I agree that the response from Housley certainly supports that
"AutoPadding" is likely a safe mode to use. I still would prefer not to see
it (keeping things simple) but don't really have any objections to it.
For KW+PKCS5, I have (unfortunately) seen this deployed in the real world
and had to assist Java developers in manually removing PKCS5 padding from
the result of KW decryption. (No OIDs were used in any parts of these
designs, so I cannot say what would have been used.) So, since I had to
help multiple people write this code already, I really cannot object to
adding support for it. The JCA supports many algorithms which shouldn't be
used but exist for compatibility and interactions with other systems (DES,
RC4, etc.). This would be yet another algorithm of that type. (Arguably,
both KW and KWP should probably be replaced by AES-GCM for modern systems,
but that is an entirely different discussion.)
As for OIDs, that seems somewhat unrelated and I don't think we need
something new. I've rarely needed to use the OIDs for KW or KWP and
suspect that we could simply choose the one that corresponds to the
algorithm we actually used.
I've also encountered HSMs which added PKCS5 padding prior to KW so that
all output was 8-byte aligned. That was very frustrating to deal with as it
was not clearly documented at the time.
Finally, I believe that the encoding question is separate from the wrapping
question. Each key type should (and generally does) define how to encode it
as an octet string. Then you apply the relevant wrapping/unwrapping
algorithm to that encoding. KW/KWP should not define how to encode keys any
more than RSA should define how to wrap a serialized RSA key. (However, I
may have misunderstood your comment "... RFC written that specifies the
default encodings for keys wrapped by this algorithm.")
Greg
On Wed, Apr 7, 2021 at 11:51 AM Michael StJohns <mstjo...@comcast.net>
wrote:
> *sigh* Minor correction in line.
>
> On 4/7/2021 2:49 PM, Michael StJohns wrote:
>
> On 4/7/2021 1:28 PM, Greg Rubin wrote:
>
> Mike,
>
> Yes, this was in response to your comment.
>
> I'm aware that the IV really serves more as an integrity check and mode
> signalling mechanism than anything else. My concern is that in the past few
> years I've seen various issues related to "in band signalling" where
> something about the ciphertext (or directly associated metadata) changes
> how the data is decrypted and authenticated. This has reached the level
> where several cryptographic forums I participate in are starting to
> consider it a full anti-pattern.
>
> The proposed "AutoPadding" mode is an example of in-band signalling in
> which an externally provided ciphertext changes how it is interpreted.
> While I cannot personally think of a specific risk in this case, I would be
> inclined not to include this mode unless there is a strong driving need
> from our users. While I have definitely seen people not knowing if their
> data was encrypted with KW or KW+PKCS5/7, I haven't personally seen
> uncertainty between KW and KWP. (I also haven't worked with all possible
> HSMs, just a few of them.) So, from a position of caution, I'd avoid
> "AutoPadding", but this is a preference based on current best-practice
> rather than a strong objection based on specific concerns or risks.
>
>
> I sent a note off to the original mode inventor - Russ Housley:
>
> Can you think of any reason why there might be an issue with providing an
> autopadding mode for KW/KWP (e.g. select which to use based on the input
> data for encrypt and determine which was used after running the unwrap
> function but before removing the initial block and any padding)?
>
> I got back:
>
> As long as every party supports both modes, you could use KW id [sic - I
> think he meant "is"]
>
> "if" not "is"
>
> the inout is a multiple of 64 bits, otherwise use KWP. Of course, the
> algorithm identifier needs to be set appropriately.
>
> Which sort of confirms what I thought, but added a question: Are there
> algorithm OIDs for KW with PKCS5 padding or do people just use the KW OID(
> 2.16.840.1.101.3.4.1.{5,25,45}? As far as I can tell, there are no OIDs
> for KW with PKCS5.
>
> Does there need to be an autopad OID?
>
> If it were me, I'd be avoiding implementing the PKCS5 padding mode here.
> I can't actually find a specification that includes it and it looks like a
> hack that was fixed by the specification of KWP. I'd prefer not to extend
> the hack's lifetime, given that RFC5649 is 10+ years old.
>
> WRT to HSM uncertainty, I ran into problems especially trying to wrap RSA
> private keys. Turned out that some encoded as 8 byte multiples and some
> did not. In any event, I mentioned HSMs, but I really care about the
> general model for the JCE. I'd *really* like to avoid having to have to
> first figure out the private key encoding length (which may be difficult as
> a provider may not choose to export an unwrapped private key even if its a
> software provider) before choosing the wrapping algorithm. Doing it that
> way just fits the JCE model better.
>
> At some point, there needs to be an RFC written that specifies the default
> encodings for keys wrapped by this algorithm.
>
> Later, Mike
>
>
>
> Thank you,
> Greg
>
> On Sat, Apr 3, 2021 at 4:38 PM Michael StJohns <mstjo...@comcast.net>
> wrote:
>
>> On 4/3/2021 11:35 AM, Greg Rubin wrote:
>> > I'd advise against the AutoPadding scheme without more careful analysis
>> and discussion. Have we seen either KW or KWP specifications which
>> recommend that behavior?
>> >
>> > My concern is that we've seen cases before where two different
>> cryptographic algorithms could be selected transparently upon decryption
>> and it lowers the security of the overall system. (A variant of in-band
>> signalling.) The general consensus that I've been seeing in the (applied)
>> cryptographic community is strongly away from in-band signalling and
>> towards the decryptor fully specifying the algorithms and behavior prior to
>> attempting decryption.
>>
>> I think this is in response to my comment?
>>
>> The wrap function can take a Key as an input and can have the unwrap
>> method produce a Key as an output - indeed it should be used primarily
>> for this rather than the more general encrypt/decrypt functions. The
>> problem is that the encoding of the key may not be known prior to the
>> attempt to wrap it - hence it's not known whether or not padding need be
>> applied. This is especially problematic with HSMs. Providing an
>> AutoPadding mode would allow the wrapping algorithm to decide whether to
>> use either of the RFC 3394 (AKA KW) Integrity Check Value (ICV) or the
>> RFC5649 (aka KWP) value and padding length.
>>
>> The key thing to remember here is that the IV (initial value - RFC
>> language) /ICV (integrity check value - NIST language)actually isn't an
>> IV(initialization vector) in the ordinary meaning, it's a flag, padding
>> and integrity indicator and will be fixed for all keys of the same
>> length that use the specified values. E.g. unlike other modes that
>> require an initialization vector, you don't need to know the ICV to
>> decrypt the underlying key stream, but you can (and for that matter
>> MUST) easily test the recovered first block against the expected ICV to
>> determine whether the output needs padding removed or not.
>>
>> FWIW, the actual cryptographic operations between padded data and
>> non-padded data (of the right multiple length) are identical. It's only
>> the pre or post processing that's looking for different data.
>>
>> Obviously, this doesn't work if someone provides their own IV - but
>> that's fairly unlikely. CF CCM and its non-normative example formatting
>> function appendix A - each and every implementation I've seen uses that
>> formatting function, even though it isn't actually required by the
>> standard. I'd be surprised if anyone decided to use a different set of
>> non-standard IV values.
>>
>> If an AutoPadding mode were implemented, I'd throw exceptions if someone
>> tried to set the IV.
>>
>> Later, Mike
>>
>>
>>
>
>
