Don't allow SHAKE128/SHAKE256 with HMAC

[openssl.git] / doc / man3 / EVP_MAC.pod

diff --git a/doc/man3/EVP_MAC.pod b/doc/man3/EVP_MAC.pod
index 32bf5896f804e7d981170d5bd4af6790c584cb0e..a55d8db259c099afde36ba441bfa63c98c68f521 100644 (file)
--- a/doc/man3/EVP_MAC.pod
+++ b/doc/man3/EVP_MAC.pod
@@ -202,6 +202,9 @@ For MAC implementations that use an underlying computation algorithm,
 these controls set what the algorithm should be, and the engine that
 implements the algorithm if needed.
 
 these controls set what the algorithm should be, and the engine that
 implements the algorithm if needed.
 

+Note that not all algorithms may support all digests. HMAC does not support
+variable output length digests such as SHAKE128 or SHAKE256.
+

 B<EVP_MAC_CTRL_SET_ENGINE> takes one argument: C<ENGINE *>
 
 B<EVP_MAC_CTRL_SET_MD> takes one argument: C<EVP_MD *>
 B<EVP_MAC_CTRL_SET_ENGINE> takes one argument: C<ENGINE *>
 
 B<EVP_MAC_CTRL_SET_MD> takes one argument: C<EVP_MD *>