docs/architecture/tls13-experimental.md

TLS 1.3 Experimental Developments
=================================

Overview
--------

Mbed TLS doesn't support the TLS 1.3 protocol yet, but a prototype is in development.
Stable parts of this prototype that can be independently tested are being successively
upstreamed under the guard of the following macro:

```
MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL
```

This macro will likely be renamed to `MBEDTLS_SSL_PROTO_TLS1_3` once a minimal viable
implementation of the TLS 1.3 protocol is available.

See the [documentation of `MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL`](../../include/mbedtls/mbedtls_config.h)
for more information.

Status
------

The following lists which parts of the TLS 1.3 prototype have already been upstreamed
together with their level of testing:

* TLS 1.3 record protection mechanisms

  The record protection routines `mbedtls_ssl_{encrypt|decrypt}_buf()` have been extended
  to support the modified TLS 1.3 record protection mechanism, including modified computation
  of AAD, IV, and the introduction of a flexible padding.

  Those record protection routines have unit tests in `test_suite_ssl` alongside the
  tests for the other record protection routines.

  TODO: Add some test vectors from RFC 8448.

- The HKDF key derivation function on which the TLS 1.3 key schedule is based,
  is already present as an independent module controlled by `MBEDTLS_HKDF_C`
  independently of the development of the TLS 1.3 prototype.

- The TLS 1.3-specific HKDF-based key derivation functions (see RFC 8446):
  * HKDF-Expand-Label
  * Derive-Secret
  - Secret evolution
  * The traffic {Key,IV} generation from secret
  Those functions are implemented in `library/ssl_tls13_keys.c` and
  tested in `test_suite_ssl` using test vectors from RFC 8448 and
  https://tls13.ulfheim.net/.

- New TLS Message Processing Stack (MPS)

  The TLS 1.3 prototype is developed alongside a rewrite of the TLS messaging layer,
  encompassing low-level details such as record parsing, handshake reassembly, and
  DTLS retransmission state machine.

  MPS has the following components:
  - Layer 1 (Datagram handling)
  - Layer 2 (Record handling)
  - Layer 3 (Message handling)
  - Layer 4 (Retransmission State Machine)
  - Reader  (Abstracted pointer arithmetic and reassembly logic for incoming data)
  - Writer  (Abstracted pointer arithmetic and fragmentation logic for outgoing data)

  Of those components, the following have been upstreamed
  as part of `MBEDTLS_SSL_PROTO_TLS1_3_EXPERIMENTAL`:

  - Reader ([`library/mps_reader.h`](../../library/mps_reader.h))


MVP definition
--------------

The TLS 1.3 MVP implements only the client side of the protocol.
The TLS 1.3 MVP does not support the handling of server HelloRetryRequest and
CertificateRequest messages. If it receives one of those messages, it aborts
the handshake with an handshake_failure closure alert and the
`mbedtls_ssl_handshake()` returns in error with the
`MBEDTLS_ERR_SSL_HANDSHAKE_FAILURE` error code.

- Supported cipher suites: depends on the library configuration. Potentially
  all of them:
  TLS_AES_128_GCM_SHA256, TLS_AES_256_GCM_SHA384, TLS_CHACHA20_POLY1305_SHA256,
  TLS_AES_128_CCM_SHA256 and TLS_AES_128_CCM_8_SHA256.

- Supported ClientHello extensions:

  | Extension                    |   MVP   | Prototype (1) |
  | ---------------------------- | ------- | ------------- |
  | server_name                  | YES     | YES           |
  | max_fragment_length          | no      | YES           |
  | status_request               | no      | no            |
  | supported_groups             | YES     | YES           |
  | signature_algorithms         | YES     | YES           |
  | use_srtp                     | no      | no            |
  | heartbeat                    | no      | no            |
  | apln                         | no      | YES           |
  | signed_certificate_timestamp | no      | no            |
  | client_certificate_type      | no      | no            |
  | server_certificate_type      | no      | no            |
  | padding                      | no      | no            |
  | key_share                    | YES (2) | YES           |
  | pre_shared_key               | no      | YES           |
  | psk_key_exchange_modes       | no      | YES           |
  | early_data                   | no      | YES           |
  | cookie                       | no      | YES           |
  | supported_versions           | YES (3) | YES           |
  | certificate_authorities      | no      | no            |
  | post_handshake_auth          | no      | no            |
  | signature_algorithms_cert    | no      | no            |

  (1) This is just for comparison.

  (2) The MVP sends one shared secret corresponding to the configured preferred
      group. The preferred group is the group of the first curve in the list of
      allowed curves as defined by the configuration. By default, it is the
      mandatory group as defined by section 9.1 of the specification,
      `secp256r1`. The list of allowed curves can be set through the
      `mbedtls_ssl_conf_curves()` API.

  (3) The MVP proposes only TLS 1.3 and does not support version negociation.
      Out-of-protocol fallback is supported though if the Mbed TLS library
      has been built to support both TLS 1.3 and TLS 1.2: just set the
      maximum of the minor version of the SSL configuration to
      MBEDTLS_SSL_MINOR_VERSION_3 (`mbedtls_ssl_conf_min_version()` API) and
      re-initiate a server handshake.

- Supported groups: depends on the library configuration.
  Minimally (as defined in section 9.1 of the TLS 1.3 specification):
  secp256r1 and x25519.

  Furthermore, depending on the library configuration, potentially:
  secp384r1 and secp521r1.

  Finite field groups (DHE) are not supported.

- Supported signature algorithms(both for certificates and CertificateVerify):
  Minimally (as defined in section 9.1 of the TLS 1.3 specification):
  rsa_pkcs1_sha256, rsa_pss_rsae_sha256 and ecdsa_secp256r1_sha256

  Furthermore, depending on the library configuration, potentially:
  ecdsa_secp384r1_sha384 and ecdsa_secp521r1_sha512

- Supported versions: only TLS 1.3, version negotiation is not supported.

- Support of Mbed TLS SSL/TLS related (not DTLS) features:

  The TLS 1.3 MVP is compatible with all TLS 1.2 configuration options in the
  sense that when enabling the TLS 1.3 MVP in the library there is no need to
  modify the configuration for TLS 1.2. Mbed TLS SSL/TLS related features are
  not supported or not applicable to the TLS 1.3 MVP:

  | Mbed TLS configuration option            | Support |
  | ---------------------------------------- | ------- |
  | MBEDTLS_SSL_ALL_ALERT_MESSAGES           | no      |
  | MBEDTLS_SSL_ASYNC_PRIVATE                | no      |
  | MBEDTLS_SSL_CONTEXT_SERIALIZATION        | no      |
  | MBEDTLS_SSL_DEBUG_ALL                    | no      |
  | MBEDTLS_SSL_ENCRYPT_THEN_MAC             | n/a     |
  | MBEDTLS_SSL_EXTENDED_MASTER_SECRET       | n/a     |
  | MBEDTLS_SSL_KEEP_PEER_CERTIFICATE        | no      |
  | MBEDTLS_SSL_RENEGOTIATION                | n/a     |
  | MBEDTLS_SSL_MAX_FRAGMENT_LENGTH          | no      |
  |                                          |         |
  | MBEDTLS_SSL_SESSION_TICKETS              | no      |
  | MBEDTLS_SSL_EXPORT_KEYS                  | no (1)  |
  | MBEDTLS_SSL_SERVER_NAME_INDICATION       | no      |
  | MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH       | no      |
  |                                          |         |
  | MBEDTLS_ECP_RESTARTABLE                  | no      |
  | MBEDTLS_ECDH_VARIANT_EVEREST_ENABLED     | no      |
  |                                          |         |
  | MBEDTLS_KEY_EXCHANGE_PSK_ENABLED         | n/a (2) |
  | MBEDTLS_KEY_EXCHANGE_DHE_PSK_ENABLED     | n/a     |
  | MBEDTLS_KEY_EXCHANGE_ECDHE_PSK_ENABLED   | n/a     |
  | MBEDTLS_KEY_EXCHANGE_RSA_PSK_ENABLED     | n/a     |
  | MBEDTLS_KEY_EXCHANGE_RSA_ENABLED         | n/a     |
  | MBEDTLS_KEY_EXCHANGE_DHE_RSA_ENABLED     | n/a     |
  | MBEDTLS_KEY_EXCHANGE_ECDHE_RSA_ENABLED   | n/a     |
  | MBEDTLS_KEY_EXCHANGE_ECDHE_ECDSA_ENABLED | n/a     |
  | MBEDTLS_KEY_EXCHANGE_ECDH_ECDSA_ENABLED  | n/a     |
  | MBEDTLS_KEY_EXCHANGE_ECDH_RSA_ENABLED    | n/a     |
  | MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED     | n/a     |
  |                                          |         |
  | MBEDTLS_USE_PSA_CRYPTO                   | no      |

  (1) Some support has already been upstreamed but it is incomplete.
  (2) Key exchange configuration options for TLS 1.3 will likely to be
      organized around the notion of key exchange mode along the line
      of the MBEDTLS_SSL_TLS13_KEY_EXCHANGE_MODE_NONE/PSK/PSK_EPHEMERAL/EPHEMERAL
      runtime configuration macros.

- Quality considerations
  - Standard Mbed TLS review bar
  - Interoperability testing with OpenSSL and GnuTLS. Test with all the
    cipher suites supported by OpenSSL/GnuTLS server with and without
    certificate base authentication.
  - Negative testing against OpenSSL/GnuTLS servers with which the
    handshake fails due to imcompatibility with the capabilities of the
    MVP: TLS 1.2 or 1.1 server, server sending an HelloRetryRequest message in
    response to the MVP ClientHello, server sending a CertificateRequest
    message ...

Coding rules checklist for TLS 1.3
----------------------------------

The following coding rules are aimed to be a checklist for TLS 1.3 upstreaming
work to reduce review rounds and the number of comments in each round. They
come along (do NOT replace) the project coding rules
(https://tls.mbed.org/kb/development/mbedtls-coding-standards). They have been
established and discussed following the review of #4882 that was the
PR upstreaming the first part of TLS 1.3 ClientHello writing code.

TLS 1.3 specific coding rules:

  - TLS 1.3 specific C modules, headers, static functions names are prefixed
    with `ssl_tls13_`. The same applies to structures and types that are
    internal to C modules.

  - TLS 1.3 specific exported functions, structures and types are
    prefixed with `mbedtls_ssl_tls13_`.

  - Use TLS1_3 in TLS 1.3 specific macros.

  - The names of macros and variables related to a field or structure in the
    TLS 1.3 specification should contain as far as possible the field name as
    it is in the specification. If the field name is "too long" and we prefer
    to introduce some kind of abbreviation of it, use the same abbreviation
    everywhere in the code.

    Example 1: #define CLIENT_HELLO_RANDOM_LEN 32, macro for the length of the
        `random` field of the ClientHello message.

    Example 2 (consistent abbreviation): `mbedtls_ssl_tls1_3_write_sig_alg_ext()`
        and `MBEDTLS_TLS_EXT_SIG_ALG`, `sig_alg` standing for
        `signature_algorithms`.

  - Regarding vectors that are represented by a length followed by their value
    in the data exchanged between servers and clients:

    - Use `<vector name>_len` for the name of a variable used to compute the
      length in bytes of the vector, where <vector name> is the name of the
      vector as defined in the TLS 1.3 specification.

    - Use `p_<vector_name>_len` for the name of a variable intended to hold
      the address of the first byte of the vector length.

    - Use `<vector_name>` for the name of a variable intended to hold the
      address of the first byte of the vector value.

    - Use `<vector_name>_end` for the name of a variable intended to hold
      the address of the first byte past the vector value.

    Those idioms should lower the risk of mis-using one of the address in place
    of another one which could potentially lead to some nasty issues.

    Example: `cipher_suites` vector of ClientHello in
             `ssl_tls1_3_write_client_hello_cipher_suites()`
    ```
    size_t cipher_suites_len;
    unsigned char *p_cipher_suites_len;
    unsigned char *cipher_suites;
    ```

  - Use of MBEDTLS_BYTE_xyz, MBEDTLS_PUT/GET_xyz, MBEDTLS_SSL_CHK_BUF_PTR
    MBEDTLS_SSL_CHK_BUF_READ_PTR macros where applicable.

    These macros were introduced after the prototype was written thus are
    likely not to be used in prototype where we now would use them in
    development.

    The two first types, MBEDTLS_BYTE_xyz and MBEDTLS_PUT/GET_xyz, improve
    the readability of the code and reduce the risk of writing or reading
    bytes in the wrong order: we should probably have only MBEDTLS_GET/PUT_*_BE
    (BE stands for Big-Endian) macros in the TLS 1.3 code.

    The two last types, `MBEDTLS_SSL_CHK_BUF_PTR` and
    `MBEDTLS_SSL_CHK_BUF_READ_PTR`, improve the readability of the code and
    reduce the risk of error in the non-completely-trivial arithmetic to
    check that we do not write or read past the end of a data buffer. The
    usage of those macros combined with the following rule mitigate the risk
    to read/write past the end of a data buffer.

    Examples:
    ```
    hs_hdr[1] = MBEDTLS_BYTE_2( total_hs_len );
    MBEDTLS_PUT_UINT16_BE( MBEDTLS_TLS_EXT_SUPPORTED_VERSIONS, p, 0 );
    MBEDTLS_SSL_CHK_BUF_PTR( p, end, 7 );
    ```

  - To mitigate what happened here
    (https://github.com/ARMmbed/mbedtls/pull/4882#discussion_r701704527) from
    happening again, use always a local variable named `p` for the reading
    pointer in functions parsing TLS 1.3 data, and for the writing pointer in
    functions writing data into an output buffer. The name `p` has been
    chosen as it was already widely used in TLS code.

  - When an TLS 1.3 structure is written or read by a function or as part of
    a function, provide as documentation the definition of the structure as
    it is in the TLS 1.3 specification.

General coding rules:

  - We prefer grouping "related statement lines" by not adding blank lines
    between them.

    Example 1:
    ```
    ret = ssl_tls13_write_client_hello_cipher_suites( ssl, buf, end, &output_len );
    if( ret != 0 )
        return( ret );
    buf += output_len;
    ```

    Example 2:
    ```
    MBEDTLS_SSL_CHK_BUF_PTR( cipher_suites_iter, end, 2 );
    MBEDTLS_PUT_UINT16_BE( cipher_suite, cipher_suites_iter, 0 );
    cipher_suites_iter += 2;
    ```

  - Use macros for constants that are used in different functions, different
    places in the code. When a constant is used only locally in a function
    (like the length in bytes of the vector lengths in functions reading and
    writing TLS handshake message) there is no need to define a macro for it.

    Example: `#define CLIENT_HELLO_RANDOM_LEN 32`

  - When declaring a pointer the dereferencing operator should be prepended to
    the pointer name not appended to the pointer type:

    Example: `mbedtls_ssl_context *ssl;`

  - Maximum line length is 80 characters.

    Exceptions:

    - string literals can extend beyond 80 characters as we do not want to
      split them to ease their search in the code base.

    - A line can be more than 80 characters by a few characters if just looking
      at the 80 first characters is enough to fully understand the line. For
      example it is generally fine if some closure characters like ";" or ")"
      are beyond the 80 characters limit.

  - When in successive lines, functions and macros parameters should be aligned
    vertically.

    Example:
    ```
    int mbedtls_ssl_tls13_start_handshake_msg( mbedtls_ssl_context *ssl,
                                               unsigned hs_type,
                                               unsigned char **buf,
                                               size_t *buf_len );
    ```