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Update control logic for BN_gcd

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PR https://github.com/openssl/openssl/pull/10122 introduced changes to
the BN_gcd function and the control logic inside it accessed `g->d[0]`
irrespective of `g->top`.

When BN_add is called, in case the result is zero, `BN_zero` is called.
The latter behaves differently depending on the API compatibility level
flag: normally `g->d[0]` is cleared but in `no-deprecated` builds only
`g->top` is set to zero.

This commit uses bitwise logic to ensure that `g` is treated as zero if
`g->top` is zero, irrespective of `g->d[0]`.

Co-authored-by: Nicola Tuveri <nic.tuv@gmail.com>

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10232)
This commit is contained in:
Cesar Pereida Garcia 2019-10-21 14:41:01 +03:00 committed by Nicola Tuveri
parent 777182a0c7
commit 8aca4bfe82
1 changed files with 7 additions and 2 deletions
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9
crypto/bn/bn_gcd.c
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@ -593,7 +593,9 @@ int BN_gcd(BIGNUM *r, const BIGNUM *in_a, const BIGNUM *in_b, BN_CTX *ctx)
for (i = 0; i < m; i++) {
/* conditionally flip signs if delta is positive and g is odd */
cond = (-delta >> (8 * sizeof(delta) - 1)) & g->d[0] & 1;
cond = (-delta >> (8 * sizeof(delta) - 1)) & g->d[0] & 1
/* make sure g->top > 0 (i.e. if top == 0 then g == 0 always) */
& (~((g->top - 1) >> (sizeof(g->top) * 8 - 1)));
delta = (-cond & -delta) | ((cond - 1) & delta);
r->neg ^= cond;
/* swap */
@ -603,7 +605,10 @@ int BN_gcd(BIGNUM *r, const BIGNUM *in_a, const BIGNUM *in_b, BN_CTX *ctx)
delta++;
if (!BN_add(temp, g, r))
goto err;
BN_consttime_swap(g->d[0] & 1, g, temp, top);
BN_consttime_swap(g->d[0] & 1 /* g is odd */
/* make sure g->top > 0 (i.e. if top == 0 then g == 0 always) */
& (~((g->top - 1) >> (sizeof(g->top) * 8 - 1))),
g, temp, top);
if (!BN_rshift1(g, g))
goto err;
}