int s2n_record_parse_composite()

in tls/s2n_record_read_composite.c [28:110]

• 50 lines of code
• 3 McCabe index (conditional complexity)

int s2n_record_parse_composite(
        const struct s2n_cipher_suite *cipher_suite,
        struct s2n_connection *conn,
        uint8_t content_type,
        uint16_t encrypted_length,
        uint8_t *implicit_iv,
        struct s2n_hmac_state *mac,
        uint8_t *sequence_number,
        struct s2n_session_key *session_key)
{
    /* Don't reduce encrypted length for explicit IV, composite decrypt expects it */
    struct s2n_blob iv = { .data = implicit_iv, .size = cipher_suite->record_alg->cipher->io.comp.record_iv_size };
    uint8_t ivpad[S2N_TLS_MAX_IV_LEN];

    /* Add the header to the HMAC */
    uint8_t *header = s2n_stuffer_raw_read(&conn->header_in, S2N_TLS_RECORD_HEADER_LENGTH);
    POSIX_ENSURE_REF(header);

    struct s2n_blob en = { .size = encrypted_length, .data = s2n_stuffer_raw_read(&conn->in, encrypted_length) };
    POSIX_ENSURE_REF(en.data);

    uint16_t payload_length = encrypted_length;
    uint8_t mac_digest_size = 0;
    POSIX_GUARD(s2n_hmac_digest_size(mac->alg, &mac_digest_size));

    POSIX_ENSURE_GTE(payload_length, mac_digest_size);
    payload_length -= mac_digest_size;

    /* Compute non-payload parts of the MAC(seq num, type, proto vers, fragment length) for composite ciphers.
     * Composite "decrypt" will MAC the actual payload data.
     */
    /* In the decrypt case, this outputs the MAC digest length:
     * https://github.com/openssl/openssl/blob/master/crypto/evp/e_aes_cbc_hmac_sha1.c#L842 */
    int mac_size = 0;
    POSIX_GUARD(cipher_suite->record_alg->cipher->io.comp.initial_hmac(session_key, sequence_number, content_type, conn->actual_protocol_version, payload_length, &mac_size));

    POSIX_ENSURE_GTE(payload_length, mac_size);
    payload_length -= mac_size;
    /* Adjust payload_length for explicit IV */
    if (conn->actual_protocol_version > S2N_TLS10) {
        uint32_t out = 0;
        POSIX_GUARD(s2n_sub_overflow(payload_length, cipher_suite->record_alg->cipher->io.comp.record_iv_size, &out));
        payload_length = out;
    }

    /* Decrypt stuff! */
    POSIX_ENSURE_NE(en.size, 0);
    POSIX_ENSURE_EQ(en.size % iv.size, 0);

    /* Copy the last encrypted block to be the next IV */
    POSIX_CHECKED_MEMCPY(ivpad, en.data + en.size - iv.size, iv.size);

    /* This will: Skip the explicit IV(if applicable), decrypt the payload, verify the MAC and padding. */
    POSIX_GUARD((cipher_suite->record_alg->cipher->io.comp.decrypt(session_key, &iv, &en, &en)));

    POSIX_CHECKED_MEMCPY(implicit_iv, ivpad, iv.size);

    /* Subtract the padding length */
    POSIX_ENSURE_GT(en.size, 0);
    uint32_t out = 0;
    POSIX_GUARD(s2n_sub_overflow(payload_length, en.data[en.size - 1] + 1, &out));
    payload_length = out;

    struct s2n_blob seq = { .data = sequence_number, .size = S2N_TLS_SEQUENCE_NUM_LEN };
    POSIX_GUARD(s2n_increment_sequence_number(&seq));

    /* O.k., we've successfully read and decrypted the record, now we need to align the stuffer
     * for reading the plaintext data.
     */
    POSIX_GUARD(s2n_stuffer_reread(&conn->in));
    POSIX_GUARD(s2n_stuffer_reread(&conn->header_in));

    /* Skip the IV, if any */
    if (conn->actual_protocol_version > S2N_TLS10) {
        POSIX_GUARD(s2n_stuffer_skip_read(&conn->in, cipher_suite->record_alg->cipher->io.comp.record_iv_size));
    }

    /* Truncate and wipe the MAC and any padding */
    POSIX_GUARD(s2n_stuffer_wipe_n(&conn->in, s2n_stuffer_data_available(&conn->in) - payload_length));
    conn->in_status = PLAINTEXT;

    return 0;
}