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-rw-r--r--doc/man3/HMAC.pod4
1 files changed, 3 insertions, 1 deletions
diff --git a/doc/man3/HMAC.pod b/doc/man3/HMAC.pod
index 65386a7baa31..30c0e6bac392 100644
--- a/doc/man3/HMAC.pod
+++ b/doc/man3/HMAC.pod
@@ -63,7 +63,9 @@ If B<md> is NULL, the digest is placed in a static array. The size of
the output is placed in B<md_len>, unless it is B<NULL>. Note: passing a NULL
value for B<md> to use the static array is not thread safe.
-B<evp_md> can be EVP_sha1(), EVP_ripemd160() etc.
+B<evp_md> is a message digest such as EVP_sha1(), EVP_ripemd160() etc. HMAC does
+not support variable output length digests such as EVP_shake128() and
+EVP_shake256().
HMAC_CTX_new() creates a new HMAC_CTX in heap memory.