// This file is dual licensed under the terms of the Apache License, Version
// 2.0, and the BSD License. See the LICENSE file in the root of this
// repository for complete details.
/* Returns the value of the input with the most-significant-bit copied to all
of the bits. */
static uint8_t Cryptography_DUPLICATE_MSB_TO_ALL(uint8_t a) {
return (1 - (a >> (sizeof(uint8_t) * 8 - 1))) - 1;
}
/* This returns 0xFF if a < b else 0x00, but does so in a constant time
fashion */
static uint8_t Cryptography_constant_time_lt(uint8_t a, uint8_t b) {
a -= b;
return Cryptography_DUPLICATE_MSB_TO_ALL(a);
}
uint8_t Cryptography_check_pkcs7_padding(const uint8_t *data,
uint8_t block_len) {
uint8_t i;
uint8_t pad_size = data[block_len - 1];
uint8_t mismatch = 0;
for (i = 0; i < block_len; i++) {
unsigned int mask = Cryptography_constant_time_lt(i, pad_size);
uint8_t b = data[block_len - 1 - i];
mismatch |= (mask & (pad_size ^ b));
}
/* Check to make sure the pad_size was within the valid range. */
mismatch |= ~Cryptography_constant_time_lt(0, pad_size);
mismatch |= Cryptography_constant_time_lt(block_len, pad_size);
/* 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;
}
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