diff --git a/library/cipher.c b/library/cipher.c
index cbbdcd510..2941f40f3 100644
--- a/library/cipher.c
+++ b/library/cipher.c
@@ -533,14 +533,19 @@ static void add_zeros_padding( unsigned char *output,
 static int get_zeros_padding( unsigned char *input, size_t input_len,
                               size_t *data_len )
 {
-    unsigned char *p = input + input_len - 1;
+    size_t i;
+    unsigned char done = 0, prev_done;
+
     if( NULL == input || NULL == data_len )
         return POLARSSL_ERR_CIPHER_BAD_INPUT_DATA;
 
-    while( *p == 0x00 && p > input )
-        --p;
-
-    *data_len = *p == 0x00 ? 0 : p - input + 1;
+    *data_len = 0;
+    for( i = input_len; i > 0; i-- )
+    {
+        prev_done = done;
+        done |= ( input[i-1] != 0 );
+        *data_len |= i * ( done != prev_done );
+    }
 
     return 0;
 }