in tls/s2n_record_write.c [215:488]
int s2n_record_writev(struct s2n_connection *conn, uint8_t content_type, const struct iovec *in, int in_count, size_t offs, size_t to_write)
{
struct s2n_blob iv = { 0 };
uint8_t padding = 0;
uint16_t block_size = 0;
uint8_t aad_iv[S2N_TLS_MAX_IV_LEN] = { 0 };
/* In TLS 1.3, handle CCS message as unprotected records */
struct s2n_crypto_parameters *current_client_crypto = conn->client;
struct s2n_crypto_parameters *current_server_crypto = conn->server;
if (conn->actual_protocol_version == S2N_TLS13 && content_type == TLS_CHANGE_CIPHER_SPEC) {
conn->client = &conn->initial;
conn->server = &conn->initial;
}
uint8_t *sequence_number = conn->server->server_sequence_number;
struct s2n_hmac_state *mac = &conn->server->server_record_mac;
struct s2n_session_key *session_key = &conn->server->server_key;
const struct s2n_cipher_suite *cipher_suite = conn->server->cipher_suite;
uint8_t *implicit_iv = conn->server->server_implicit_iv;
if (conn->mode == S2N_CLIENT) {
sequence_number = conn->client->client_sequence_number;
mac = &conn->client->client_record_mac;
session_key = &conn->client->client_key;
cipher_suite = conn->client->cipher_suite;
implicit_iv = conn->client->client_implicit_iv;
}
/* The NULL stream cipher MUST NEVER be used for ApplicationData.
* Writing ApplicationData unencrypted defeats the purpose of TLS. */
if (cipher_suite->record_alg->cipher == &s2n_null_cipher) {
POSIX_ENSURE(content_type != TLS_APPLICATION_DATA, S2N_ERR_ENCRYPT);
}
const int is_tls13_record = cipher_suite->record_alg->flags & S2N_TLS13_RECORD_AEAD_NONCE;
s2n_stack_blob(aad, is_tls13_record ? S2N_TLS13_AAD_LEN : S2N_TLS_MAX_AAD_LEN, S2N_TLS_MAX_AAD_LEN);
S2N_ERROR_IF(s2n_stuffer_data_available(&conn->out), S2N_ERR_RECORD_STUFFER_NEEDS_DRAINING);
uint8_t mac_digest_size;
POSIX_GUARD(s2n_hmac_digest_size(mac->alg, &mac_digest_size));
/* Before we do anything, we need to figure out what the length of the
* fragment is going to be.
*/
uint16_t max_write_payload_size = 0;
POSIX_GUARD_RESULT(s2n_record_max_write_payload_size(conn, &max_write_payload_size));
const uint16_t data_bytes_to_take = MIN(to_write, max_write_payload_size);
uint16_t extra = 0;
POSIX_GUARD_RESULT(s2n_tls_record_overhead(conn, &extra));
/* If we have padding to worry about, figure that out too */
if (cipher_suite->record_alg->cipher->type == S2N_CBC) {
block_size = cipher_suite->record_alg->cipher->io.cbc.block_size;
if (((data_bytes_to_take + extra) % block_size)) {
padding = block_size - ((data_bytes_to_take + extra) % block_size);
}
} else if (cipher_suite->record_alg->cipher->type == S2N_COMPOSITE) {
block_size = cipher_suite->record_alg->cipher->io.comp.block_size;
}
/* Start the MAC with the sequence number */
POSIX_GUARD(s2n_hmac_update(mac, sequence_number, S2N_TLS_SEQUENCE_NUM_LEN));
if (s2n_stuffer_is_freed(&conn->out)) {
/* If the output buffer has not been allocated yet, allocate enough memory to hold
* a record with the local maximum fragment length. Because this only occurs if the
* output buffer has not been allocated, it does NOT resize existing buffers.
*
* The maximum fragment length is:
* 1) The local default configured for new connections
* 2) The local value set by the user via s2n_connection_prefer_throughput()
* or s2n_connection_prefer_low_latency()
* 3) On the server, the minimum of the local value and the value negotiated with the
* client via the max_fragment_length extension
*/
uint16_t max_wire_record_size = 0;
POSIX_GUARD_RESULT(s2n_record_max_write_size(conn, max_write_payload_size, &max_wire_record_size));
POSIX_GUARD(s2n_stuffer_growable_alloc(&conn->out, max_wire_record_size));
}
/* Now that we know the length, start writing the record */
POSIX_GUARD(s2n_stuffer_write_uint8(&conn->out, is_tls13_record ?
/* tls 1.3 opaque type */ TLS_APPLICATION_DATA :
/* actual content_type */ content_type ));
POSIX_GUARD(s2n_record_write_protocol_version(conn));
/* First write a header that has the payload length, this is for the MAC */
POSIX_GUARD(s2n_stuffer_write_uint16(&conn->out, data_bytes_to_take));
if (conn->actual_protocol_version > S2N_SSLv3) {
POSIX_GUARD(s2n_hmac_update(mac, conn->out.blob.data, S2N_TLS_RECORD_HEADER_LENGTH));
} else {
/* SSLv3 doesn't include the protocol version in the MAC */
POSIX_GUARD(s2n_hmac_update(mac, conn->out.blob.data, 1));
POSIX_GUARD(s2n_hmac_update(mac, conn->out.blob.data + 3, 2));
}
/* Compute non-payload parts of the MAC(seq num, type, proto vers, fragment length) for composite ciphers.
* Composite "encrypt" will MAC the payload data and fill in padding.
*/
if (cipher_suite->record_alg->cipher->type == S2N_COMPOSITE) {
/* Only fragment length is needed for MAC, but the EVP ctrl function needs fragment length + eiv len. */
uint16_t payload_and_eiv_len = data_bytes_to_take;
if (conn->actual_protocol_version > S2N_TLS10) {
payload_and_eiv_len += block_size;
}
/* Outputs number of extra bytes required for MAC and padding */
int pad_and_mac_len;
POSIX_GUARD(cipher_suite->record_alg->cipher->io.comp.initial_hmac(session_key, sequence_number, content_type, conn->actual_protocol_version,
payload_and_eiv_len, &pad_and_mac_len));
extra += pad_and_mac_len;
}
/* TLS 1.3 protected record occupies one extra byte for content type */
if (is_tls13_record) {
extra += S2N_TLS_CONTENT_TYPE_LENGTH;
}
/* Rewrite the length to be the actual fragment length */
const uint16_t actual_fragment_length = data_bytes_to_take + padding + extra;
/* ensure actual_fragment_length + S2N_TLS_RECORD_HEADER_LENGTH <= max record length */
const uint16_t max_record_length = is_tls13_record ? S2N_TLS13_MAXIMUM_RECORD_LENGTH : S2N_TLS_MAXIMUM_RECORD_LENGTH;
S2N_ERROR_IF(actual_fragment_length + S2N_TLS_RECORD_HEADER_LENGTH > max_record_length, S2N_ERR_RECORD_LENGTH_TOO_LARGE);
POSIX_GUARD(s2n_stuffer_wipe_n(&conn->out, 2));
POSIX_GUARD(s2n_stuffer_write_uint16(&conn->out, actual_fragment_length));
/* If we're AEAD, write the sequence number as an IV, and generate the AAD */
if (cipher_suite->record_alg->cipher->type == S2N_AEAD) {
struct s2n_stuffer iv_stuffer = {0};
s2n_blob_init(&iv, aad_iv, sizeof(aad_iv));
POSIX_GUARD(s2n_stuffer_init(&iv_stuffer, &iv));
if (cipher_suite->record_alg->flags & S2N_TLS12_AES_GCM_AEAD_NONCE) {
/* Partially explicit nonce. See RFC 5288 Section 3 */
POSIX_GUARD(s2n_stuffer_write_bytes(&conn->out, sequence_number, S2N_TLS_SEQUENCE_NUM_LEN));
POSIX_GUARD(s2n_stuffer_write_bytes(&iv_stuffer, implicit_iv, cipher_suite->record_alg->cipher->io.aead.fixed_iv_size));
POSIX_GUARD(s2n_stuffer_write_bytes(&iv_stuffer, sequence_number, S2N_TLS_SEQUENCE_NUM_LEN));
} else if (cipher_suite->record_alg->flags & S2N_TLS12_CHACHA_POLY_AEAD_NONCE || is_tls13_record) {
/* Fully implicit nonce. See RFC7905 Section 2 */
uint8_t four_zeroes[4] = { 0 };
POSIX_GUARD(s2n_stuffer_write_bytes(&iv_stuffer, four_zeroes, 4));
POSIX_GUARD(s2n_stuffer_write_bytes(&iv_stuffer, sequence_number, S2N_TLS_SEQUENCE_NUM_LEN));
for(int i = 0; i < cipher_suite->record_alg->cipher->io.aead.fixed_iv_size; i++) {
aad_iv[i] = aad_iv[i] ^ implicit_iv[i];
}
} else {
POSIX_BAIL(S2N_ERR_INVALID_NONCE_TYPE);
}
/* Set the IV size to the amount of data written */
iv.size = s2n_stuffer_data_available(&iv_stuffer);
if (is_tls13_record) {
POSIX_GUARD_RESULT(s2n_tls13_aead_aad_init(data_bytes_to_take + S2N_TLS_CONTENT_TYPE_LENGTH, cipher_suite->record_alg->cipher->io.aead.tag_size, &aad));
} else {
POSIX_GUARD_RESULT(s2n_aead_aad_init(conn, sequence_number, content_type, data_bytes_to_take, &aad));
}
} else if (cipher_suite->record_alg->cipher->type == S2N_CBC || cipher_suite->record_alg->cipher->type == S2N_COMPOSITE) {
s2n_blob_init(&iv, implicit_iv, block_size);
/* For TLS1.1/1.2; write the IV with random data */
if (conn->actual_protocol_version > S2N_TLS10) {
POSIX_GUARD_RESULT(s2n_get_public_random_data(&iv));
if (cipher_suite->record_alg->cipher->type == S2N_COMPOSITE) {
/* Write a separate random block to the record. This will be used along with the previously generated
* iv blob to generate the final explicit_iv for this record.
*
* How? Openssl's AES-CBC stitched encrypt populates the first block of application data with:
* AES(Key, XOR(iv, initial_block))
*
* If we make initial_block a random block unrelated to random_iv, explicit IV for this record
* is random value based on the two random blobs we just generated:
* AES(Key, XOR(random_iv, explicit_iv_placeholder) == AES(Key, XOR(random_iv, random_iv2))
*
* NOTE: We can't use the same random IV blob as both the initial block and IV since it will result in:
* AES(Key, XOR(random_iv, random_iv)) == AES(Key, 0), which will be shared by all records in this session.
*/
struct s2n_blob explicit_iv_placeholder;
uint8_t zero_block[S2N_TLS_MAX_IV_LEN] = { 0 };
POSIX_GUARD(s2n_blob_init(&explicit_iv_placeholder, zero_block, block_size));
POSIX_GUARD_RESULT(s2n_get_public_random_data(&explicit_iv_placeholder));
POSIX_GUARD(s2n_stuffer_write(&conn->out, &explicit_iv_placeholder));
} else {
/* We can write the explicit IV directly to the record for non composite CBC because
* s2n starts AES *after* the explicit IV.
*/
POSIX_GUARD(s2n_stuffer_write(&conn->out, &iv));
}
}
}
/* We are done with this sequence number, so we can increment it */
struct s2n_blob seq = {.data = sequence_number,.size = S2N_TLS_SEQUENCE_NUM_LEN };
POSIX_GUARD(s2n_increment_sequence_number(&seq));
/* Write the plaintext data */
POSIX_GUARD(s2n_stuffer_writev_bytes(&conn->out, in, in_count, offs, data_bytes_to_take));
void *orig_write_ptr = conn->out.blob.data + conn->out.write_cursor - data_bytes_to_take;
POSIX_GUARD(s2n_hmac_update(mac, orig_write_ptr, data_bytes_to_take));
/* Write the digest */
uint8_t *digest = s2n_stuffer_raw_write(&conn->out, mac_digest_size);
POSIX_ENSURE_REF(digest);
POSIX_GUARD(s2n_hmac_digest(mac, digest, mac_digest_size));
POSIX_GUARD(s2n_hmac_reset(mac));
/* Write content type for TLS 1.3 record (RFC 8446 Section 5.2) */
if (is_tls13_record) {
POSIX_GUARD(s2n_stuffer_write_uint8(&conn->out, content_type));
}
if (cipher_suite->record_alg->cipher->type == S2N_CBC) {
/* Include padding bytes, each with the value 'p', and
* include an extra padding length byte, also with the value 'p'.
*/
for (int i = 0; i <= padding; i++) {
POSIX_GUARD(s2n_stuffer_write_uint8(&conn->out, padding));
}
}
/* Rewind to rewrite/encrypt the packet */
POSIX_GUARD(s2n_stuffer_rewrite(&conn->out));
/* Skip the header */
POSIX_GUARD(s2n_stuffer_skip_write(&conn->out, S2N_TLS_RECORD_HEADER_LENGTH));
uint16_t encrypted_length = data_bytes_to_take + mac_digest_size;
switch (cipher_suite->record_alg->cipher->type) {
case S2N_AEAD:
POSIX_GUARD(s2n_stuffer_skip_write(&conn->out, cipher_suite->record_alg->cipher->io.aead.record_iv_size));
encrypted_length += cipher_suite->record_alg->cipher->io.aead.tag_size;
if (is_tls13_record) {
/* one extra byte for content type */
encrypted_length += S2N_TLS_CONTENT_TYPE_LENGTH;
}
break;
case S2N_CBC:
if (conn->actual_protocol_version > S2N_TLS10) {
/* Leave the IV alone and unencrypted */
POSIX_GUARD(s2n_stuffer_skip_write(&conn->out, iv.size));
}
/* Encrypt the padding and the padding length byte too */
encrypted_length += padding + 1;
break;
case S2N_COMPOSITE:
/* Composite CBC expects a pointer starting at explicit IV: [Explicit IV | fragment | MAC | padding | padding len ]
* extra will account for the explicit IV len(if applicable), MAC digest len, padding len + padding byte.
*/
encrypted_length += extra;
break;
default:
break;
}
/* Check that stuffer have enough space to write encrypted record, because raw_write cannot expand tainted stuffer */
S2N_ERROR_IF(s2n_stuffer_space_remaining(&conn->out) < encrypted_length, S2N_ERR_RECORD_STUFFER_SIZE);
/* Do the encryption */
struct s2n_blob en = { .size = encrypted_length, .data = s2n_stuffer_raw_write(&conn->out, encrypted_length) };
POSIX_GUARD(s2n_record_encrypt(conn, cipher_suite, session_key, &iv, &aad, &en, implicit_iv, block_size));
if (conn->actual_protocol_version == S2N_TLS13 && content_type == TLS_CHANGE_CIPHER_SPEC) {
conn->client = current_client_crypto;
conn->server = current_server_crypto;
}
conn->wire_bytes_out += actual_fragment_length + S2N_TLS_RECORD_HEADER_LENGTH;
return data_bytes_to_take;
}