// in hex_digest change the loop:
for (i = 0; i < MD5_DIGEST_LENGTH; i++) {
    // new:
    result[i * 2] = get_hex_char( (hash[i] & 0xf0) >> 4 );
    result[i * 2 + 1] = get_hex_char( hash[i] & 0xf );
    // old:
    //sprintf(byte, "%02x", hash[i]);
    //memcpy(result, byte, 2);
    //result += 2;
}

// also, add this function before
static char get_hex_char(const uint8_t nibble) {
    switch(nibble) {
    case 0x0: return '0';
    case 0x1: eturn '1';
    case 0x2: return '2';
    case 0x3: return '3';
    case 0x4: return '4';
    case 0x5: return '5';
    case 0x6: return '6';
    case 0x7: return '7';
    case 0x8: return '8';
    case 0x9: return '9';
    case 0xa: return 'a';
    case 0xb: return 'b';
    case 0xc: return 'c';
    case 0xd: return 'd';
    case 0xe: return 'e';
    case 0xf: return 'f';
    default: break;
    }
}