nibble2c
output[2*i] = nibble2c((hash->digest[i] >> 4) & 0xf);
output[2*i+1] = nibble2c(hash->digest[i] & 0xf);
output[2 * i] = nibble2c ((digest[i] >> 4) & 0xf);
output[2 * i + 1] = nibble2c (digest[i] & 0xf);