 29/09/17 15:17, Johannes Schindelin wrote:

Hi Gilles,

On Tue, 19 Sep 2017, Gilles Van Assche wrote:


On 19/09/17 00:16, Johannes Schindelin wrote:

SHA-256 got much more cryptanalysis than SHA3-256 […].

I do not think this is true.

Please read what I said again: SHA-256 got much more cryptanalysis
than SHA3-256.

Indeed. What I meant is that SHA3-256 got at least as much cryptanalysis
as SHA-256. :-)

Oh? I got the opposite impression... I got the impression that *everybody*
in the field banged on all the SHA-2 candidates because everybody was
worried that SHA-1 would be utterly broken soon, and I got the impression
that after this SHA-2 competition, people were less worried?

Besides, I would expect that the difference in age (at *least* 7 years by
my humble arithmetic skills) to make a difference...


I never said that SHA3-256 got little cryptanalysis. Personally, I
think that SHA3-256 got a ton more cryptanalysis than SHA-1, and that
SHA-256 *still* got more cryptanalysis. But my opinion does not count,
really. However, the two experts I pestered with questions over
questions left me with that strong impression, and their opinion does
count.

OK, I respect your opinion and that of your two experts. Yet, the "much
more" part of your statement, in particular, is something that may
require a bit more explanations.

I would also like to point out the ubiquitousness of SHA-256. I have been
asked to provide SHA-256 checksums for the downloads of Git for Windows,
but not SHA3-256...

And this is a practically-relevant thing: the more users of an algorithm
there are, the more high-quality implementations you can choose from. And
this becomes relevant, say, when you have to switch implementations due to
license changes (*cough, cough looking in OpenSSL's direction*). Or when
you have to support the biggest Git repository on this planet and have to
eek out 5-10% more performance using the latest hardware. All of a sudden,
your consideration cannot only be "security of the algorithm" any longer.

Having said that, I am *really* happy to have SHA3-256 as a valid fallback
option in case SHA-256 should be broken.

Ciao,
Johannes

Re: RFC v3: Another proposed hash function transition plan

[Johannes Schindelin]

Hi Gilles,

On Tue, 19 Sep 2017, Gilles Van Assche wrote:

> On 19/09/17 00:16, Johannes Schindelin wrote:
> >>> SHA-256 got much more cryptanalysis than SHA3-256 […].
> >>
> >> I do not think this is true.
> >
> > Please read what I said again: SHA-256 got much more cryptanalysis
> > than SHA3-256.
> 
> Indeed. What I meant is that SHA3-256 got at least as much cryptanalysis
> as SHA-256. :-)

Oh? I got the opposite impression... I got the impression that *everybody*
in the field banged on all the SHA-2 candidates because everybody was
worried that SHA-1 would be utterly broken soon, and I got the impression
that after this SHA-2 competition, people were less worried?

Besides, I would expect that the difference in age (at *least* 7 years by
my humble arithmetic skills) to make a difference...

> > I never said that SHA3-256 got little cryptanalysis. Personally, I
> > think that SHA3-256 got a ton more cryptanalysis than SHA-1, and that
> > SHA-256 *still* got more cryptanalysis. But my opinion does not count,
> > really. However, the two experts I pestered with questions over
> > questions left me with that strong impression, and their opinion does
> > count.
> 
> OK, I respect your opinion and that of your two experts. Yet, the "much
> more" part of your statement, in particular, is something that may
> require a bit more explanations.

I would also like to point out the ubiquitousness of SHA-256. I have been
asked to provide SHA-256 checksums for the downloads of Git for Windows,
but not SHA3-256...

And this is a practically-relevant thing: the more users of an algorithm
there are, the more high-quality implementations you can choose from. And
this becomes relevant, say, when you have to switch implementations due to
license changes (*cough, cough looking in OpenSSL's direction*). Or when
you have to support the biggest Git repository on this planet and have to
eek out 5-10% more performance using the latest hardware. All of a sudden,
your consideration cannot only be "security of the algorithm" any longer.

Having said that, I am *really* happy to have SHA3-256 as a valid fallback
option in case SHA-256 should be broken.

Ciao,
Johannes

Re: RFC v3: Another proposed hash function transition plan

[Jonathan Nieder]

Hi,

Johannes Schindelin wrote:

> Sorry, you are asking cryptography experts to spend their time on the Git
> mailing list. I tried to get them to speak out on the Git mailing list.
> They respectfully declined.
>
> I can't fault them, they have real jobs to do, and none of their managers
> would be happy for them to educate the Git mailing list on matters of
> cryptography, not after what happened in 2005.

Fortunately we have had a few public comments from crypto specialists:

https://public-inbox.org/git/91a34c5b-7844-3db2-cf29-411df5bcf...@noekeon.org/
https://public-inbox.org/git/CAL9PXLzhPyE+geUdcLmd=pidt5p8efebbsgx_ds88knz2q_...@mail.gmail.com/
https://public-inbox.org/git/CAL9PXLxMHG1nP5_GQaK_WSJTNKs=_qbaL6V5v2GzVG=9vu2...@mail.gmail.com/
https://public-inbox.org/git/59bfb95d.1030...@st.com/
https://public-inbox.org/git/59c149a3.6080...@st.com/

[...]
> Let's be realistic. Git is pretty important to us, but it is not important
> enough to sway, say, Intel into announcing hardware support for SHA3.

Yes, I agree with this.  (Adoption by Git could lead to adoption by
some other projects, leading to more work on high quality software
implementations in projects like OpenSSL, but I am not convinced that
that would be a good thing for the world anyway.  There are downsides
to a proliferation of too many crypto primitives.  This is the basic
argument described in more detail at [1].)

[...]
> On Tue, 26 Sep 2017, Jason Cooper wrote:

>> For my use cases, as a user of git, I have a plan to maintain provable
>> integrity of existing objects stored in git under sha1 while migrating
>> away from sha1.  The same plan works for migrating away from SHA2 or
>> SHA3 when the time comes.
>
> Please do not make the mistake of taking your use case to be a template
> for everybody's use case.

That said, I'm curious at what plan you are alluding to.  Is it
something that could benefit others on the list?

Thanks,
Jonathan

[1] https://www.imperialviolet.org/2017/05/31/skipsha3.html

Re: RFC v3: Another proposed hash function transition plan

[Johannes Schindelin]

Hi Jason,

On Tue, 26 Sep 2017, Jason Cooper wrote:

> On Thu, Sep 14, 2017 at 08:45:35PM +0200, Johannes Schindelin wrote:
> > On Wed, 13 Sep 2017, Linus Torvalds wrote:
> > > On Wed, Sep 13, 2017 at 6:43 AM, demerphq  wrote:
> > > > SHA3 however uses a completely different design where it mixes a 1088
> > > > bit block into a 1600 bit state, for a leverage of 2:3, and the excess
> > > > is *preserved between each block*.
> > > 
> > > Yes. And considering that the SHA1 attack was actually predicated on
> > > the fact that each block was independent (no extra state between), I
> > > do think SHA3 is a better model.
> > > 
> > > So I'd rather see SHA3-256 than SHA256.
> 
> Well, for what it's worth, we need to be aware that SHA3 is *different*.
> In crypto, "different" = "bugs haven't been found yet".  :-P
> 
> And SHA2 is *known*.  So we have a pretty good handle on how it'll
> weaken over time.

Here, you seem to agree with me.

> > SHA-256 got much more cryptanalysis than SHA3-256, and apart from the
> > length-extension problem that does not affect Git's usage, there are no
> > known weaknesses so far.
> 
> While I think that statement is true on it's face (particularly when
> including post-competition analysis), I don't think it's sufficient
> justification to chose one over the other.

And here you don't.

I find that very confusing.

> > It would seem that the experts I talked to were much more concerned about
> > that amount of attention than the particulars of the algorithm. My
> > impression was that the new features of SHA3 were less studied than the
> > well-known features of SHA2, and that the new-ness of SHA3 is not
> > necessarily a good thing.
> 
> The only thing I really object to here is the abstract "experts".  We're
> talking about cryptography and integrity here.  It's no longer
> sufficient to cite anonymous experts.  Either they can put their
> thoughts, opinions and analysis on record here, or it shouldn't be
> considered.  Sorry.

Sorry, you are asking cryptography experts to spend their time on the Git
mailing list. I tried to get them to speak out on the Git mailing list.
They respectfully declined.

I can't fault them, they have real jobs to do, and none of their managers
would be happy for them to educate the Git mailing list on matters of
cryptography, not after what happened in 2005.

> Other than their anonymity, though, I do agree with your experts
> assessments.

I know what our in-house cryptography experts have to prove to start
working at Microsoft. Forgive me, but you are not a known entity to me.

> However, whether we chose SHA2 or SHA3 doesn't matter.

To you, it does not matter.

To me, it matters. To the several thousand developers working on Windows,
probably the largest Git repository in active use, it matters. It matters
because the speed difference that has little impact on you has a lot more
impact on us.

> Moving away from SHA1 does.  Once the object_id code is in place to
> facilitate that transition, the problem is solved from git's
> perspective.

Uh oh. You forgot the mapping. And the protocol. And pretty much
everything except the oid.

> If SHA3 is chosen as the successor, it's going to get a *lot* more
> adoption, and thus, a lot more analysis.  If cracks start to show, the
> hard work of making git flexible is already done.  We can migrate to
> SHA4/5/whatever in an orderly fashion with far less effort than the
> transition away from SHA1.

Sure. And if XYZ789 is chosen, it's going to get a *lot* more adoption,
too.

We think.

Let's be realistic. Git is pretty important to us, but it is not important
enough to sway, say, Intel into announcing hardware support for SHA3.

And if you try to force through *any* hash function only so that it gets
more adoption and hence more support, in the short run you will make life
harder for developers on more obscure platforms, who may not easily get
high-quality, high-speed implementations of anything but the very
mainstream (which is, let's face it, MD5, SHA-1 and SHA-256). I know I
would have cursed you for such a decision back when I had to work on AIX
and IRIX.

> For my use cases, as a user of git, I have a plan to maintain provable
> integrity of existing objects stored in git under sha1 while migrating
> away from sha1.  The same plan works for migrating away from SHA2 or
> SHA3 when the time comes.

Please do not make the mistake of taking your use case to be a template
for everybody's use case.

Migrating a large team away from any hash function to another one *will*
be painful, and costly.

Migrating will be very costly for hosting companies like GitHub, Microsoft
and BitBucket, too.

Ciao,
Johannes

Re: RFC v3: Another proposed hash function transition plan

[Jason Cooper]

Hi all,

Sorry for late commentary...

On Thu, Sep 14, 2017 at 08:45:35PM +0200, Johannes Schindelin wrote:
> On Wed, 13 Sep 2017, Linus Torvalds wrote:
> > On Wed, Sep 13, 2017 at 6:43 AM, demerphq  wrote:
> > > SHA3 however uses a completely different design where it mixes a 1088
> > > bit block into a 1600 bit state, for a leverage of 2:3, and the excess
> > > is *preserved between each block*.
> > 
> > Yes. And considering that the SHA1 attack was actually predicated on
> > the fact that each block was independent (no extra state between), I
> > do think SHA3 is a better model.
> > 
> > So I'd rather see SHA3-256 than SHA256.

Well, for what it's worth, we need to be aware that SHA3 is *different*.
In crypto, "different" = "bugs haven't been found yet".  :-P

And SHA2 is *known*.  So we have a pretty good handle on how it'll
weaken over time.

> SHA-256 got much more cryptanalysis than SHA3-256, and apart from the
> length-extension problem that does not affect Git's usage, there are no
> known weaknesses so far.

While I think that statement is true on it's face (particularly when
including post-competition analysis), I don't think it's sufficient
justification to chose one over the other.

> It would seem that the experts I talked to were much more concerned about
> that amount of attention than the particulars of the algorithm. My
> impression was that the new features of SHA3 were less studied than the
> well-known features of SHA2, and that the new-ness of SHA3 is not
> necessarily a good thing.

The only thing I really object to here is the abstract "experts".  We're
talking about cryptography and integrity here.  It's no longer
sufficient to cite anonymous experts.  Either they can put their
thoughts, opinions and analysis on record here, or it shouldn't be
considered.  Sorry.

Other than their anonymity, though, I do agree with your experts
assessments.

However, whether we chose SHA2 or SHA3 doesn't matter.  Moving away from
SHA1 does.  Once the object_id code is in place to facilitate that
transition, the problem is solved from git's perspective.

If SHA3 is chosen as the successor, it's going to get a *lot* more
adoption, and thus, a lot more analysis.  If cracks start to show, the
hard work of making git flexible is already done.  We can migrate to
SHA4/5/whatever in an orderly fashion with far less effort than the
transition away from SHA1.

For my use cases, as a user of git, I have a plan to maintain provable
integrity of existing objects stored in git under sha1 while migrating
away from sha1.  The same plan works for migrating away from SHA2 or
SHA3 when the time comes.


thx,

Jason.

Re: RFC v3: Another proposed hash function transition plan

[Gilles Van Assche]

Hi Johannes,

Thanks for your feedback.

On 19/09/17 00:16, Johannes Schindelin wrote:
>>> SHA-256 got much more cryptanalysis than SHA3-256 […]. 
>>
>> I do not think this is true. 
>
> Please read what I said again: SHA-256 got much more cryptanalysis
> than SHA3-256.

Indeed. What I meant is that SHA3-256 got at least as much cryptanalysis
as SHA-256. :-)

> I never said that SHA3-256 got little cryptanalysis. Personally, I
> think that SHA3-256 got a ton more cryptanalysis than SHA-1, and that
> SHA-256 *still* got more cryptanalysis. But my opinion does not count,
> really. However, the two experts I pestered with questions over
> questions left me with that strong impression, and their opinion does
> count.

OK, I respect your opinion and that of your two experts. Yet, the "much
more" part of your statement, in particular, is something that may
require a bit more explanations.

Kind regards,
Gilles

Re: RFC v3: Another proposed hash function transition plan

[Jonathan Nieder]

Hi,

Gilles Van Assche wrote:
> Hi Johannes,

>> SHA-256 got much more cryptanalysis than SHA3-256 […].
>
> I do not think this is true. Keccak/SHA-3 actually got (and is still
> getting) a lot of cryptanalysis, with papers published at renowned
> crypto conferences [1].
>
> Keccak/SHA-3 is recognized to have a significant safety margin. E.g.,
> one can cut the number of rounds in half (as in Keyak or KangarooTwelve)
> and still get a very strong function. I don't think we could say the
> same for SHA-256 or SHA-512…

I just wanted to thank you for paying attention to this conversation
and weighing in.

Most of the regulars in the git project are not crypto experts.  This
kind of extra information (and e.g. [2]) is very useful to us.

Thanks,
Jonathan

> Kind regards,
> Gilles, for the Keccak team
>
> [1] https://keccak.team/third_party.html
[2] 
https://public-inbox.org/git/91a34c5b-7844-3db2-cf29-411df5bcf...@noekeon.org/

Re: RFC v3: Another proposed hash function transition plan

[Johannes Schindelin]

Hi Gilles,

On Mon, 18 Sep 2017, Gilles Van Assche wrote:

> > SHA-256 got much more cryptanalysis than SHA3-256 […].
> 
> I do not think this is true.

Please read what I said again: SHA-256 got much more cryptanalysis than
SHA3-256.

I never said that SHA3-256 got little cryptanalysis. Personally, I think
that SHA3-256 got a ton more cryptanalysis than SHA-1, and that SHA-256
*still* got more cryptanalysis. But my opinion does not count, really.
However, the two experts I pestered with questions over questions left me
with that strong impression, and their opinion does count.

> Keccak/SHA-3 actually got (and is still getting) a lot of cryptanalysis,
> with papers published at renowned crypto conferences [1].
> 
> Keccak/SHA-3 is recognized to have a significant safety margin. E.g.,
> one can cut the number of rounds in half (as in Keyak or KangarooTwelve)
> and still get a very strong function. I don't think we could say the
> same for SHA-256 or SHA-512…

Again, I do not want to criticize SHA3/Keccak. Personally, I have a lot of
respect for Keccak.

I also have a lot of respect for everybody who scrutinized the SHA2 family
of algorithms.

I also respect the fact that there are more implementations of SHA-256,
and thanks to everybody seeming to demand SHA-256 checksums instead of
SHA-1 or MD5 for downloads, bugs in those implementations are probably
discovered relatively quickly, and I also cannot ignore the prospect of
hardware support for SHA-256.

In any case, having SHA3 as a fallback in case SHA-256 gets broken seems
like a very good safety net to me.

Ciao,
Johannes

Re: RFC v3: Another proposed hash function transition plan

[Gilles Van Assche]

Hi Johannes,

> SHA-256 got much more cryptanalysis than SHA3-256 […].

I do not think this is true. Keccak/SHA-3 actually got (and is still
getting) a lot of cryptanalysis, with papers published at renowned
crypto conferences [1].

Keccak/SHA-3 is recognized to have a significant safety margin. E.g.,
one can cut the number of rounds in half (as in Keyak or KangarooTwelve)
and still get a very strong function. I don't think we could say the
same for SHA-256 or SHA-512…

Kind regards,
Gilles, for the Keccak team

[1] https://keccak.team/third_party.html

Re: RFC v3: Another proposed hash function transition plan

[Philip Oakley]

Hi Jonathan,

"Jonathan Nieder"  wrote;

Johannes Schindelin wrote:

On Wed, 13 Sep 2017, Jonathan Nieder wrote:



As a side note, I am probably misreading, but I found this set of
paragraphs a bit condescending.  It sounds to me like you are saying
"You are making the wrong choice of hash function and everything else
you are describing is irrelevant when compared to that monumental
mistake.  Please stop working on things I don't consider important".
With that reading it is quite demotivating to read.


I am sorry you read it that way. I did not feel condescending when I 
wrote

that mail, I felt annoyed by the side track, and anxious. In my mind, the
transition is too important for side tracking, and I worry that we are 
not

fast enough (imagine what would happen if a better attack was discovered
that is not as easily detected as the one we know about?).


Thanks for clarifying.  That makes sense.

[...]

As to *my* opinion: after reading https://goo.gl/gh2Mzc (is it really
correct that its last update has been on March 6th?), my only concern is
really that it still talks about SHA3-256 when I think that the
performance benefits of SHA-256 (think: "Git at scale", and also hardware
support) really make the latter a better choice.

In order to be "ironed out", I think we need to talk about the
implementation detail "Translation table". This is important. It needs to
be *fast*.

Speaking of *fast*, I could imagine that it would make sense to store the
SHA-1 objects on disk, still, instead of converting them on the fly. I am
not sure whether this is something we need to define in the document,
though, as it may very well be premature optimization; Maybe mention that
we could do this if necessary?

Apart from that, I would *love* to see this document as The Official Plan
that I can Show To The Manager so that I can ask to Allocate Time.


Sounds promising!

Thanks much for this feedback.  This is very helpful for knowing what
v4 of the doc needs.

The discussion of the translation table in [1] didn't make it to the
doc.  You're right that it needs to.

Caching SHA-1 objects (and the pros and cons involved) makes sense to
mention in an "ideas for future work" section.

An implementation plan with well-defined pieces for people to take on
and estimates of how much work each involves may be useful for Showing
To The Manager.  So I'll include a sketch of that for reviewers to
poke holes in, too.

Another thing the doc doesn't currently describe is how Git protocol
would work.  That's worth sketching in a "future work" section as
well.

Sorry it has been taking so long to get this out.  I think we should
have something ready to send on Monday.


I had a look at the current doc  https://goo.gl/gh2Mzc and thought that the 
selection of the "NewHash" should be separated out into a section of it's 
own as a 'separation of concerns', so that the general transition plan only 
refers to the "NewHash", so as not to accidentally pre-judge that selection.


I did look up the arguments regarding sha2 (sha256) versus sha3-256 and 
found these two Q items


https://security.stackexchange.com/questions/152360/should-we-be-using-sha3-2017

https://security.stackexchange.com/questions/86283/how-does-sha3-keccak-shake-compare-to-sha2-should-i-use-non-shake-parameter

with an onward link to this:
https://www.imperialviolet.org/2012/10/21/nist.html

"NIST may not have you in mind (21 Oct 2012)"

"A couple of weeks back, NIST announced that Keccak would be SHA-3. Keccak 
has somewhat disappointing software performance but is a gift to hardware 
implementations."


which does appear to cover some of the concerns that dscho had noted, and 
speed does appear to be a core Git selling point.


It would be worth at least covering these trade offs in the "select a 
NewHash" section of the document, as at the end of the day it will be a 
political judgement about what the future might hold regarding the 
contenders.


What may also be worth noting is the fall back plan should the chosen 
NewHash be the first to fail, perhaps spectacularly, as having a ready plan 
could support the choice at risk.




Thanks,
Jonathan

[1] 
https://public-inbox.org/git/CAJo=hJtoX9=aylhhpujs7fuev9ciz_mnpnephuz8whui6g9...@mail.gmail.com/


--
Philip 

Re: RFC v3: Another proposed hash function transition plan

[Johannes Schindelin]

Hi Jonathan,

On Thu, 14 Sep 2017, Jonathan Nieder wrote:

> Johannes Schindelin wrote:
> > On Wed, 13 Sep 2017, Jonathan Nieder wrote:
> 
> >> [3] https://www.imperialviolet.org/2017/05/31/skipsha3.html,
> >
> > I had read this short after it was published, and had missed the updates.
> > One link in particular caught my eye:
> >
> > https://eprint.iacr.org/2012/476
> >
> > Essentially, the authors demonstrate that using SIMD technology can speed
> > up computation by factor 2 for longer messages (2kB being considered
> > "long" already). It is a little bit unclear to me from a cursory look
> > whether their fast algorithm computes SHA-256, or something similar.
> 
> The latter: that paper is about a variant on SHA-256 called SHA-256x4
> (or SHA-256x16 to take advantage of newer instructions).  It's a
> different hash function.  This is what I was alluding to at [1].

Thanks for the explanation!

> > As the author of that paper is also known to have contributed to OpenSSL,
> > I had a quick look and it would appear that a comment in
> > crypto/sha/asm/sha256-mb-x86_64.pl speaking about "lanes" suggests that
> > OpenSSL uses the ideas from the paper, even if b783858654 (x86_64 assembly
> > pack: add multi-block AES-NI, SHA1 and SHA256., 2013-10-03) does not talk
> > about the paper specifically.
> >
> > The numbers shown in
> > https://github.com/openssl/openssl/blob/master/crypto/sha/asm/keccak1600-x86_64.pl#L28
> > and in
> > https://github.com/openssl/openssl/blob/master/crypto/sha/asm/sha256-mb-x86_64.pl#L17
> >
> > are sufficiently satisfying.
> 
> This one is about actual SHA-256, but computing the hash of multiple
> streams in a single funtion call.  The paper to read is [2].  We could
> probably take advantage of it for e.g. bulk-checkin and index-pack.
> Most other code paths that compute hashes wouldn't be able to benefit
> from it.

Again, thanks for the explanation.

Ciao,
Dscho

> [1] 
> https://public-inbox.org/git/20170616212414.gc133...@aiede.mtv.corp.google.com/
> [2] https://eprint.iacr.org/2012/371
> 

Re: RFC v3: Another proposed hash function transition plan

[Johannes Schindelin]

Hi,

On Thu, 14 Sep 2017, demerphq wrote:

> On 14 September 2017 at 17:23, Johannes Schindelin
>  wrote:
> >
> > SHA-256 has been hammered on a lot more than SHA3-256.
> 
> Last year that was even more true of SHA1 than it is true of SHA-256
> today.

I hope you are not deliberately trying to annoy me. I say that because you
seemed to be interested enough in cryptography to know that the known
attacks on SHA-256 *today* are unlikely to extend to Git's use case,
whereas the known attacks on SHA-1 *in 2005* were already raising doubts.

So while SHA-1 has been hammered on for longer than SHA-256, the latter
came out a lot less scathed than the former.

Besides, you are totally missing the point here that the choice is *not*
between SHA-1 and SHA-256, but between SHA-256 and SHA3-256.

After all, we would not consider any hash algorithm with known problems
(as far as Git's usage is concerned). The amount of scrutiny with which
the algorithm was investigated would only be a deciding factor among the
remaining choices, yes?

In any case, don't trust me on cryptography (just like I do not trust you
on that matter). Trust the cryptographers. I contacted some of my
colleagues who are responsible for crypto, and the two who seem to
disagree on pretty much everything agreed on this one thing: that SHA-256
would be a good choice for Git (and one of them suggested that it would be
much better than SHA3-256, because SHA-256 saw more cryptanalysis).

Ciao,
Johannes

Re: RFC v3: Another proposed hash function transition plan

[Jonathan Nieder]

Johannes Schindelin wrote:
> On Wed, 13 Sep 2017, Jonathan Nieder wrote:

>> As a side note, I am probably misreading, but I found this set of
>> paragraphs a bit condescending.  It sounds to me like you are saying
>> "You are making the wrong choice of hash function and everything else
>> you are describing is irrelevant when compared to that monumental
>> mistake.  Please stop working on things I don't consider important".
>> With that reading it is quite demotivating to read.
>
> I am sorry you read it that way. I did not feel condescending when I wrote
> that mail, I felt annoyed by the side track, and anxious. In my mind, the
> transition is too important for side tracking, and I worry that we are not
> fast enough (imagine what would happen if a better attack was discovered
> that is not as easily detected as the one we know about?).

Thanks for clarifying.  That makes sense.

[...]
> As to *my* opinion: after reading https://goo.gl/gh2Mzc (is it really
> correct that its last update has been on March 6th?), my only concern is
> really that it still talks about SHA3-256 when I think that the
> performance benefits of SHA-256 (think: "Git at scale", and also hardware
> support) really make the latter a better choice.
>
> In order to be "ironed out", I think we need to talk about the
> implementation detail "Translation table". This is important. It needs to
> be *fast*.
>
> Speaking of *fast*, I could imagine that it would make sense to store the
> SHA-1 objects on disk, still, instead of converting them on the fly. I am
> not sure whether this is something we need to define in the document,
> though, as it may very well be premature optimization; Maybe mention that
> we could do this if necessary?
>
> Apart from that, I would *love* to see this document as The Official Plan
> that I can Show To The Manager so that I can ask to Allocate Time.

Sounds promising!

Thanks much for this feedback.  This is very helpful for knowing what
v4 of the doc needs.

The discussion of the translation table in [1] didn't make it to the
doc.  You're right that it needs to.

Caching SHA-1 objects (and the pros and cons involved) makes sense to
mention in an "ideas for future work" section.

An implementation plan with well-defined pieces for people to take on
and estimates of how much work each involves may be useful for Showing
To The Manager.  So I'll include a sketch of that for reviewers to
poke holes in, too.

Another thing the doc doesn't currently describe is how Git protocol
would work.  That's worth sketching in a "future work" section as
well.

Sorry it has been taking so long to get this out.  I think we should
have something ready to send on Monday.

Thanks,
Jonathan

[1] 
https://public-inbox.org/git/CAJo=hJtoX9=aylhhpujs7fuev9ciz_mnpnephuz8whui6g9...@mail.gmail.com/

Re: RFC v3: Another proposed hash function transition plan

[Johannes Schindelin]

Hi Linus,

On Wed, 13 Sep 2017, Linus Torvalds wrote:

> On Wed, Sep 13, 2017 at 6:43 AM, demerphq  wrote:
> >
> > SHA3 however uses a completely different design where it mixes a 1088
> > bit block into a 1600 bit state, for a leverage of 2:3, and the excess
> > is *preserved between each block*.
> 
> Yes. And considering that the SHA1 attack was actually predicated on
> the fact that each block was independent (no extra state between), I
> do think SHA3 is a better model.
> 
> So I'd rather see SHA3-256 than SHA256.

SHA-256 got much more cryptanalysis than SHA3-256, and apart from the
length-extension problem that does not affect Git's usage, there are no
known weaknesses so far.

It would seem that the experts I talked to were much more concerned about
that amount of attention than the particulars of the algorithm. My
impression was that the new features of SHA3 were less studied than the
well-known features of SHA2, and that the new-ness of SHA3 is not
necessarily a good thing.

You will have to deal with the fact that I trust the crypto experts'
opinion on this a lot more than your opinion. Sure, you learned from the
fact that you had been warned about SHA-1 already seeing theoretical
attacks in 2005 and still choosing to hard-wire it into Git. And yet, you
are still no more of a cryptography expert than I am.

Ciao,
Dscho

Re: RFC v3: Another proposed hash function transition plan

[Jonathan Nieder]

Hi,

Johannes Schindelin wrote:
> On Wed, 13 Sep 2017, Jonathan Nieder wrote:

>> [3] https://www.imperialviolet.org/2017/05/31/skipsha3.html,
>
> I had read this short after it was published, and had missed the updates.
> One link in particular caught my eye:
>
>   https://eprint.iacr.org/2012/476
>
> Essentially, the authors demonstrate that using SIMD technology can speed
> up computation by factor 2 for longer messages (2kB being considered
> "long" already). It is a little bit unclear to me from a cursory look
> whether their fast algorithm computes SHA-256, or something similar.

The latter: that paper is about a variant on SHA-256 called SHA-256x4
(or SHA-256x16 to take advantage of newer instructions).  It's a
different hash function.  This is what I was alluding to at [1].

> As the author of that paper is also known to have contributed to OpenSSL,
> I had a quick look and it would appear that a comment in
> crypto/sha/asm/sha256-mb-x86_64.pl speaking about "lanes" suggests that
> OpenSSL uses the ideas from the paper, even if b783858654 (x86_64 assembly
> pack: add multi-block AES-NI, SHA1 and SHA256., 2013-10-03) does not talk
> about the paper specifically.
>
> The numbers shown in
> https://github.com/openssl/openssl/blob/master/crypto/sha/asm/keccak1600-x86_64.pl#L28
> and in
> https://github.com/openssl/openssl/blob/master/crypto/sha/asm/sha256-mb-x86_64.pl#L17
>
> are sufficiently satisfying.

This one is about actual SHA-256, but computing the hash of multiple
streams in a single funtion call.  The paper to read is [2].  We could
probably take advantage of it for e.g. bulk-checkin and index-pack.
Most other code paths that compute hashes wouldn't be able to benefit
from it.

Thanks,
Jonathan

[1] 
https://public-inbox.org/git/20170616212414.gc133...@aiede.mtv.corp.google.com/
[2] https://eprint.iacr.org/2012/371

Re: RFC v3: Another proposed hash function transition plan

[Johannes Schindelin]

Hi Jonathan,

On Wed, 13 Sep 2017, Jonathan Nieder wrote:

> [3] https://www.imperialviolet.org/2017/05/31/skipsha3.html,

I had read this short after it was published, and had missed the updates.
One link in particular caught my eye:

https://eprint.iacr.org/2012/476

Essentially, the authors demonstrate that using SIMD technology can speed
up computation by factor 2 for longer messages (2kB being considered
"long" already). It is a little bit unclear to me from a cursory look
whether their fast algorithm computes SHA-256, or something similar.

As the author of that paper is also known to have contributed to OpenSSL,
I had a quick look and it would appear that a comment in
crypto/sha/asm/sha256-mb-x86_64.pl speaking about "lanes" suggests that
OpenSSL uses the ideas from the paper, even if b783858654 (x86_64 assembly
pack: add multi-block AES-NI, SHA1 and SHA256., 2013-10-03) does not talk
about the paper specifically.

The numbers shown in
https://github.com/openssl/openssl/blob/master/crypto/sha/asm/keccak1600-x86_64.pl#L28
and in
https://github.com/openssl/openssl/blob/master/crypto/sha/asm/sha256-mb-x86_64.pl#L17
are sufficiently satisfying.

Ciao,
Dscho

Re: RFC v3: Another proposed hash function transition plan

[Brandon Williams]

On 09/14, Johannes Schindelin wrote:
> Hi Jonathan,
> 
> On Wed, 13 Sep 2017, Jonathan Nieder wrote:
> 
> > As a side note, I am probably misreading, but I found this set of
> > paragraphs a bit condescending.  It sounds to me like you are saying
> > "You are making the wrong choice of hash function and everything else
> > you are describing is irrelevant when compared to that monumental
> > mistake.  Please stop working on things I don't consider important".
> > With that reading it is quite demotivating to read.
> 
> I am sorry you read it that way. I did not feel condescending when I wrote
> that mail, I felt annoyed by the side track, and anxious. In my mind, the
> transition is too important for side tracking, and I worry that we are not
> fast enough (imagine what would happen if a better attack was discovered
> that is not as easily detected as the one we know about?).
> 
> > An alternative reading is that you are saying that the transition plan
> > described in this thread is not ironed out.  Can you spell that out
> > more?  What particular aspect of the transition plan (which is of
> > course orthogonal to the choice of hash function) are you discontent
> > with?
> 
> My impression from reading Junio's mail was that he does not consider the
> transition plan ironed out yet, and that he wants to spend time on
> discussing generation numbers right now.
> 
> I was in particularly frightened by the suggestion to "reboot" [*1*].
> Hopefully I misunderstand and he meant "finishing touches" instead.
> 
> As to *my* opinion: after reading https://goo.gl/gh2Mzc (is it really
> correct that its last update has been on March 6th?), my only concern is
> really that it still talks about SHA3-256 when I think that the
> performance benefits of SHA-256 (think: "Git at scale", and also hardware
> support) really make the latter a better choice.
> 
> In order to be "ironed out", I think we need to talk about the
> implementation detail "Translation table". This is important. It needs to
> be *fast*.

Agreed, when that document was written it was hand waved as an
implementation detail but once we should probably stare ironing out
those details soon so that we have a concrete plan in place.

> 
> Speaking of *fast*, I could imagine that it would make sense to store the
> SHA-1 objects on disk, still, instead of converting them on the fly. I am
> not sure whether this is something we need to define in the document,
> though, as it may very well be premature optimization; Maybe mention that
> we could do this if necessary?
> 
> Apart from that, I would *love* to see this document as The Official Plan
> that I can Show To The Manager so that I can ask to Allocate Time.

Speaking of having a concrete plan, we discussed in office the other day
about finally converting the doc into a Documentation patch.  That was
always are intention but after writing up the doc we got busy working on
other projects.  Getting it in as a patch (with a more concrete road map)
is probably the next step we'd need to take.

I do want to echo what jonathan has said in other parts of this thread,
that the transition plan itself doesn't depend on which hash function we
end up going with in the end.  I fully expect that for the transition
plan to succeed that we'll have infrastructure for dropping in different
hash functions so that we can do some sort of benchmarking before
selecting one to use.  This would also give us the ability to more
easily transition to another hash function when the time comes.

-- 
Brandon Williams

Re: RFC v3: Another proposed hash function transition plan

[demerphq]

On 14 September 2017 at 17:23, Johannes Schindelin
 wrote:
> Hi Junio,
>
> On Thu, 14 Sep 2017, Junio C Hamano wrote:
>
>> Jonathan Nieder  writes:
>>
>> > In other words, a long lifetime for the hash absolutely is a design
>> > goal.  Coping well with an unexpectedly short lifetime for the hash is
>> > also a design goal.
>> >
>> > If the hash function lasts 10 years then I am happy.
>>
>> Absolutely.  When two functions have similar expected remaining life
>> and are equally widely supported, then faster is better than slower.
>> Otherwise our primary goal when picking the function from candidates
>> should be to optimize for its remaining life and wider availability.
>
> SHA-256 has been hammered on a lot more than SHA3-256.

Last year that was even more true of SHA1 than it is true of SHA-256 today.

Anyway,
Yves
-- 
perl -Mre=debug -e "/just|another|perl|hacker/"

Re: RFC v3: Another proposed hash function transition plan

[Johannes Schindelin]

Hi Junio,

On Thu, 14 Sep 2017, Junio C Hamano wrote:

> Jonathan Nieder  writes:
> 
> > In other words, a long lifetime for the hash absolutely is a design
> > goal.  Coping well with an unexpectedly short lifetime for the hash is
> > also a design goal.
> >
> > If the hash function lasts 10 years then I am happy.
> 
> Absolutely.  When two functions have similar expected remaining life
> and are equally widely supported, then faster is better than slower.
> Otherwise our primary goal when picking the function from candidates
> should be to optimize for its remaining life and wider availability.

SHA-256 has been hammered on a lot more than SHA3-256.

That would be a strong point in favor of SHA2.

Ciao,
Dscho

Re: RFC v3: Another proposed hash function transition plan

[Johannes Schindelin]

Hi Jonathan,

On Wed, 13 Sep 2017, Jonathan Nieder wrote:

> As a side note, I am probably misreading, but I found this set of
> paragraphs a bit condescending.  It sounds to me like you are saying
> "You are making the wrong choice of hash function and everything else
> you are describing is irrelevant when compared to that monumental
> mistake.  Please stop working on things I don't consider important".
> With that reading it is quite demotivating to read.

I am sorry you read it that way. I did not feel condescending when I wrote
that mail, I felt annoyed by the side track, and anxious. In my mind, the
transition is too important for side tracking, and I worry that we are not
fast enough (imagine what would happen if a better attack was discovered
that is not as easily detected as the one we know about?).

> An alternative reading is that you are saying that the transition plan
> described in this thread is not ironed out.  Can you spell that out
> more?  What particular aspect of the transition plan (which is of
> course orthogonal to the choice of hash function) are you discontent
> with?

My impression from reading Junio's mail was that he does not consider the
transition plan ironed out yet, and that he wants to spend time on
discussing generation numbers right now.

I was in particularly frightened by the suggestion to "reboot" [*1*].
Hopefully I misunderstand and he meant "finishing touches" instead.

As to *my* opinion: after reading https://goo.gl/gh2Mzc (is it really
correct that its last update has been on March 6th?), my only concern is
really that it still talks about SHA3-256 when I think that the
performance benefits of SHA-256 (think: "Git at scale", and also hardware
support) really make the latter a better choice.

In order to be "ironed out", I think we need to talk about the
implementation detail "Translation table". This is important. It needs to
be *fast*.

Speaking of *fast*, I could imagine that it would make sense to store the
SHA-1 objects on disk, still, instead of converting them on the fly. I am
not sure whether this is something we need to define in the document,
though, as it may very well be premature optimization; Maybe mention that
we could do this if necessary?

Apart from that, I would *love* to see this document as The Official Plan
that I can Show To The Manager so that I can ask to Allocate Time.

Ciao,
Dscho

Footnote *1*:
https://public-inbox.org/git/xmqqa828733s@gitster.mtv.corp.google.com/

Re: RFC v3: Another proposed hash function transition plan

[Junio C Hamano]

Jonathan Nieder  writes:

> In other words, a long lifetime for the hash absolutely is a design
> goal.  Coping well with an unexpectedly short lifetime for the hash is
> also a design goal.
>
> If the hash function lasts 10 years then I am happy.

Absolutely.  When two functions have similar expected remaining life
and are equally widely supported, then faster is better than slower.
Otherwise our primary goal when picking the function from candidates
should be to optimize for its remaining life and wider availability.

Thanks.

Re: RFC v3: Another proposed hash function transition plan

[Junio C Hamano]

Jonathan Nieder  writes:

> The NewHash-based signature is included in the SHA-1 content as well,
> for the sake of round-tripping.  It is not stripped out.

Ah, OK, that allays my worries.  We rely on the fact that unknown
object headers from the future are ignored.  We use something other
than "gpgsig" header (say, "gpgsigN") to store NewHash based
signature on a commit object created in the NewHash world, so that
SHA-1 clients will ignore it but still include in the signature
computation---is that the idea?

Existing versions of Git that live in the SHA-1 world may still need
to learn to ignore/drop "gpgsigN" while amending a commit that
originally was created in the NewHash world.  Or to force upgrade we
may freeze the SHA-1 only versions of Git and stop updating them
altogether.  I dunno.

We also need to use something other than "mergetag" when carrying
over the contents of a tag being merged in the NewHash world, but
I'd imagine that you've thought about this already.

Thanks.

Re: RFC v3: Another proposed hash function transition plan

[Linus Torvalds]

On Wed, Sep 13, 2017 at 6:43 AM, demerphq  wrote:
>
> SHA3 however uses a completely different design where it mixes a 1088
> bit block into a 1600 bit state, for a leverage of 2:3, and the excess
> is *preserved between each block*.

Yes. And considering that the SHA1 attack was actually predicated on
the fact that each block was independent (no extra state between), I
do think SHA3 is a better model.

So I'd rather see SHA3-256 than SHA256.

  Linus

Re: RFC v3: Another proposed hash function transition plan

[Jonathan Nieder]

Hi,

Yves wrote:
> On 13 September 2017 at 14:05, Johannes Schindelin

>> For example, I am still in favor of SHA-256 over SHA3-256, after learning
>> some background details from in-house cryptographers: it provides
>> essentially the same level of security, according to my sources, while
>> hardware support seems to be coming to SHA-256 a lot sooner than to
>> SHA3-256.
>
> FWIW, and I know it is not worth much, as far as I can tell there is
> at least some security/math basis to prefer SHA3-256 to SHA-256.

Thanks for spelling this out.  From my (very cursory) understanding of
the math, what you are saying makes sense.  I think there were some
hints of this topic on-list before, but not made so explicit before.

Here's my summary of the discussion of other aspects of the choice of
hash functions so far:

My understanding from asking cryptographers matches what Dscho said.
One of the lessons of the history of hash functions is that some kinds
of attempts to improve the security margin of a hash function do not
help as much as expected once a function is broken.

In practice, what we are looking for is

- is the algorithm broken, or likely to be broken soon
- do the algorithm's guarantees match the application
- is the algorithm fast enough
- are high quality implementations widely available

On that first question, every well informed person I have asked has
assured me that SHA-256, SHA-512, SHA-512/256, SHA-256x16, SHA3-256,
K12, BLAKE2bp-256, etc are equally likely to be broken in the next 10
years.  The main difference for the longevity question is that some of
those algorithms have had more scrutiny than others, but all have had
significant scrutiny.  See [1] and the surrounding thread for more
discussion on that.

On the second question, SHA-256 is vulnerable to length extension
attacks, which means it would not be usable as a MAC directly (instead
of using the HMAC construction).  Fortunately Git doesn't use its hash
function that way.

On the third question, SHA-256 is one of the slower ones, even with
hardware accelaration, but it should be fast enough.

On the fourth question, SHA-256 shines.  See [2].  That is where I had
thought the conversation ended up.

For what it's worth, I'm pretty happy both with the level of scrutiny
we've given to this question and SHA-256 as an answer.  Luckily even
if at the last minute we learn something that changes the choice of
hash function, that would not significantly affect the transition
plan, so we have a chance to learn more.

See also [3].

Thanks,
Jonathan

[1] 
https://public-inbox.org/git/CAL9PXLzhPyE+geUdcLmd=pidt5p8efebbsgx_ds88knz2q_...@mail.gmail.com/#t
[2] https://public-inbox.org/git/xmqq37azy7ru@gitster.mtv.corp.google.com/
[3] https://www.imperialviolet.org/2017/05/31/skipsha3.html,
https://news.ycombinator.com/item?id=14453622

Re: RFC v3: Another proposed hash function transition plan

[Jonathan Nieder]

Junio C Hamano wrote:

> In the proposed transition plan, the treatment of various signatures
> (deliberately) makes the conversion not quite roundtrip.

That's not precisely true.  Details below.

> When existing SHA-1 history in individual clones are converted to
> NewHash, we obviously cannot re-sign the corresponding NewHash
> contents with the same PGP key, so these converted objects will
> carry only signature on SHA-1 contents.  They can still be validated
> when they are exported back to SHA-1 world via the fetch/push
> protocol, and can be validated locally by converting them back to
> SHA-1 contents and then passing the result to gpgv.

Correct.

> The plan also states, if I remember what I read correctly, that
> newly created and signed objects (this includes signed commits and
> signed tags; mergetags merely carry over what the tag object that
> was merged was signed with, so we do not have to worry about them
> unless the resulting commit that has mergetag is signed itself, but
> that is already covered by how we handle signed commits) would be
> signed both for NewHash contents and its corresponding SHA-1
> contents (after internally convering it to SHA-1 contents).

Also correct.

> would allow us to strip the signature over NewHash contents and
> derive the SHA-1 contents to be shown to the outside world while
> migration is going on and I'd imagine it would be a good practice;
> it would allow us to sign something that allows everybody to verify,
> when some participants of the project are not yet NewHash capable.

The NewHash-based signature is included in the SHA-1 content as well,
for the sake of round-tripping.  It is not stripped out.

> But the signing over SHA-1 contents has to stop at some point, when
> everybody's Git becomes completely unaware of SHA-1.  We may want to
> have a guideline in the transition plan to (1) encourage signing for
> both for quite some time, and (2) the criteria for us to decide when
> to stop.

Yes, spelling out a rough schedule is a good idea.  I'll add that.

A version of Git that is aware of NewHash should be able to verify
NewHash signatures even for users that are using SHA-1 locally for the
sake of faster fetches and pushes to SHA-1 based peers.

In addition to a new enough Git, this requires the translation table
to translate to NewHash to be present.

So the criterion (2) is largely based on how up-to-date the Git used
by users wanting to verify signatures is and whether they are willing
to tolerate the performance implications of having a translation
table.  My hope is that when communicating with peers using the same
hash function, the translation table will not add too much performance
overhead.

Thank you,
Jonathan

Re: RFC v3: Another proposed hash function transition plan

[Jonathan Nieder]

Hi,

Stefan Beller wrote:
> On Wed, Sep 13, 2017 at 2:52 PM, Junio C Hamano  wrote:

>> This is a tangent, but it may be fine for a shallow clone to treat
>> the cut-off points in the history as if they are root commits and
>> compute generation numbers locally, just like everybody else does.
[...]
> Locally it helps for some operations such as correct walks.
> For the network case however, it doesn't really help either.
>
> If we had global generation numbers, one could imagine that they
> are used in the pack negotiation (server advertises the maximum
> generation number or even gen number per branch; client
> could binary search in there for the fork point)
>
> I wonder if locally generated generation numbers (for the shallow
> case) could be used somehow to still improve network operations.

I have a different concern about locally generated generation numbers in
a shallow clone.  My concern is that it is slow to recompute them when
deepening the shallow clone.

However:

 1. That only affects performance and for some use cases could be
mitigated e.g. by introducing some laziness, and, more
convincingly,

 2. With a small protocol change, the server could communicate the
generation numbers for commit objects at the edge of a shallow
clone, avoiding this trouble.

So I am not too concerned.

More generally, unless there is a very very compelling reason to, I
don't want to couple other changes into the hash function transition.
If they're worthwhile enough to do, they're worthwhile enough to do
whether we're transitioning to a new hash function or not: I have not
heard a convincing example yet of a "while at it" that is worth the
complexity of such coupling.

(That said, if two format changes are worth doing and happen to be
implemented at the same time, then we can save users the trouble of
experiencing two format change transitions.  That is a kind of
coupling from the end user's point of view.  But from the perspective
of someone writing the code, there is no need to count on that, and it
is not likely to happen anyway.)

> If we'd get the transition somewhat right, the next transition will
> be easier than the current transition, such that I am not that concerned
> about longevity. I am rather concerned about the complexity that is added
> to the code base (whilst accumulating technical debt instead of clearer
> abstraction layers)

During the transition, users have to suffer reencoding overhead, so it
is not good for such transitions to need to happen very often.  If the
new hash function breaks early, then we have to cope with it and as
you say, having the framework in place means we'd be ready for that.
But I still don't want the chosen hash function to break early.

In other words, a long lifetime for the hash absolutely is a design
goal.  Coping well with an unexpectedly short lifetime for the hash is
also a design goal.

If the hash function lasts 10 years then I am happy.

Thanks,
Jonathan

Re: RFC v3: Another proposed hash function transition plan

[Junio C Hamano]

Junio C Hamano  writes:

> Jonathan Nieder  writes:
>
>> Treating generation numbers as derived data (as in Jeff King's
>> preferred design, if I have understood his replies correctly) would
>> also be possible but it does not interact well with shallow clone or
>> narrow clone.
>
> Just like we have skewed committer timestamps, there is no reason to
> believe that generation numbers embedded in objects are trustable,
> and there is no way for narrow clients to even verify their correctness.
>
> So I agree with Peff that having generation numbers in object is
> pointless; I agree any other derivables like corresponding sha-1
> name is also pointless to have.
>
> This is a tangent, but it may be fine for a shallow clone to treat
> the cut-off points in the history as if they are root commits and
> compute generation numbers locally, just like everybody else does.
> As generation numbers won't have to be global (because we will not
> be embedding them in objects), nobody gets hurt if they do not match
> across repositories---just like often-mentioned rename detection
> cache, it can be kept as a mere local performance aid and does not
> have to participate in the object model.
>
>> All that said, for simplicity I still lean against including
>> generation numbers as part of a hash function transition.
>
> Good.

In the proposed transition plan, the treatment of various signatures
(deliberately) makes the conversion not quite roundtrip.

When existing SHA-1 history in individual clones are converted to
NewHash, we obviously cannot re-sign the corresponding NewHash
contents with the same PGP key, so these converted objects will
carry only signature on SHA-1 contents.  They can still be validated
when they are exported back to SHA-1 world via the fetch/push
protocol, and can be validated locally by converting them back to
SHA-1 contents and then passing the result to gpgv.

The plan also states, if I remember what I read correctly, that
newly created and signed objects (this includes signed commits and
signed tags; mergetags merely carry over what the tag object that
was merged was signed with, so we do not have to worry about them
unless the resulting commit that has mergetag is signed itself, but
that is already covered by how we handle signed commits) would be
signed both for NewHash contents and its corresponding SHA-1
contents (after internally convering it to SHA-1 contents).  That
would allow us to strip the signature over NewHash contents and
derive the SHA-1 contents to be shown to the outside world while
migration is going on and I'd imagine it would be a good practice;
it would allow us to sign something that allows everybody to verify,
when some participants of the project are not yet NewHash capable.

But the signing over SHA-1 contents has to stop at some point, when
everybody's Git becomes completely unaware of SHA-1.  We may want to
have a guideline in the transition plan to (1) encourage signing for
both for quite some time, and (2) the criteria for us to decide when
to stop.

Thanks.

Re: RFC v3: Another proposed hash function transition plan

[Stefan Beller]

On Wed, Sep 13, 2017 at 2:52 PM, Junio C Hamano  wrote:
> Jonathan Nieder  writes:
>
>> Treating generation numbers as derived data (as in Jeff King's
>> preferred design, if I have understood his replies correctly) would
>> also be possible but it does not interact well with shallow clone or
>> narrow clone.
>
> Just like we have skewed committer timestamps, there is no reason to
> believe that generation numbers embedded in objects are trustable,
> and there is no way for narrow clients to even verify their correctness.
>
> So I agree with Peff that having generation numbers in object is
> pointless; I agree any other derivables like corresponding sha-1
> name is also pointless to have.
>
> This is a tangent, but it may be fine for a shallow clone to treat
> the cut-off points in the history as if they are root commits and
> compute generation numbers locally, just like everybody else does.
> As generation numbers won't have to be global (because we will not
> be embedding them in objects), nobody gets hurt if they do not match
> across repositories---just like often-mentioned rename detection
> cache, it can be kept as a mere local performance aid and does not
> have to participate in the object model.

Locally it helps for some operations such as correct walks.
For the network case however, it doesn't really help either.

If we had global generation numbers, one could imagine that they
are used in the pack negotiation (server advertises the maximum
generation number or even gen number per branch; client
could binary search in there for the fork point)

I wonder if locally generated generation numbers (for the shallow
case) could be used somehow to still improve network operations.



>> My assumption based on previous conversations (and other external
>> conversations like [1]) is that we are going to use SHA2-256 and have
>> a pretty strong consensus for that.  Don't worry!
>
> Hmph, I actually re-read the thread recently, and my impression was
> that we didn't quite have a consensus but were leaning towards
> SHA3-256.
>
> I do not personally have a strong preference myself and I would say
> that anything will do as long as it is with good longevity and
> availability.  SHA2 family would be a fine choice due to its age on
> both counts, being scrutinized longer and having a chance to be
> implemented in many places, even though its age itself may have to
> be subtracted from the longevity factor.

If we'd get the transition somewhat right, the next transition will
be easier than the current transition, such that I am not that concerned
about longevity. I am rather concerned about the complexity that is added
to the code base (whilst accumulating technical debt instead of clearer
abstraction layers)

Re: RFC v3: Another proposed hash function transition plan

[Junio C Hamano]

Jonathan Nieder  writes:

> Treating generation numbers as derived data (as in Jeff King's
> preferred design, if I have understood his replies correctly) would
> also be possible but it does not interact well with shallow clone or
> narrow clone.

Just like we have skewed committer timestamps, there is no reason to
believe that generation numbers embedded in objects are trustable,
and there is no way for narrow clients to even verify their correctness.

So I agree with Peff that having generation numbers in object is
pointless; I agree any other derivables like corresponding sha-1
name is also pointless to have.

This is a tangent, but it may be fine for a shallow clone to treat
the cut-off points in the history as if they are root commits and
compute generation numbers locally, just like everybody else does.
As generation numbers won't have to be global (because we will not
be embedding them in objects), nobody gets hurt if they do not match
across repositories---just like often-mentioned rename detection
cache, it can be kept as a mere local performance aid and does not
have to participate in the object model.

> All that said, for simplicity I still lean against including
> generation numbers as part of a hash function transition.

Good.

> This is unrelated to Brandon's message, except for his use of SHA3 as
> a placeholder for "the next hash function".
>
> My assumption based on previous conversations (and other external
> conversations like [1]) is that we are going to use SHA2-256 and have
> a pretty strong consensus for that.  Don't worry!

Hmph, I actually re-read the thread recently, and my impression was
that we didn't quite have a consensus but were leaning towards
SHA3-256.

I do not personally have a strong preference myself and I would say
that anything will do as long as it is with good longevity and
availability.  SHA2 family would be a fine choice due to its age on
both counts, being scrutinized longer and having a chance to be
implemented in many places, even though its age itself may have to
be subtracted from the longevity factor.

Thanks.

Re: RFC v3: Another proposed hash function transition plan

[Jonathan Nieder]

Hi Dscho,

Johannes Schindelin wrote:

> So even if the code to generate a bidirectional old <-> new hash mapping
> might be with us forever, it *definitely* should be optional ("optional"
> at least as in "config setting"), allowing developers who only work with
> new-hash repositories to save the time and electrons.

Agreed.  This is a good reason not to store the sha1 inside the
sha256-encoded objects.  I think that is exactly what Brandon was saying
in response to Junio --- did you read it differently?

[...]
> ... or Git would simply handle the absence of the generation number header
> gracefully, so that sha1-content == sha3-content...

Part of the sha1-content is references to other objects using their
sha1-name, so it is not possible to have sha1-content == sha3-content.

That said, I am also leaning against including generation numbers as
part of this design.

There is an argument for including generation numbers.  It is much
simpler to have generation numbers in *all* commit objects than only in
some, since it means the slop-based heuristics for faking generation
numbers using commit timestamp can be completely avoided for a
repository using such a format.  Including generation numbers in all
commit objects is a painless thing to do during a format change, since
it can happen without harming round-tripping.

Treating generation numbers as derived data (as in Jeff King's
preferred design, if I have understood his replies correctly) would
also be possible but it does not interact well with shallow clone or
narrow clone.

All that said, for simplicity I still lean against including
generation numbers as part of a hash function transition.  Nothing
stops us from having another format change later.

This is a particularly hard decision because I don't have a strong
preference.  That leads me to err on the side of simplicity.

I will make sure to discuss this issue in my patch to
Documentation/technical/, so we don't have to repeat the same
conversations again and again.

[...]
> Taking a step back, though, it may be a good idea to leave the generation
> number business for later, as much fun as it is to get side tracked and
> focus on relatively trivial stuff instead of the far more difficult and
> complex task to get the transition plan to a new hash ironed out.
>
> For example, I am still in favor of SHA-256 over SHA3-256, after learning
> some background details from in-house cryptographers: it provides
> essentially the same level of security, according to my sources, while
> hardware support seems to be coming to SHA-256 a lot sooner than to
> SHA3-256.
>
> Which hash algorithm to choose is a tough question to answer, and
> discussing generation numbers will sadly not help us answer it any quicker.

This is unrelated to Brandon's message, except for his use of SHA3 as
a placeholder for "the next hash function".

My assumption based on previous conversations (and other external
conversations like [1]) is that we are going to use SHA2-256 and have
a pretty strong consensus for that.  Don't worry!

As a side note, I am probably misreading, but I found this set of
paragraphs a bit condescending.  It sounds to me like you are saying
"You are making the wrong choice of hash function and everything else
you are describing is irrelevant when compared to that monumental
mistake.  Please stop working on things I don't consider important".
With that reading it is quite demotivating to read.

An alternative reading is that you are saying that the transition plan
described in this thread is not ironed out.  Can you spell that out
more?  What particular aspect of the transition plan (which is of
course orthogonal to the choice of hash function) are you discontent
with?

Thanks and hope that helps,
Jonathan

[1] https://www.imperialviolet.org/2017/05/31/skipsha3.html

Re: RFC v3: Another proposed hash function transition plan

[demerphq]

On 13 September 2017 at 14:05, Johannes Schindelin
 wrote:
> For example, I am still in favor of SHA-256 over SHA3-256, after learning
> some background details from in-house cryptographers: it provides
> essentially the same level of security, according to my sources, while
> hardware support seems to be coming to SHA-256 a lot sooner than to
> SHA3-256.

FWIW, and I know it is not worth much, as far as I can tell there is
at least some security/math basis to prefer SHA3-256 to SHA-256.

The SHA1 and SHA-256 hash functions, (iirc along with their older
cousins MD5 and MD2) all have a common design feature where they mix a
relatively large block size into a much smaller state *each block*. So
for instance SHA-256 mixes a 512 bit block into a 256 bit state with a
2:1 "leverage" between the block being read and the state. In SHA1
this was worse, mixing a 512 bit block into a 160 bit state, closer to
3:1 leverage.

SHA3 however uses a completely different design where it mixes a 1088
bit block into a 1600 bit state, for a leverage of 2:3, and the excess
is *preserved between each block*.

Assuming everything else is equal between SHA-256 and SHA3 this
difference alone would seem to justify choosing SHA3 over SHA-256. We
know that there MUST be collisions when compressing a 512 bit block
into a 256 bit space, however one cannot say the same about mixing
1088 bits into a 1600 bit state. The excess state which is not
directly modified by the input block makes a big difference when
reading the next block.

Of course in both cases we end up compressing the entire source
document down to the same number of bits, however SHA3 does that
*once*, in finalization only, whereas SHA-256 does it *every* block
read. So it seems to me that the opportunity for collisions is *much*
higher in SHA-256 than it is in SHA3-256. (Even if they should be
vanishingly rare regardless.)

For this reason if I had a vote I would definitely vote SHA3-256, or
even for SHA3-512. The latter has an impressive 1:2 leverage between
block and state, and much better theoretical security levels.

cheers,
Yves
Note: I am not a cryptographer, although I am probably pretty well
informed as far hobby-hash-function-enthusiasts go.
-- 
perl -Mre=debug -e "/just|another|perl|hacker/"

Re: RFC v3: Another proposed hash function transition plan

[Johannes Schindelin]

Hi Brandon,

On Mon, 11 Sep 2017, Brandon Williams wrote:

> On 09/08, Junio C Hamano wrote:
> > Junio C Hamano  writes:
> > 
> > > One thing I still do not know how I feel about after re-reading the
> > > thread, and I didn't find the above doc, is Linus's suggestion to
> > > use the objects themselves as NewHash-to-SHA-1 mapper [*1*].  
> > > ...
> > > [Reference]
> > >
> > > *1* 
> > 
> > I think this falls into the same category as the often-talked-about
> > addition of the "generation number" field.  It is very tempting to add
> > these "mechanically derivable but expensive to compute" pieces of
> > information to the sha3-content while converting from sha1-content and
> > creating anew.  
> 
> We didn't discuss that in the doc since this particular transition plan
> we made uses an external NewHash-to-SHA1 map instead of an internal one
> because we believe that at some point we would be able to drop
> compatibility with SHA1.

Is there even a question about that? I mean, why would *any* project that
switches entirely to SHA-256 want to carry the SHA-1 baggage around?

So even if the code to generate a bidirectional old <-> new hash mapping
might be with us forever, it *definitely* should be optional ("optional"
at least as in "config setting"), allowing developers who only work with
new-hash repositories to save the time and electrons.

> Now I suspect that wont happen for a long time but I think it would be
> preferable over carrying the SHA1 luggage indefinitely.

It should be possible to push back the SHA-1 ginny into a small gin bottle
inside Git's source code, so to say, i.e. encapsulate it to the point
where it is a compile-time option, in addition to a runtime option.

Of course, that's only unless the SHA-1 calculation is made mandatory as
suggested above. I really shudder at the idea of requiring SHA-1 to be
required forever. We ignored advice in 2005 against making ourselves too
dependent on SHA-1, and I would hope that we would learn from this.

> At some point, then, we would be able to stop hashing objects twice
> (once with SHA1 and once with NewHash) instead of always requiring that
> we hash them with each hash function which was used historically.

Yes, please.

> > Because the "sha1-name" or the "generation number" can mechanically
> > be computed,

... as long as a shallow clone you do not have, of course...

> > as long as everybody agrees to _always_ place them in the
> > sha3-content, the same sha1-content will be converted into exactly the
> > same sha3-content without ambiguity, and converting them back to
> > sha1-content while pushing to an older repository will correctly
> > produce the original sha1-content, as it would just be the matter of
> > simply stripping these extra pieces of information.

... or Git would simply handle the absence of the generation number header
gracefully, so that sha1-content == sha3-content...

> > The same thing could happen if we decide to bake "generation number"
> > in the SHA-3 commit objects.  One possible definition would be that a
> > root commit will have gen #0; a commit with 1 or more parents will get
> > max(parents' gen numbers) + 1 as its gen number.  But somebody may
> > botch the counting and records sum(parents' gen numbers) as its gen
> > number.
> > 
> > In these cases, not just the SHA3-content but also the resulting SHA-3
> > object name would be different from the name of the object that would
> > have recorded the same contents correctly.  So converting back to
> > SHA-1 world from these botched SHA-3 contents may produce the original
> > contents, but we may end up with multiple "plausibly looking" set of
> > SHA-3 objects that (clain to) correspond to a single SHA-1 object,
> > only one of which is a valid one.
> > 
> > Our "git fsck" already treats certain brokenness (like a tree whose
> > entry has mode that is 0-padded to the left) as broken but still
> > tolerate them.  I am not sure if it is sufficient to diagnose and
> > declare broken and invalid when we see sha3-content that records
> > these "mechanically derivable but expensive to compute" pieces of
> > information incorrectly.
> > 
> > I am leaning towards saying "yes, catching in fsck is enough" and
> > suggesting to add generation number to sha3-content of the commit
> > objects, and to add even the "original sha1 name" thing if we find
> > good use of it.  But I cannot shake this nagging feeling off that I
> > am missing some huge problems that adding these fields and opening
> > ourselves to more classes of broken objects.
> > 
> > Thoughts?

Seeing as current Git versions would always ignore the generation number
(and therefore work perfectly even with erroneous baked-in generation
numbers), and seeing as it would be easy to add a config option to force
Git to ignore the embedded generation numbers, I would consider `fsck`
catching those problems the best idea.

It 

Re: RFC v3: Another proposed hash function transition plan

[Brandon Williams]

On 09/08, Junio C Hamano wrote:
> Junio C Hamano  writes:
> 
> > One thing I still do not know how I feel about after re-reading the
> > thread, and I didn't find the above doc, is Linus's suggestion to
> > use the objects themselves as NewHash-to-SHA-1 mapper [*1*].  
> > ...
> > [Reference]
> >
> > *1* 
> 
> I think this falls into the same category as the often-talked-about
> addition of the "generation number" field.  It is very tempting to
> add these "mechanically derivable but expensive to compute" pieces
> of information to the sha3-content while converting from
> sha1-content and creating anew.  

We didn't discuss that in the doc since this particular transition plan
we made uses an external NewHash-to-SHA1 map instead of an internal one
because we believe that at some point we would be able to drop
compatibility with SHA1.  Now I suspect that wont happen for a long time
but I think it would be preferable over carrying the SHA1 luggage
indefinitely.  At some point, then, we would be able to stop hashing
objects twice (once with SHA1 and once with NewHash) instead of always
requiring that we hash them with each hash function which was used
historically.

> 
> Because the "sha1-name" or the "generation number" can mechanically
> be computed, as long as everybody agrees to _always_ place them in
> the sha3-content, the same sha1-content will be converted into
> exactly the same sha3-content without ambiguity, and converting them
> back to sha1-content while pushing to an older repository will
> correctly produce the original sha1-content, as it would just be the
> matter of simply stripping these extra pieces of information.
> 
> The reason why I still feel a bit uneasy about adding these things
> (aside from the fact that sha1-name thing will be a baggage we would
> need to carry forever even after we completely wean ourselves off of
> the old hash) is because I am not sure what we should do when we
> encounter sha3-content in the wild that has these things _wrong_.
> An object that exists today in the SHA-1 world is fetched into the
> new repository and converted to SHA-3 contents, and Linus's extra
> "original SHA-1 name" field is added to the object's header while
> recording the SHA-3 content.  But for whatever reason, the original
> SHA-1 name is recorded incorrectly in the resulting SHA-3 object.

This wasn't one of the issues that I thought of but it just makes the
argument against adding sha1's to the sha3 content stronger.

> 
> The same thing could happen if we decide to bake "generation number"
> in the SHA-3 commit objects.  One possible definition would be that
> a root commit will have gen #0; a commit with 1 or more parents will
> get max(parents' gen numbers) + 1 as its gen number.  But somebody
> may botch the counting and records sum(parents' gen numbers) as its
> gen number.
> 
> In these cases, not just the SHA3-content but also the resulting
> SHA-3 object name would be different from the name of the object
> that would have recorded the same contents correctly.  So converting
> back to SHA-1 world from these botched SHA-3 contents may produce
> the original contents, but we may end up with multiple "plausibly
> looking" set of SHA-3 objects that (clain to) correspond to a single
> SHA-1 object, only one of which is a valid one.
> 
> Our "git fsck" already treats certain brokenness (like a tree whose
> entry has mode that is 0-padded to the left) as broken but still
> tolerate them.  I am not sure if it is sufficient to diagnose and
> declare broken and invalid when we see sha3-content that records
> these "mechanically derivable but expensive to compute" pieces of
> information incorrectly.
> 
> I am leaning towards saying "yes, catching in fsck is enough" and
> suggesting to add generation number to sha3-content of the commit
> objects, and to add even the "original sha1 name" thing if we find
> good use of it.  But I cannot shake this nagging feeling off that I
> am missing some huge problems that adding these fields and opening
> ourselves to more classes of broken objects.
> 
> Thoughts?
> 
> 

-- 
Brandon Williams

Re: RFC v3: Another proposed hash function transition plan

[Jeff King]

On Fri, Sep 08, 2017 at 11:40:21AM +0900, Junio C Hamano wrote:

> Our "git fsck" already treats certain brokenness (like a tree whose
> entry has mode that is 0-padded to the left) as broken but still
> tolerate them.  I am not sure if it is sufficient to diagnose and
> declare broken and invalid when we see sha3-content that records
> these "mechanically derivable but expensive to compute" pieces of
> information incorrectly.
> 
> I am leaning towards saying "yes, catching in fsck is enough" and
> suggesting to add generation number to sha3-content of the commit
> objects, and to add even the "original sha1 name" thing if we find
> good use of it.  But I cannot shake this nagging feeling off that I
> am missing some huge problems that adding these fields and opening
> ourselves to more classes of broken objects.

I share your nagging feeling.

I have two thoughts on the "fsck can catch it" line of reasoning.

  1. It's harder to fsck generation numbers than other syntactic
 elements of an object, because it inherently depends on the links.
 So I can't fsck a commit object in isolation. I have to open its
 parents and check _their_ generation numbers.

 In some sense that isn't a big deal. A real fsck wants to know that
 we _have_ the parents in the first place. But traditionally we've
 separated "is this syntactically valid" from "do we have full
 connectivity". And features like shallow clones rely on us fudging
 the latter but not the former. A shallow history could never
 properly fsck the generation numbers.

 A multiple-hash field doesn't have this problem. It's purely a
 function of the bytes in the object.

  2. I wouldn't classify the current fsck checks as a wild success in
 containing breakages. If a buggy implementation produces invalid
 objects, the same buggy implementation generally lets people (and
 their colleagues) unwittingly build on top of those objects. It's
 only later (sometimes much later) that they interact with a
 non-buggy implementation whose fsck complains.

 And what happens then? If they're lucky, the invalid objects
 haven't spread far, and the worst thing is that they have to learn
 to use filter-branch (which itself is punishment enough). But
 sometimes a significant bit of history has been built on top, and
 it's awkward or impossible to rewrite it.

 That puts the burden on whoever is running the non-buggy
 implementation that wants to reject the objects. Do they accept
 these broken objects? If so, what do they do to mitigate the wrong
 answers that Git will return?

I'm much more in favor of keeping that data outside the object-hash
computation, and caching the pre-computed results as necessary. Those
cache can disagree with the objects, of course, but the cost to dropping
and re-building them is much lower than a history rewrite.

I'm speaking primarily to the generation-number thing, where I really
don't think there's any benefit to embedding it in the object beyond the
obvious "well, it has to go _somewhere_, and this saves us implementing
a local cache layer".  I haven't thought hard enough on the
multiple-hash thing to know if there's some other benefit to having it
inside the objects.

-Peff

Re: RFC v3: Another proposed hash function transition plan

[Junio C Hamano]

Junio C Hamano  writes:

> One thing I still do not know how I feel about after re-reading the
> thread, and I didn't find the above doc, is Linus's suggestion to
> use the objects themselves as NewHash-to-SHA-1 mapper [*1*].  
> ...
> [Reference]
>
> *1* 

I think this falls into the same category as the often-talked-about
addition of the "generation number" field.  It is very tempting to
add these "mechanically derivable but expensive to compute" pieces
of information to the sha3-content while converting from
sha1-content and creating anew.  

Because the "sha1-name" or the "generation number" can mechanically
be computed, as long as everybody agrees to _always_ place them in
the sha3-content, the same sha1-content will be converted into
exactly the same sha3-content without ambiguity, and converting them
back to sha1-content while pushing to an older repository will
correctly produce the original sha1-content, as it would just be the
matter of simply stripping these extra pieces of information.

The reason why I still feel a bit uneasy about adding these things
(aside from the fact that sha1-name thing will be a baggage we would
need to carry forever even after we completely wean ourselves off of
the old hash) is because I am not sure what we should do when we
encounter sha3-content in the wild that has these things _wrong_.
An object that exists today in the SHA-1 world is fetched into the
new repository and converted to SHA-3 contents, and Linus's extra
"original SHA-1 name" field is added to the object's header while
recording the SHA-3 content.  But for whatever reason, the original
SHA-1 name is recorded incorrectly in the resulting SHA-3 object.

The same thing could happen if we decide to bake "generation number"
in the SHA-3 commit objects.  One possible definition would be that
a root commit will have gen #0; a commit with 1 or more parents will
get max(parents' gen numbers) + 1 as its gen number.  But somebody
may botch the counting and records sum(parents' gen numbers) as its
gen number.

In these cases, not just the SHA3-content but also the resulting
SHA-3 object name would be different from the name of the object
that would have recorded the same contents correctly.  So converting
back to SHA-1 world from these botched SHA-3 contents may produce
the original contents, but we may end up with multiple "plausibly
looking" set of SHA-3 objects that (clain to) correspond to a single
SHA-1 object, only one of which is a valid one.

Our "git fsck" already treats certain brokenness (like a tree whose
entry has mode that is 0-padded to the left) as broken but still
tolerate them.  I am not sure if it is sufficient to diagnose and
declare broken and invalid when we see sha3-content that records
these "mechanically derivable but expensive to compute" pieces of
information incorrectly.

I am leaning towards saying "yes, catching in fsck is enough" and
suggesting to add generation number to sha3-content of the commit
objects, and to add even the "original sha1 name" thing if we find
good use of it.  But I cannot shake this nagging feeling off that I
am missing some huge problems that adding these fields and opening
ourselves to more classes of broken objects.

Thoughts?

Re: RFC v3: Another proposed hash function transition plan

[Junio C Hamano]

Jonathan Nieder  writes:

> Linus Torvalds wrote:
>> On Fri, Mar 3, 2017 at 5:12 PM, Jonathan Nieder  wrote:
>
>>> This document is still in flux but I thought it best to send it out
>>> early to start getting feedback.
>>
>> This actually looks very reasonable if you can implement it cleanly
>> enough.
>
> Thanks for the kind words on what had quite a few flaws still.  Here's
> a new draft.  I think the next version will be a patch against
> Documentation/technical/.

Can we reboot the discussion and advance this to v4 state?

> As before, comments welcome, both here and inline at
>
>   https://goo.gl/gh2Mzc

I think what you have over there looks pretty-much ready as the
final outline.

One thing I still do not know how I feel about after re-reading the
thread, and I didn't find the above doc, is Linus's suggestion to
use the objects themselves as NewHash-to-SHA-1 mapper [*1*].  

It does not help the reverse mapping that is needed while pushing
things out (the SHA-1 receiver tells us what they have in terms of
SHA-1 names; we need to figure out where we stop sending based on
that).  While it does help maintaining itself (while constructing
SHA3-content, we'd be required to find out its SHA1 name but the
SHA3 objects that we refer to all know their SHA-1 names), if it is
not useful otherwise, then that does not count as a plus.  Also
having to bake corresponding SHA-1 name in the object would mean
mistakes can easily propagate and cannot be corrected without
rewriting the history, which would be a huge downside.  So perhaps
we are better off without it, I guess.


[Reference]

*1* 

Re: RFC v3: Another proposed hash function transition plan

[Jonathan Nieder]

Jeff King wrote:
> On Thu, Mar 09, 2017 at 12:24:08PM -0800, Jonathan Nieder wrote:

>>> SHA-1 to SHA-3: lookup SHA-1 in .msha1, reverse .idx, find offset to
>>> read the SHA-3.
>>> SHA-3 to SHA-1: lookup SHA-3 in .idx, and reverse the .msha1 file to
>>> translate offset to SHA-1.
>>
>> Thanks for this suggestion.  I was initially vaguely nervous about
>> lookup times in an idx-style file, but as you say, object reads from a
>> packfile already have to deal with this kind of lookup and work fine.
>
> Not exactly. The "reverse .idx" step has to build the reverse mapping on
> the fly, and it's non-trivial.

Sure.  To be clear, I was handwaving over that since adding an on-disk
reverse .idx is a relatively small change.

[...]
> So I think it's solvable, but I suspect we would want an extension to
> the .idx format to store the mapping array, in order to keep it log-n.

i.e., this.

The loose object side is the more worrying bit, since we currently don't
have any practical bound on the number of loose objects.

One way to deal with that is to disallow loose objects completely.
Use packfiles for new objects, batching the objects produced by a
single process into a single packfile.  Teach "git gc --auto" a
behavior similar to Martin Fick's "git exproll" to combine packfiles
between full gcs to maintain reasonable performance.  For unreachable
objects, instead of using loose objects, use "unreachable garbage"
packs explicitly labeled as such, with similar semantics to what
JGit's DfsRepository backend uses (described in the discussion at
https://git.eclipse.org/r/89455).

That's a direction that I want in the long term anyway.  I was hoping
not to couple such changes with the hash transition but it might be
one of the simpler ways to go.

Jonathan

Re: RFC v3: Another proposed hash function transition plan

[Jeff King]

On Thu, Mar 09, 2017 at 12:24:08PM -0800, Jonathan Nieder wrote:

> > SHA-1 to SHA-3: lookup SHA-1 in .msha1, reverse .idx, find offset to
> > read the SHA-3.
> > SHA-3 to SHA-1: lookup SHA-3 in .idx, and reverse the .msha1 file to
> > translate offset to SHA-1.
> 
> Thanks for this suggestion.  I was initially vaguely nervous about
> lookup times in an idx-style file, but as you say, object reads from a
> packfile already have to deal with this kind of lookup and work fine.

Not exactly. The "reverse .idx" step has to build the reverse mapping on
the fly, and it's non-trivial. For instance, try:

  sha1=$(git rev-parse HEAD)
  time echo $sha1 | git cat-file --batch-check='%(objectsize)'
  time echo $sha1 | git cat-file --batch-check='%(objectsize:disk)'

on a large repo (where HEAD is in a big pack). The on-disk size is
conceptually simpler, as we only need to look at the offset of the
object versus the offset of the object after it. But in practice it
takes much longer, because it has to build the revindex on the fly (I
get 7ms versus 179ms on linux.git).

The effort is linear in the number of objects (we create the revindex
with a radix sort).

The reachability bitmaps suffer from this, too, as they need the
revindex to know which object is at which bit position. At GitHub we
added an extension to the .bitmap files that stores this "bit cache".
Here are timings before and after on linux.git:

  $ time git rev-list --use-bitmap-index --count master
  659371

  real  0m0.182s
  user  0m0.136s
  sys   0m0.044s

  $ time git.gh rev-list --use-bitmap-index --count master
  659371

  real  0m0.016s
  user  0m0.008s
  sys   0m0.004s

It's not a full revindex, but it's enough for bitmap use. You can also
use it to generate the revindex slightly more quickly, because you can
skip the sorting step (you just insert the entries in the correct order
by walking the bit cache and dereferencing the offsets from the .idx
portion). So it's still linear, but with a smaller constant factor.

I think for the purposes here, though, we don't actually care about the
offsets. For the cost of one uint32_t per object, you can keep a list
mapping positions in the sha1 index into the sha3 index. So then you do
the log-n binary search to find the sha1, a constant-time lookup in the
mapping array, and that gives you the position in the sha3 index, from
which you can then access the sha3 (or the actual pack offset, for that
matter).

So I think it's solvable, but I suspect we would want an extension to
the .idx format to store the mapping array, in order to keep it log-n.

-Peff

Re: RFC v3: Another proposed hash function transition plan

[Jonathan Nieder]

Hi,

Shawn Pearce wrote:
> On Mon, Mar 6, 2017 at 4:17 PM, Jonathan Nieder  wrote:

>> Alongside the packfile, a sha3 repository stores a bidirectional
>> mapping between sha3 and sha1 object names. The mapping is generated
>> locally and can be verified using "git fsck". Object lookups use this
>> mapping to allow naming objects using either their sha1 and sha3 names
>> interchangeably.
>
> I saw some discussion about using LevelDB for this mapping table. I
> think any existing database may be overkill.
>
> For packs, you may be able to simplify by having only one file
> (pack-*.msha1) that maps SHA-1 to pack offset; idx v2. The CRC32 table
> in v2 is unnecessary, but you need the 64 bit offset support.
>
> SHA-1 to SHA-3: lookup SHA-1 in .msha1, reverse .idx, find offset to
> read the SHA-3.
> SHA-3 to SHA-1: lookup SHA-3 in .idx, and reverse the .msha1 file to
> translate offset to SHA-1.

Thanks for this suggestion.  I was initially vaguely nervous about
lookup times in an idx-style file, but as you say, object reads from a
packfile already have to deal with this kind of lookup and work fine.

> For loose objects, the loose object directories should have only
> O(4000) entries before auto gc is strongly encouraging
> packing/pruning. With 256 shards, each given directory has O(16) loose
> objects in it. When writing a SHA-3 loose object, Git could also
> append a line "$sha3 $sha1\n" to objects/${first_byte}/sha1, which
> GC/prune rewrites to remove entries. With O(16) objects in a
> directory, these files should only have O(16) entries in them.

Insertion time is what worries me.  When writing a small number of
objects using a command like "git commit", I don't want to have to
regenerate an entire idx file.  I don't want to move the pain to
O(loose objects) work at read time, either --- some people disable
auto gc, and others have a large number of loose objects due to gc
ejecting unreachable objects.

But some kind of simplification along these lines should be possible.
I'll experiment.

Jonathan

Re: RFC v3: Another proposed hash function transition plan

[Shawn Pearce]

On Mon, Mar 6, 2017 at 4:17 PM, Jonathan Nieder  wrote:
> Linus Torvalds wrote:
>> On Fri, Mar 3, 2017 at 5:12 PM, Jonathan Nieder  wrote:
>
>>> This document is still in flux but I thought it best to send it out
>>> early to start getting feedback.
>>
>> This actually looks very reasonable if you can implement it cleanly
>> enough.
>
> Thanks for the kind words on what had quite a few flaws still.  Here's
> a new draft.  I think the next version will be a patch against
> Documentation/technical/.

FWIW, I like this approach.

> Alongside the packfile, a sha3 repository stores a bidirectional
> mapping between sha3 and sha1 object names. The mapping is generated
> locally and can be verified using "git fsck". Object lookups use this
> mapping to allow naming objects using either their sha1 and sha3 names
> interchangeably.

I saw some discussion about using LevelDB for this mapping table. I
think any existing database may be overkill.

For packs, you may be able to simplify by having only one file
(pack-*.msha1) that maps SHA-1 to pack offset; idx v2. The CRC32 table
in v2 is unnecessary, but you need the 64 bit offset support.

SHA-1 to SHA-3: lookup SHA-1 in .msha1, reverse .idx, find offset to
read the SHA-3.
SHA-3 to SHA-1: lookup SHA-3 in .idx, and reverse the .msha1 file to
translate offset to SHA-1.


For loose objects, the loose object directories should have only
O(4000) entries before auto gc is strongly encouraging
packing/pruning. With 256 shards, each given directory has O(16) loose
objects in it. When writing a SHA-3 loose object, Git could also
append a line "$sha3 $sha1\n" to objects/${first_byte}/sha1, which
GC/prune rewrites to remove entries. With O(16) objects in a
directory, these files should only have O(16) entries in them.

SHA-3 to SHA-1: open objects/${sha3_first_byte}/sha1 and scan until a
match is found.
SHA-1 to SHA-3: brute force read 256 files. Callers performing this
mapping may load all 256 files into a table in memory.