Remove HKDF-Extract/Expand
Being resolved in https://github.com/Mbed-TLS/mbedtls/issues/5784
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/docs/architecture/psa-migration/psa-limitations.md b/docs/architecture/psa-migration/psa-limitations.md
index d72f795..c3e62b8 100644
--- a/docs/architecture/psa-migration/psa-limitations.md
+++ b/docs/architecture/psa-migration/psa-limitations.md
@@ -323,47 +323,7 @@
in the meantime. Such an extension seems inconvenient and not motivated by
strong security arguments, so it's unclear whether it would be accepted.
-HKDF: Expand not exposed on its own (TLS 1.3)
----------------------------------------------
-
-The HKDF function uses an Extract-then-Expand approach, that is:
-
- HKDF(x, ...) = HKDF-Expand(HKDF-Extract(x, ...), ...)
-
-Only the full HKDF function is safe in general, however there are cases when
-one can safely use the individual Extract and Expand; the TLS 1.3 key
-schedule does so. Specifically, looking at the [hierarchy of secrets][13hs]
-is seems that Expand and Extract are always chained, so that this hierarchy
-can be implemented using only the full HKDF. However, looking at the
-derivation of traffic keys (7.3) and the update mechanism (7.2) it appears
-that calls to HKDF-Expand are iterated without any intermediated call to
-HKDF-Extract : that is, the traffic keys are computed as
-
- HKDF-Expand(HKDF-Expand(HKDF-Extract(...)))
-
-(with possibly more than two Expands in a row with update).
-
-[13hs]: https://datatracker.ietf.org/doc/html/rfc8446#page-93
-
-For now (early 2022), we've worked around that by re-implementing HKDF
-in `ssl_tls13_keys.c` based on the `psa_mac_` APIs (for HMAC).
-
-In the long term, it is desirable to extend the PSA API. See
-https://github.com/ARM-software/psa-crypto-api/issues/539
-
Limitations relevant for G2 (isolation of long-term secrets)
============================================================
-HKDF: Expand not exposed on its own (TLS 1.3)
----------------------------------------------
-
-See the section with the same name in the G1 part above for background.
-
-The work-around mentioned there works well enough just for acceleration, but
-is not sufficient for key isolation or generally proper key management (it
-requires marking keys are usable for HMAC while they should only be used for
-key derivation).
-
-The obvious long-term solution is to make HKDF-Expand available as a new KDF
-(in addition to the full HKDF) in PSA (with appropriate warnings in the
-documentation).
+Currently none.
