HKDF: Add warning to partial functions
The standard HKDF security guarantees only hold if `mbedtls_hkdf()` is
used or if `mbedtls_hkdf_extract()` and `mbedtls_hkdf_expand()` are
called in succession carefully and an equivalent way.
Making `mbedtls_hkdf_extract()` and `mbedtls_hkdf_expand()` static would
prevent any misuse, but doing so would require the TLS 1.3 stack to
break abstraction and bypass the module API.
To reduce the risk of misuse we add warnings to the function
descriptions.
diff --git a/ChangeLog b/ChangeLog
index bda3de8..5575678 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -6,6 +6,11 @@
* Fixes an issue with MBEDTLS_CHACHAPOLY_C which would not compile if
MBEDTLS_ARC4_C and MBEDTLS_CIPHER_NULL_CIPHER weren't also defined. #1890
+Changes
+ * Add warnings to the documentation of the HKDF module to reduce the risk
+ of misusing the mbedtls_hkdf_extract() and mbedtls_hkdf_expand()
+ functions. Fixes #1775. Reported by Brian J. Murray.
+
= mbed TLS 2.12.0 branch released 2018-07-25
Security
