Re: [kitten] SPAKE preauth: generation of SPAKE2 secret input (proposed text)

Greg Hudson <ghudson@mit.edu> Tue, 19 May 2015 16:16 UTC

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Date: Tue, 19 May 2015 12:14:46 -0400
From: Greg Hudson <ghudson@mit.edu>
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Subject: Re: [kitten] SPAKE preauth: generation of SPAKE2 secret input (proposed text)
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Based on the results of this discussion, here is the wording I came up
with for my pull request.  In the prerequisites section:

   Both KDCs and clients MUST implement one or more groups for which the
   discrete logarithm problem is hard, each with an corresponding object
   identifer.  Group object identifiers MUST refer to one of the
   following:

   o  One of the eight elliptic curves specified in [SEC2] section 2,
      using the corresponding object identifier from appendix A.2.1.

   o  A group with an unambiguous specification for representing group
      elements as octet strings, and an unambiguous specification for
      converting an octet string of a specific length into an integer
      for use as a scalar multiplier.

and then in a new section titled "SPAKE Parameters and Conversions":

   The SPAKE algorithm requires a shared secret input w to be used as a
   scalar multiplier (see [I-D.irtf-cfrg-spake2] section 2).  This value
   MUST be produced from the client principal's long-term key as
   follows:

   1.  Determine a scalar octet string input length for the chosen
       group.  For SEC 2 curves over a prime field F(p), this is
       ceil(log2(p)/8).  For other groups, this length is specified by
       the group.

   2.  Produce an octet string of the above length with PRF+(K,
       "SPAKEsecret"), where K is the long-term key and PRF+ is defined
       in [RFC6113] section 5.1.

   3.  If the chosen group is secp521r1, set the high seven bits of the
       first byte to 0 so that there are at most 521 significant bits in
       the octet string.

   4.  For SEC 2 curves, decode the octet string as a big-endian
       positive integer as specified in [SEC1] section 2.3.8.  For other
       groups, decode the octet string as specified by the group.

Salient points:

* SEC 2 curves over Fp (which include NIST P-256/P-384/P-521) get to use
existing OIDs with conversions specified in this draft, using SEC 1
encoding primitives.  Other groups need to use an OID which
unambiguously specifies the needed conversions.  I expect the
forthcoming CFRG curves to slot in here, although I'm not sure if they
plan to designate OIDs for their curves.

* Because the eight SEC 2 curves all have cofactor 1, we don't have to
worry about cofactors in the conversions we specify.  Except for
secp521r1, they all use primes close to a power of 2^8.

* The w values won't be uniformly distributed even if the long-term key
is.  But as Watson noted, SPAKE doesn't assume that w is uniformly
distributed.

* A small proportion of w values will be larger than the group order.
Implementations need to be able to handle these values correctly and
without side channels.  I will try to make a note of this in the
security considerations, and will try to make sure that there are test
vectors exercising this possibility.  OpenSSL contains P-256 and P-521
implementations which are supposed to be constant-time if the input
bignum doesn't contain more than 256 or 521 significant bits respectively.

* It's possible, though vanishingly unlikely, for w to be equal to 0 or
to the group order.  I believe all of the computations should work in
this case, so there is no need to reject those values.