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: