diff --git a/library/ecdsa.c b/library/ecdsa.c
index 8892317..70fd202 100644
--- a/library/ecdsa.c
+++ b/library/ecdsa.c
@@ -396,6 +396,9 @@
                               &ctx->Q, &r, &s ) ) != 0 )
         goto cleanup;
 
+    /* At this point we know that the buffer starts with a valid signature.
+     * Return 0 if the buffer just contains the signature, and a specific
+     * error code if the valid signature is followed by more data. */
     if( p != end )
         ret = MBEDTLS_ERR_ECP_SIG_LEN_MISMATCH;