diff --git a/library/rsa.c b/library/rsa.c
index fba68dd..881805e 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -720,6 +720,10 @@
      */
     hlen = mbedtls_md_get_size( md_info );
 
+    // checking for integer underflow
+    if( 2 * hlen + 2 > ilen )
+        return( MBEDTLS_ERR_RSA_BAD_INPUT_DATA );
+
     mbedtls_md_init( &md_ctx );
     mbedtls_md_setup( &md_ctx, md_info, 0 );