RFR: 8248268: Support KWP in addition to KW [v4]

Michael StJohns mstjohns at comcast.net
Wed Apr 7 18:49:11 UTC 2021


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"] 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 <mstjohns at comcast.net 
> <mailto:mstjohns at 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
>
>

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/security-dev/attachments/20210407/b498eacf/attachment.htm>


More information about the security-dev mailing list