/* A TLS CBC record looks like ..
*
* [ Payload data ] [ HMAC ] [ Padding ] [ Padding length byte ]
*
* Each byte in the padding is expected to be set to the same value
* as the padding length byte. So if the padding length byte is '2'
* then the padding will be [ '2', '2' ] (there'll be three bytes
* set to that value if you include the padding length byte).
*
* The goal of s2n_verify_cbc() is to verify that the padding and hmac
* are correct, without leaking (via timing) how much padding there
* actually is: as this is considered secret.
*
* In addition to our efforts here though, s2n also wraps any CBC
* verification error (or record parsing error in general) with
* a randomized delay of between 1ms and 10 seconds. See s2n_connection.c.
* This amount of delay randomization is sufficient to increase the
* complexity of attack for even a 1 microsecond timing leak (which
* is quite large) by a factor of around 83 trillion.
*/
int s2n_verify_cbc(struct s2n_connection *conn, struct s2n_hmac_state *hmac, struct s2n_blob *decrypted)
{
struct s2n_hmac_state copy;
int mac_digest_size = s2n_hmac_digest_size(hmac->alg);
/* The record has to be at least big enough to contain the MAC,
* plus the padding length byte */
gt_check(decrypted->size, mac_digest_size);
int payload_and_padding_size = decrypted->size - mac_digest_size;
/* Determine what the padding length is */
uint8_t padding_length = decrypted->data[decrypted->size - 1];
int payload_length = MAX(payload_and_padding_size - padding_length - 1, 0);
/* Update the MAC */
GUARD(s2n_hmac_update(hmac, decrypted->data, payload_length));
GUARD(s2n_hmac_copy(©, hmac));
/* Check the MAC */
uint8_t check_digest[S2N_MAX_DIGEST_LEN];
lte_check(mac_digest_size, sizeof(check_digest));
GUARD(s2n_hmac_digest_two_compression_rounds(hmac, check_digest, mac_digest_size));
int mismatches = s2n_constant_time_equals(decrypted->data + payload_length, check_digest, mac_digest_size) ^ 1;
/* Compute a MAC on the rest of the data so that we perform the same number of hash operations */
GUARD(s2n_hmac_update(©, decrypted->data + payload_length + mac_digest_size, decrypted->size - payload_length - mac_digest_size - 1));
/* SSLv3 doesn't specify what the padding should actually be */
if (conn->actual_protocol_version == S2N_SSLv3) {
return 0 - mismatches;
}
/* Check the maximum amount that could theoritically be padding */
int check = MIN(255, (payload_and_padding_size - 1));
int cutoff = check - padding_length;
for (int i = 0, j = decrypted->size - 1 - check; i < check && j < decrypted->size; i++, j++) {
uint8_t mask = ~(0xff << ((i >= cutoff) * 8));
mismatches |= (decrypted->data[j] ^ padding_length) & mask;
}
if (mismatches) {
S2N_ERROR(S2N_ERR_CBC_VERIFY);
}
return 0;
}
int s2n_client_finished_recv(struct s2n_connection *conn)
{
uint8_t *our_version;
our_version = conn->handshake.client_finished;
uint8_t *their_version = s2n_stuffer_raw_read(&conn->handshake.io, S2N_TLS_FINISHED_LEN);
notnull_check(their_version);
if (!s2n_constant_time_equals(our_version, their_version, S2N_TLS_FINISHED_LEN)) {
S2N_ERROR(S2N_ERR_BAD_MESSAGE);
}
conn->handshake.next_state = SERVER_CHANGE_CIPHER_SPEC;
return 0;
}
int s2n_hmac_digest_verify(const void *a, uint32_t alen, const void *b, uint32_t blen)
{
return 0 - (!s2n_constant_time_equals(a, b, alen) | !!(alen - blen));
}
