python-cryptography 0.6.1-1+deb8u1 / usr / lib / python2.7 / dist-packages / cryptography / hazmat / primitives / src / constant_time.c

// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//    http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
// implied.
// See the License for the specific language governing permissions and
// limitations under the License.

uint8_t Cryptography_constant_time_bytes_eq(uint8_t *a, size_t len_a,
                                            uint8_t *b, size_t len_b) {
    size_t i = 0;
    uint8_t mismatch = 0;
    if (len_a != len_b) {
        return 0;
    }
    for (i = 0; i < len_a; i++) {
        mismatch |= a[i] ^ b[i];
    }

    /* Make sure any bits set are copied to the lowest bit */
    mismatch |= mismatch >> 4;
    mismatch |= mismatch >> 2;
    mismatch |= mismatch >> 1;
    /* Now check the low bit to see if it's set */
    return (mismatch & 1) == 0;
}