diff options
Diffstat (limited to 'deps/openssl/openssl/crypto/ec/ecp_nistp521.c')
| -rw-r--r-- | deps/openssl/openssl/crypto/ec/ecp_nistp521.c | 11 |
1 files changed, 8 insertions, 3 deletions
diff --git a/deps/openssl/openssl/crypto/ec/ecp_nistp521.c b/deps/openssl/openssl/crypto/ec/ecp_nistp521.c index 2f47772a34..e31b85c5f7 100644 --- a/deps/openssl/openssl/crypto/ec/ecp_nistp521.c +++ b/deps/openssl/openssl/crypto/ec/ecp_nistp521.c @@ -357,10 +357,15 @@ static void felem_diff64(felem out, const felem in) static void felem_diff_128_64(largefelem out, const felem in) { /* - * In order to prevent underflow, we add 0 mod p before subtracting. + * In order to prevent underflow, we add 64p mod p (which is equivalent + * to 0 mod p) before subtracting. p is 2^521 - 1, i.e. in binary a 521 + * digit number with all bits set to 1. See "The representation of field + * elements" comment above for a description of how limbs are used to + * represent a number. 64p is represented with 8 limbs containing a number + * with 58 bits set and one limb with a number with 57 bits set. */ - static const limb two63m6 = (((limb) 1) << 62) - (((limb) 1) << 5); - static const limb two63m5 = (((limb) 1) << 62) - (((limb) 1) << 4); + static const limb two63m6 = (((limb) 1) << 63) - (((limb) 1) << 6); + static const limb two63m5 = (((limb) 1) << 63) - (((limb) 1) << 5); out[0] += two63m6 - in[0]; out[1] += two63m5 - in[1]; |