Reversing using C

A simple way to demonstrate common reversing tricks using C code:

1.XOR cipher execution
simple repeating-key XOR
extremely common in obfuscation

2.Known-plaintext attack
if you know part of the plaintext, you can recover the key

3.Key length recovery
detect repeating patterns in the keystream

#include <stdint.h>
#include <stddef.h>
#include <stdio.h>
#include <string.h>

static void
xmem(uint8_t *b, size_t n, const uint8_t *k, size_t klen)
{
    if (!b || !k || klen == 0)
        return;

    size_t i = 0, j = 0;

    for (; i < n; i++) {
        b[i] ^= k[j++];
        if (j == klen)
            j = 0;
    }
}

/* derive raw keystream */
static size_t
xks(uint8_t *out, size_t cap,
    const uint8_t *c, size_t clen,
    const uint8_t *p, size_t plen)
{
    if (!out || !c || !p)
        return 0;

    size_t n = clen < plen ? clen : plen;
    if (n > cap)
        n = cap;

    for (size_t i = 0; i < n; i++)
        out[i] = c[i] ^ p[i];

    return n;
}

/* naive period scan */
static size_t
xper(const uint8_t *k, size_t len)
{
    if (!k || !len)
        return 0;

    for (size_t p = 1; p < len; p++) {
        size_t i = p;
        for (; i < len; i++) {
            if (k[i] != k[i % p])
                break;
        }
        if (i == len)
            return p;
    }

    return len;
}

static void
dh(const uint8_t *b, size_t n)
{
    for (size_t i = 0; i < n; i++)
        printf("%02x ", b[i]);
    putchar('\n');
}

int
main(void)
{
    uint8_t c[] = {0x5a, 0x3b, 0x4c, 0x1d};
    uint8_t k[] = {0x42, 0x31};

    /* decrypt in place */
    xmem(c, sizeof c, k, sizeof k);
    dh(c, sizeof c);

    uint8_t known[] = "Hello";
    uint8_t ks[128];

    size_t n = xks(ks, sizeof ks,
                   c, sizeof c,
                   known, strlen((char *)known));

    dh(ks, n);

    size_t p = xper(ks, n);
    printf("p=%zu\n", p);

    return 0;
}