On: Show HN: Base32H, a human-friendly duotrigesimal n...

[yellowapple]

Anything you'd recommend to mitigate those side-channel attacks? I was going more for simplicity and portability for the reference implementations, but should there be a security-focused implementation (e.g. as part of some library like libsodium) it'd be useful to know the attack surfaces.

[dependenttypes → yellowapple]

Do not use the input as indices, do not branch depending on the input, and do not use division, mod, and even multiplication on the input. Check how libsodium does it. Here is a safe rot13 implementation in C if it is any help (note: it assumes a safe islower and isupper implementation).

    #include <ctype.h>
    
    #define IFTHENELSE(c, t, e) ((-(!!(c)) & (t)) | (-(!(c)) & (e)))
    
    unsigned char
    mod26 (unsigned char x)
    {
      x -= IFTHENELSE(x >= 26 * 4,
        26 * 4,
        0);
      x -= IFTHENELSE(x >= 26 * 2,
        26 * 2,
        0);
      x -= IFTHENELSE(x >= 26 * 2,
        26 * 2,
        0);
      x -= IFTHENELSE(x >= 26,
        26,
        0);
      return x;
    }
    
    unsigned char
    rot13 (unsigned char x)
    {
      return IFTHENELSE(islower(x), mod26(x - 'a' + 13) + 'a',
          IFTHENELSE(isupper(x), mod26(x - 'A' + 13) + 'A',
              x));
    }

implementing base32h should be much easier as you do not need to perform mod with a non-power of 2.