[openssl-commits] Errored: openssl/openssl#16033 (master - 39eeb64)

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Thu Feb 1 21:17:58 UTC 2018

Build Update for openssl/openssl

Build: #16033
Status: Errored

Duration: 24 minutes and 13 seconds
Commit: 39eeb64 (master)
Author: David Benjamin
Message: Don't leak the exponent bit width in BN_mod_exp_mont_consttime.

The exponent here is one of d, dmp1, or dmq1 for RSA. This value and its
bit length are both secret. The only public upper bound is the bit width
of the corresponding modulus (RSA n, p, and q, respectively).

Although BN_num_bits is constant-time (sort of; see bn_correct_top notes
in preceding patch), this does not fix the root problem, which is that
the windows are based on the minimal bit width, not the upper bound. We
could use BN_num_bits(m), but BN_mod_exp_mont_consttime is public API
and may be called with larger exponents. Instead, use all top*BN_BITS2
bits in the BIGNUM. This is still sensitive to the long-standing
bn_correct_top leak, but we need to fix that regardless.

This may cause us to do a handful of extra multiplications for RSA keys
which are just above a whole number of words, but that is not a standard
RSA key size.

Reviewed-by: Paul Dale <paul.dale at oracle.com>
Reviewed-by: Rich Salz <rsalz at openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5154)

View the changeset: https://github.com/openssl/openssl/compare/3c5a61dd0f9d...39eeb64f59ff

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