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+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/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/.