Re: [PATCH] hex2bin: make the function hex_to_bin constant-time

[Joe Perches <joe@perches.com>]

On Sun, 2022-04-24 at 16:54 -0400, Mikulas Patocka wrote:
> This patch changes the function hex_to_bin so that it contains no branches
> and no memory accesses.
[]
> +++ linux-2.6/lib/hexdump.c	2022-04-24 18:51:20.000000000 +0200
[]
> + * the next line is similar to the previous one, but we need to decode both
> + *	uppercase and lowercase letters, so we use (ch & 0xdf), which converts
> + *	lowercase to uppercase
>   */
>  int hex_to_bin(char ch)
>  {
> -	if ((ch >= '0') && (ch <= '9'))
> -		return ch - '0';
> -	ch = tolower(ch);
> -	if ((ch >= 'a') && (ch <= 'f'))
> -		return ch - 'a' + 10;
> -	return -1;
> +	return -1 +
> +		((ch - '0' + 1) & (((ch - '9' - 1) & ('0' - 1 - ch)) >> 8)) +
> +		(((ch & 0xdf) - 'A' + 11) & ((((ch & 0xdf) - 'F' - 1) & ('A' - 1 - (ch & 0xdf))) >> 8));

probably easier to read using a temporary for ch & 0xdf

	int CH = ch & 0xdf;

	return -1 +
	       ((ch - '0' +  1) & (((ch - '9' - 1) & ('0' - 1 - ch)) >> 8)) +
	       ((CH - 'A' + 11) & (((CH - 'F' - 1) & ('A' - 1 - CH)) >> 8));