From eceb33d3afe6b36061df9c94fb28e0e08f15ea94 Mon Sep 17 00:00:00 2001 From: Adam Langley Date: Fri, 20 Jun 2014 12:00:00 -0700 Subject: [PATCH] bignum: fix boundary condition in montgomery logic It's not clear whether this inconsistency could lead to an actual computation error, but it involved a BIGNUM being passed around the montgomery logic in an inconsistent state. This was found using flags -DBN_DEBUG -DBN_DEBUG_RAND, and working backwards from this assertion in 'ectest'; ectest: bn_mul.c:960: BN_mul: Assertion `(_bnum2->top == 0) || (_bnum2->d[_bnum2->top - 1] != 0)' failed (Imported from upstream's 3cc546a3bbcbf26cd14fc45fb133d36820ed0a75) --- crypto/bn/exponentiation.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/crypto/bn/exponentiation.c b/crypto/bn/exponentiation.c index ac47e4b3..83f1667a 100644 --- a/crypto/bn/exponentiation.c +++ b/crypto/bn/exponentiation.c @@ -692,6 +692,9 @@ int BN_mod_exp_mont(BIGNUM *rr, const BIGNUM *a, const BIGNUM *p, for (i = 1; i < j; i++) r->d[i] = (~m->d[i]) & BN_MASK2; r->top = j; + /* Upper words will be zero if the corresponding words of 'm' + * were 0xfff[...], so decrement r->top accordingly. */ + bn_correct_top(r); } else if (!BN_to_montgomery(r, BN_value_one(), mont, ctx)) { goto err; }