1 files changed, 9 insertions, 0 deletions
diff --git a/ref/blake2s-ref.c b/ref/blake2s-ref.c
index 2bab23f..a10f7ef 100644
--- a/ref/blake2s-ref.c
+++ b/ref/blake2s-ref.c
@@ -53,6 +53,11 @@ static inline int blake2s_clear_lastnode( blake2s_state *S )
 }
 
 /* Some helper functions, not necessarily useful */
+static inline int blake2s_is_lastblock( const blake2s_state *S )
+{
+  return S->f[0] != 0;
+}
+
 static inline int blake2s_set_lastblock( blake2s_state *S )
 {
   if( S->last_node ) blake2s_set_lastnode( S );
@@ -306,6 +311,10 @@ int blake2s_final( blake2s_state *S, uint8_t *out, uint8_t outlen )
   if( out == NULL || outlen == 0 || outlen > BLAKE2S_OUTBYTES )
     return -1;
 
+  if( blake2s_is_lastblock( S ) )
+    return -1;
+
+
   if( S->buflen > BLAKE2S_BLOCKBYTES )
   {
     blake2s_increment_counter( S, BLAKE2S_BLOCKBYTES );