+function B<evp_md> and the key B<key> which is B<key_len> bytes
+long. It is deprecated and only included for backward compatibility
+with OpenSSL 0.9.6b.
+
+HMAC_Init_ex() initializes or reuses a B<HMAC_CTX> structure to use
+the function B<evp_md> and key B<key>. Either can be NULL, in which
+case the existing one will be reused. HMAC_CTX_init() must have been
+called before the first use of an B<HMAC_CTX> in this
+function. B<N.B. HMAC_Init() had this undocumented behaviour in
+previous versions of OpenSSL - failure to switch to HMAC_Init_ex() in
+programs that expect it will cause them to stop working>.