XOR Encryption code produces wrong results

So as said, there's nothing really wrong about your encryption code, you just can't print all of those bytes.

Here's a slight rework that

• moves the allocation outside the XORCipher function (so the caller is responsible for memory management)
• adds an utility function to print byte values as well as the ASCII text

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

static void XORCipher(const char *data, const char *key, char *dest, int dataLen, int keyLen) {
  for (int i = 0; i < dataLen; ++i) {
    dest[i] = data[i] ^ key[i % keyLen];
  }
}

static void print_hex(const char *header, const unsigned char *data, int length) {
    int i = 0;
    while(i < length) {
        char asc_line[17] = {0};
        printf("%s(%04x): ", header, i);
        for(int j = 0; j < 16; i++, j++) {
            if(i < length) {
                printf("%02x ", data[i]);
                asc_line[j] = (data[i] >= 32 && data[i] <= 127 ? data[i] : '.');
            } else {
                printf("   ");
            }
        }
        printf("| %-16s |\n", asc_line);
    }
    printf("%s\n----\n", data);
}

int main(void) {

  char *text = "The quick brown fox jumps over the lazy dog.";
  char *key = "secret";
  int dataLen = strlen((char *)text);
  int keyLen = strlen((char *)key);
  char *output = (char *)malloc(sizeof(char) * dataLen);
  print_hex("Original text", text, dataLen);
  XORCipher(text, key, output, dataLen, keyLen);
  print_hex("Ciphered text", output, dataLen);
  XORCipher(output, key, output, dataLen, keyLen);
  print_hex("Deciphered text", output, dataLen);
}

The output is

Original text(0000): 54 68 65 20 71 75 69 63 6b 20 62 72 6f 77 6e 20 | The quick brown  |
Original text(0010): 66 6f 78 20 6a 75 6d 70 73 20 6f 76 65 72 20 74 | fox jumps over t |
Original text(0020): 68 65 20 6c 61 7a 79 20 64 6f 67 2e             | he lazy dog.     |
The quick brown fox jumps over the lazy dog.
----
Ciphered text(0000): 27 0d 06 52 14 01 1a 06 08 52 07 06 1c 12 0d 52 | '..R.....R.....R |
Ciphered text(0010): 03 1b 0b 45 09 07 08 04 00 45 0c 04 00 06 53 11 | ...E.....E....S. |
Ciphered text(0020): 0b 17 45 18 12 1f 1a 52 01 1b 14 4b             | ..E....R...K     |
'
RR
RE  
----
Deciphered text(0000): 54 68 65 20 71 75 69 63 6b 20 62 72 6f 77 6e 20 | The quick brown  |
Deciphered text(0010): 66 6f 78 20 6a 75 6d 70 73 20 6f 76 65 72 20 74 | fox jumps over t |
Deciphered text(0020): 68 65 20 6c 61 7a 79 20 64 6f 67 2e             | he lazy dog.     |
The quick brown fox jumps over the lazy dog.
----

(Note I'm deciphering the output back into output – that's fine in this instance since we know the cipher function works byte by byte.)