diff --git a/library/sha4.c b/library/sha4.c
index c214543..699cca7 100644
--- a/library/sha4.c
+++ b/library/sha4.c
@@ -417,6 +417,15 @@
 }
 
 /*
+ * SHA-512 HMAC context reset
+ */
+void sha4_hmac_reset( sha4_context *ctx )
+{
+    sha4_starts( ctx, ctx->is384 );
+    sha4_update( ctx, ctx->ipad, 128 );
+}
+
+/*
  * output = HMAC-SHA-512( hmac key, input buffer )
  */
 void sha4_hmac( const unsigned char *key, int keylen,