Make get_one_and_zeros_padding() constant-time
diff --git a/library/cipher.c b/library/cipher.c
index 17368a1..cbbdcd5 100644
--- a/library/cipher.c
+++ b/library/cipher.c
@@ -456,20 +456,24 @@
static int get_one_and_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, bad;
if( NULL == input || NULL == data_len )
return POLARSSL_ERR_CIPHER_BAD_INPUT_DATA;
- while( *p == 0x00 && p > input )
- --p;
+ bad = 0xFF;
+ *data_len = 0;
+ for( i = input_len; i > 0; i-- )
+ {
+ prev_done = done;
+ done |= ( input[i-1] != 0 );
+ *data_len |= ( i - 1 ) * ( done != prev_done );
+ bad &= ( input[i-1] ^ 0x80 ) | ( done == prev_done );
+ }
- if( *p != 0x80 )
- return POLARSSL_ERR_CIPHER_INVALID_PADDING;
+ return POLARSSL_ERR_CIPHER_INVALID_PADDING * (bad != 0);
- *data_len = p - input;
-
- return 0;
}
#endif /* POLARSSL_CIPHER_PADDING_ONE_AND_ZEROS */