Break repeating-key XOR

challenge_6.c

//
//  challenge_6.c
//  Matasano Crypto Challenge

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

char* HEX_LOOKUP = "0123456789abcdef";
int MIN_KEYSIZE = 2;
int MAX_KEYSIZE = 40;
int LINE_LENGTH = 60;

void formatAsHex(char* out, char* str) {
    for (int i = 0; i < strlen(str); ++i) {
        out[i*2] =   HEX_LOOKUP[(str[i] & 0xf0) >> 4];
        out[i*2+1] = HEX_LOOKUP[(str[i] & 0xf)];
    }
}

void repeatedXOR(char* encrypted, char* text, char* key) {
    for (int i = 0; i < strlen(text); i++) {
        encrypted[i] = text[i] ^ key[i % strlen(key)];
    }
}

void blockToSingleByte(char* byteBlocks[], char* str, unsigned int keysize) {
    for (int i = 0; i < strlen(str); i++) {
        byteBlocks[i % keysize][i / keysize] = str[i];
    }
}

int keyedEditDistance(char* enc, unsigned int nBytes) {
    if (strlen(enc) < 2 * nBytes) {
        perror("[-] Not enough characters in given encrypted string");
        return -1;
    }
    
    int distance = 0;
    for (int i = 0; i < nBytes; ++i) {
        int xor = enc[i] ^ enc[nBytes + i];
        do {
            if (xor & 0x1) distance++;
        } while((xor = xor >> 1));
    }
    
    return distance;
}

unsigned int findKeysize(char* enc) {
    unsigned int keysize = 0;
    unsigned int smallestDistance = (unsigned int) -1;
    unsigned int distance = 0;
    for (int i = MIN_KEYSIZE; i < MAX_KEYSIZE; i++) {
        if ((distance = keyedEditDistance(enc, i)) < smallestDistance) {
            smallestDistance = distance;
            keysize = i;
        }
    }
    return keysize;
}

void base64Decode(char* out, char* str) {
    
}

void readFile(char* out, char* filename) {
    char line[LINE_LENGTH];
    
    FILE* f = fopen(filename, "r");
    
    if (f == NULL) {
        perror("[-] Could not open specified file");
        exit(1);
    }
    
    while(fgets(line, 62, f) != NULL)
    {
        // realloc?
    }
}

int main() {
    // readFile()
    // base64Decode()
    // keysize = findKeysize()
    // char* byteBlocks[keysize];
    // for i in [0 : keysize - 1]:
    //      byteBlocks[i] = malloc(strlen(str) / keysize)
    // blockToSingleByte(byteBlocks, str, keysize)
    // for each block:
    //      find key
}