[curves] XEdDSA specification

[Mike Hamburg]

> On Oct 29, 2016, at 3:47 PM, Trevor Perrin <trevp at trevp.net> wrote:
> 
> On Fri, Oct 28, 2016 at 12:47 PM, Trevor Perrin <trevp at trevp.net> wrote:
>> On Fri, Oct 28, 2016 at 2:40 AM, Brian Smith <brian at briansmith.org> wrote:
>> [...]
>>> Consider this "best of both worlds" scheme:
>>> 
>>>     r = hash1(a || [first half of Z] || M || [second half of Z])
> 
> Samuel Neves mentioned (off-list) that with SHA512's 128-byte block
> size, a and M would still be mingled together in the first block.
> 
> So I'm thinking about this:
> 
>  a || Z || pad || M
> 
> where "pad" adds zero bytes to fill the hash block (so 32 bytes with
> 25519 and SHA512, since |a|=32 and |Z|=64).
> 
> Rationale:
> (1) In the Random Oracle Model, this is no different from hashing "a
> || M || Z".
> (2) Processing the secret inputs (a and Z) in a separate block (or
> blocks) from M seems cleaner
> (3) Mixing the secret key with secret random data (Z) prior to mixing
> it with known input (M) is better for resisting physical sidechannels
> (power, EM).
> (4) The prior rationale for hashing Z at the end was weak:  It might
> help protect a very weak hash where the attacker was able to choose M
> to force biases or collisions, even with unknown and randomized
> prefix.  But I think the sidechannel threat is more plausible.
> - We could consider a || Z1 || pad || M || Z2, but the risk of (4) is
> low enough that I doubt that's worth the complexity
> 
> Thoughts?
> 
> Trevor
> _______________________________________________
> Curves mailing list
> Curves at moderncrypto.org
> https://moderncrypto.org/mailman/listinfo/curves

I like the change to a || Z || pad || M, for the reasons you listed.

— Mike



-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 3693 bytes
Desc: not available
URL: <http://moderncrypto.org/mail-archive/curves/attachments/20161029/28e03a3b/attachment.bin>