Is there anything wrong with checking two hex values using == in C? [closed]

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Code:

#include<stdio.h>
#include<stdlib.h>

unsigned char 
s(const unsigned char x) {
    static const unsigned char aes_s_box[16][16] = {
        {0x63, 0x7C, 0x77, 0x7B, 0xF2, 0x6B, 0x6F, 0xC5, 0x30, 0x01, 0x67, 0x2B, 0xFE, 0xD7, 0xAB, 0x76},
        {0xCA, 0x82, 0xC9, 0x7D, 0xFA, 0x59, 0x47, 0xF0, 0xAD, 0xD4, 0xA2, 0xAF, 0x9C, 0xA4, 0x72, 0xC0},
        {0xB7, 0xFD, 0x93, 0x26, 0x36, 0x3F, 0xF7, 0xCC, 0x34, 0xA5, 0xE5, 0xF1, 0x71, 0xD8, 0x31, 0x15},
        {0x04, 0xC7, 0x23, 0xC3, 0x18, 0x96, 0x05, 0x9A, 0x07, 0x12, 0x80, 0xE2, 0xEB, 0x27, 0xB2, 0x75},
        {0x09, 0x83, 0x2C, 0x1A, 0x1B, 0x6E, 0x5A, 0xA0, 0x52, 0x3B, 0xD6, 0xB3, 0x29, 0xE3, 0x2F, 0x84},
        {0x53, 0xD1, 0x00, 0xED, 0x20, 0xFC, 0xB1, 0x5B, 0x6A, 0xCB, 0xBE, 0x39, 0x4A, 0x4C, 0x58, 0xCF},
        {0xD0, 0xEF, 0xAA, 0xFB, 0x43, 0x4D, 0x33, 0x85, 0x45, 0xF9, 0x02, 0x7F, 0x50, 0x3C, 0x9F, 0xA8},
        {0x51, 0xA3, 0x40, 0x8F, 0x92, 0x9D, 0x38, 0xF5, 0xBC, 0xB6, 0xDA, 0x21, 0x10, 0xFF, 0xF3, 0xD2},
        {0xCD, 0x0C, 0x13, 0xEC, 0x5F, 0x97, 0x44, 0x17, 0xC4, 0xA7, 0x7E, 0x3D, 0x64, 0x5D, 0x19, 0x73},
        {0x60, 0x81, 0x4F, 0xDC, 0x22, 0x2A, 0x90, 0x88, 0x46, 0xEE, 0xB8, 0x14, 0xDE, 0x5E, 0x0B, 0xDB},
        {0xE0, 0x32, 0x3A, 0x0A, 0x49, 0x06, 0x24, 0x5C, 0xC2, 0xD3, 0xAC, 0x62, 0x91, 0x95, 0xE4, 0x79},
        {0xE7, 0xC8, 0x37, 0x6D, 0x8D, 0xD5, 0x4E, 0xA9, 0x6C, 0x56, 0xF4, 0xEA, 0x65, 0x7A, 0xAE, 0x08},
        {0xBA, 0x78, 0x25, 0x2E, 0x1C, 0xA6, 0xB4, 0xC6, 0xE8, 0xDD, 0x74, 0x1F, 0x4B, 0xBD, 0x8B, 0x8A},
        {0x70, 0x3E, 0xB5, 0x66, 0x48, 0x03, 0xF6, 0x0E, 0x61, 0x35, 0x57, 0xB9, 0x86, 0xC1, 0x1D, 0x9E},
        {0xE1, 0xF8, 0x98, 0x11, 0x69, 0xD9, 0x8E, 0x94, 0x9B, 0x1E, 0x87, 0xE9, 0xCE, 0x55, 0x28, 0xDF},
        {0x8C, 0xA1, 0x89, 0x0D, 0xBF, 0xE6, 0x42, 0x68, 0x41, 0x99, 0x2D, 0x0F, 0xB0, 0x54, 0xBB, 0x16}
    };
    const unsigned int row = x & 0xF0;
    const unsigned int col = x & 0x0F;
    return aes_s_box[row][col];
}

void
main(void) {
    unsigned count = 0;
    for(unsigned char i = 0x0; i <= 0xFF; i++) {
        for(unsigned char j = 0x0; j <= 0xFF; j++) {
            const unsigned char s1 = s(i);
            const unsigned char s2 = s(j);
            const unsigned char xor = i ^ j; // only for debugging
            const unsigned char s3 = s(i ^ j);
            const unsigned char lhs_check = s1 ^ s2; // only for debugging
            if(s1 ^ s2 == s3) {
                printf("x1 = %x, x2 = %x\n", i, j);
                count++;
            }
        }
    }
    printf("count = %u\n", count);
}

The code is simply supposed to check all occurrences in an aes s-box where property of linearity is satisfied.

For those unfamiliar with the concept of S-box and linearity, s-boxes are used in AES (advanced encryption standard) to achieve confusion. They are inherently non-linear, meaning, sbox(hex1) \xor sbox(x2) != sbox(x1 \xor x2). This program checks this property, meaning, nothing should ideally be printed on the screen (except for the obvious diagonal case). However, the program returns true even for lhs = 31 and rhs = 124. I wanted to know if there's something inherently wrong with using the == operator when working with hexadecimal values.

(s1 ^ s2) == s3 Bitwise operations are lower precedence than ==. Turn up the warnings in your compiler and pay attention to them. Those for loops will never end. godbolt.org/z/hWKPhndh5 — Retired Ninja
– Retired Ninja, Commented Mar 16, 2024 at 23:56
const unsigned int row = x & 0xF0; should be const unsigned int row = x >> 4; — pmacfarlane
– pmacfarlane, Commented Mar 16, 2024 at 23:56
"I wanted to know if there's something inherently wrong with using the == operator when working with hexadecimal values" - No. What makes you think there may be? i <= 0xFF and j <= 0xFF will always be true (on all common machines) though so the loops are infinite. — Ted Lyngmo
– Ted Lyngmo, Commented Mar 16, 2024 at 23:56
They will run forever if the loop counter is an unsigned char with a range of 0-255. i can never contain a value that doesn't meet the loop condition. 255 is incremented and becomes 0 and the circle continues. — Retired Ninja
– Retired Ninja, Commented Mar 17, 2024 at 0:35

wohlstad · Accepted Answer · 2024-03-17 12:15:50Z

1

There is nothing inherently wrong with comparing hex values with ==.
Moreover hex values are actually just integers.

But there are 2 issues in your code:

Operator precedence in your if statement:
== has higher precedence than ^,
and therefore this expression: s1 ^ s2 == s3
Is actually interpreted as: s1 ^ (s2 == s3) which is not what you want.

To fix it you should use:
```
if ((s1 ^ s2) == s3) { /*...*/ }
```
Both your for loops are infinite loops:
unsigned char i and unsigned char j will always be in the range [0x0 .. 0xFF] because an unsigned char will wrap back to 0x0 (0) after a 0xFF (255) value will be incremented.

To fix it:
(1) Use a larger integer, e.g. unsigned int for i and j.
(2) When calling s with expressions based on i, j cast them to unsigned char (to avoid a "possible loss of data" or similar warning).

Demo - Godbolt (output truncated)

edited Mar 17, 2024 at 12:15

answered Mar 17, 2024 at 6:43

wohlstad

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