mbedtls/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 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)

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_tls1_3_. The same applies to structures and types that are internal to C modules.

• TLS 1.3 specific exported functions, macros, structures and types are prefixed with mbedtls_ssl_tls1_3_.

• 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 is the name of the vector as defined in the TLS 1.3 specification.

  • Use <vector_name>_len_ptr for the name of a variable intended to hold the address of the first byte of the vector length.

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

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

  Those two last idioms should lower the risk of mis-using one of the address in place of the other 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 *cipher_suites_len_ptr;
       unsigned char *cipher_suites_ptr;
  

• 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 );