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Fix BN_mod_word bug

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On systems where we do not have BN_ULLONG (e.g. typically 64 bit systems)
then BN_mod_word() can return incorrect results if the supplied modulus is
too big.

RT#4501

Reviewed-by: Andy Polyakov <appro@openssl.org>
This commit is contained in:
Matt Caswell 2016-05-31 11:28:14 +01:00
parent 4692340e31
commit 37258dadaa
1 changed files with 22 additions and 0 deletions
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22
crypto/bn/bn_word.c
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@ -22,10 +22,32 @@ BN_ULONG BN_mod_word(const BIGNUM *a, BN_ULONG w)
if (w == 0)
return (BN_ULONG)-1;
#ifndef BN_LLONG
/*
* If |w| is too long and we don't have BN_ULLONG then we need to fall
* back to using BN_div_word
*/
if (w > ((BN_ULONG)1 << BN_BITS4)) {
BIGNUM *tmp = BN_dup(a);
if (tmp == NULL)
return (BN_ULONG)-1;
ret = BN_div_word(tmp, w);
BN_free(tmp);
return ret;
}
#endif
bn_check_top(a);
w &= BN_MASK2;
for (i = a->top - 1; i >= 0; i--) {
#ifndef BN_LLONG
/*
* We can assume here that | w <= ((BN_ULONG)1 << BN_BITS4) | and so
* | ret < ((BN_ULONG)1 << BN_BITS4) | and therefore the shifts here are
* safe and will not overflow
*/
ret = ((ret << BN_BITS4) | ((a->d[i] >> BN_BITS4) & BN_MASK2l)) % w;
ret = ((ret << BN_BITS4) | (a->d[i] & BN_MASK2l)) % w;
#else