On 11/27/2017 1:03 AM, Jamil Nimeh wrote:
One additional topic for discussion: Late in the week we talked
about the current state of the API internally and one item to
revisit is where the DerivationParameterSpec objects are passed. It
was brought up by a couple people that it would be better to provide
the DPS objects pertaining to keys at the time they are called for
through deriveKey() and deriveKeys() (and possibly deriveData).
Originally we had them all grouped in a List in the init method. One
reason for needing it up there was to know the total length of
material to generate. If we can provide the total length through
the AlgorithmParameterSpec passed in via init() then things like:
Key deriveKey(DerivationParameterSpec param);
List<Key> deriveKeys(List<DerivationParameterSpec> params);
become possible. To my eyes at least it does make it more clear
what DPS you're processing since they're provided at derive time,
rather than the caller having to keep track in their heads where in
the DPS list they might be with each successive deriveKey or
deriveKeys calls. And I think we could do away with
deriveKeys(int), too.
See above - the key stream is logically produced in its entirety
before any assignment of that stream is made to any cryptographic
objects because the mixins (except for the round differentiator) are
the same for each key stream production round. Simply passing in
the total length may not give you the right result if the KDF
requires a per component length (and it should to defeat (5) or it
should only produce a single key).
From looking at 800-108, I don't see any place where the KDF needs a
per-component length. It looks like it takes L (total length) as an
input and that is applied to each round of the PRF. HKDF takes L
up-front as an input too, though it doesn't use it as an input to the
HMAC function itself. For TLS 1.3 that component length becomes part
of the context info (HkdfLabel) through the HKDF-Expand-Label
function...and it's only doing one key for a given label which is also
part of that context specific info, necessitating an init() call.
Seems like the length can go into the APS provided via init (for those
KDFs that need it at least) and you shouldn't need a DPS list up-front.
HKDF and SP800-108 only deal with the creation of the key stream and
ignore the issues with assigning the key stream to cryptographic
objects. In the TLS version of HDKF, the L value is mandatory and only
a single object is assigned per init/call to the KDF. An HSM can look
at the HKDF label information and set the appropriate policies for the
assigned cryptographic object (because if any of the label data changes,
the entire key stream changes). That's not the case for the raw HKDF
nor for any KDF that allows for multiple objects to be extracted out of
a single key stream. Hence the per-component length values.
Ideally, there should be a complete object spec for each object to be
generated that is part of the mixins (label and context) for any KDF.
That allows an HSM to rely upon the object spec when setting policy
controls for each generated object - and incidentally allows for a KDF
to generate both public and non-public data in a secure way.
So as long as you allow for the specification of all of the production
objects as part of the .init() I'm good. A given KDF might not require
this - but I can't see any way of fixing the current KDFs to work in
HSMs without something like this.
As far as your (5) scenario goes, I can see how you can twiddle the
lengths to get the keystream output with zero-length keys and large IV
buffers. But that scenario really glosses over what should be a big
hurdle and a major access control issue that stands outside the KDF
API: That the attacker shouldn't have access to the input keying
material in the first place. Protect the input keying material
properly and their attack cannot be done.
Let me give you an example. I'm running an embedded HSM - to protect
TLS keys and to do all of the crypto. An attacker compromises the TLS
server and now has access to the HSM. No problem - I'm going to notice
if the attacker starts extraditing large amounts of data from the server
(e.g. copies of the TLS in the clear but possibly reencrypted data
stream) so this isn't a threat or is it? Smart attacker does an
extraction attack on the TLS 1.2 and before KDF and turns all of the key
stream material into IV material and exports it from the HSM. The
attacker now has the much smaller key material so he can send a few
messages with those keys and allow for the passive external interception
of the traffic and decryption thereof without the risk of detection of
all that traffic being sent. Alternately, I can place the key material
in a picture via steganography and publish it as part of the server data.
The idea is to protect extraction of the key material from an HSM _*even
from authorized users of that key material*_.
KDFs don't currently do this well. Adding the overall length and per
component length stuff as well as a per component spec to the data used
to derive the key stream means that 1) changes to any of those change
the entire key stream, 2) the per component spec data may be used by the
security module policy engine to enforce restrictions and 3) because of
(1) and (2) calling the KDF a second time gets me exactly the same
objects rather than just the same key stream. The last isn't very
important in a software based security domain, but turns out to have
real implications for policy enforcing security modules.
This gets worse when you realize that the KDF key is under it all either
a HASH HMAC or CMAC key and all of those algorithms produce public
data. Ideally you need a way of preventing a KDF key from calling the
raw HASH/HMAC/CMAC functions directly (and vice versa).
I would rather see the DPS provided in the deriveKey. It couples what
you want out with the call that makes the object and it makes a lot
more sense to keep those two together than try to remember where in
the submitted list of DPS objects you are.
95% of the time this will be a call to produce a single key. 4% of
the time it will be a call to produce multiple keys. Only 1% of the
time will it need to intermix key, data and object productions.
Anybody who is doing that is going to write a wrapper around this
class to make sure they get the key and data production order correct
for each call. So I'm not all that bothered by keeping the
complexity as a price for keeping flexibility.
You could have a Key deriveKey(Key k, DerivationParameterSpec param)
for some things like TLS1.3 (where you can only make a single call to
derive key between inits) , but then you'd also need at least a
byte[] deriveData (Key k, DerivationParameterSpec param) and an
Object deriveObject(Key k, DerivationParameterSpec param).
I don't think those are necessary. If you're just doing HKDF-Expand
(for the HKDF-Expand-Label TLS 1.3 key derivation) then you can
provide the input key, label and max length and any other context info
that goes into that HkdfLabel structure...all of that would go into
init(). Then provide the key alg and desired length via the DPS at
deriveKey time. Any subsequent keys in the TLS 1.3 key schedule would
need a new init call anyway since the labels change and possibly the
output length.
Over the next day or so I'm going to have to make some final decisions
on this API as there are internal projects that are waiting on this
API to proceed. I'm already past the cut-off date I set, but I
recognize these discussions are important to have and I appreciate the
input you and others have provided.
--Jamil
Reading this last I think I've lost the context. Here's where I think
we are:
1) Get instance gets the default configuration of a given KDF (and that
default will be attached to the instance name defintion)
2) .setParameter() may be used to update the KDF configuration - once.
3) .init() takes at least the key, it may optionally take a set of
derivation parameters. The derivation parameters provided in .init()
are intended for use in forming the label and context mixins for the
KDF. They may provide - for example - the total length of the key
stream, the objects to be derived, the length of the objects, protection
parameters for each of the objects etc.
4) A kdf generate a free-running or fixed length key stream depending on
the derivation parameters (e.g. if "L" is not a mixin to the KDF then it
is free-running and may produce as much key stream as desired or if the
production object specifications are not part of the derivation mixins).
Doing (4) is mostly not a good idea, but someone might want to do
this. In that case it may make the most sense to just allow them to do
deriveData(int length) calls as the only function (a keyed PRNG basically).
Re the last version of your api - if you add the .setParameter()
.getParameter() calls to both KeyDerivation and KeyDerivationSpi I think
I'm happy with this part of the API. I'm wondering if we should talk
about KeyAgreement though.
