Detect single-character XOR

gistfile1.c

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

int most_freq_chars(char *decrypt_attempt)
{
	struct high_freq_characters {
		int E;
		int T;
		int A;
		int O;
		int I;
		int N;
		int space;
	};
	
	struct high_freq_characters hfq = {0,0,0,0,0,0};
	
	struct low_freq_characters {
		int V;
		int K;
		int J;
		int X;
		int Q;
		int Z;
	};
	
	struct low_freq_characters lfq = {0,0,0,0,0,0};
	
	int j = 0;
	for (j = 0; decrypt_attempt[j] != '\0'; j++)
	{
		switch(decrypt_attempt[j])
		{
			case 'E': hfq.E++; break;
			case 'e': hfq.E++; break;
			case 'T': hfq.T++; break;
			case 't': hfq.E++; break;
			case 'A': hfq.A++; break;
			case 'a': hfq.E++; break;
			case 'O': hfq.O++; break;
			case 'o': hfq.E++; break;
			case 'I': hfq.I++; break;
			case 'i': hfq.E++; break;
			case 'N': hfq.N++; break;
			case 'n': hfq.E++; break;
		}
		switch(decrypt_attempt[j])
		{
			case 'V': lfq.V--; break;
			case 'v': lfq.V--; break;
			case 'K': lfq.K--; break;
			case 'k': lfq.V--; break;
			case 'J': lfq.J--; break;
			case 'j': lfq.V--; break;
			case 'X': lfq.X--; break;
			case 'x': lfq.V--; break;
			case 'Q': lfq.Q--; break;
			case 'q': lfq.V--; break;
			case 'Z': lfq.Z--; break;
			case 'z': lfq.V--; break;
		}
	}
	int high_freq_total = hfq.E + hfq.T + hfq.A + hfq.O + hfq.I + hfq.N;
	int low_freq_total = lfq.V + lfq.K + lfq.J + lfq.X + lfq.Q + lfq.Z;
	int total = high_freq_total + low_freq_total;
	return total;
}

int max = 0;
char result[128];
char original_hex[128];

void hex_to_int(char *cmd_line_arg) 
{
	int i = 0;
	int j = 0; 
	int hex_result;
	char xor_string[strlen(cmd_line_arg) + 1];
	for (j = 0; j < 128; j++)
	{
		int k = 0;
		char xor_string[128];
		for (i = 0; cmd_line_arg[i] != '\0'; i+=2)
		{
			char temp[3];
			int hex = 0;
			temp[0] = cmd_line_arg[i];
			temp[1] = cmd_line_arg[i + 1];
			temp[2] = '\0';
			sscanf(temp, "%x", &hex);
			int xor_i = hex ^ j;
			char xor_c = xor_i + '\0';
			xor_string[k] = xor_c;
			k++;
		}
		xor_string[k] = '\0';
		if (most_freq_chars(xor_string) > max)
		{
			max = most_freq_chars(xor_string);
			strcpy(result, xor_string);
      strcpy(original_hex, cmd_line_arg);
		}
	}
}

int decrypt(char argv[])
{
	hex_to_int(argv);
	return 0;
}

int main() 
{
  char * buffer = 0;
  long length;
  FILE *f;
  f = fopen("challenge_four_data.txt", "r");
  
  if (f)
  {
    fseek (f, 0, SEEK_END);
    length = ftell (f);
    fseek (f, 0, SEEK_SET);
    buffer = malloc (length);
    if (buffer)
    {
      fread (buffer, 1, length, f);
    }
    fclose (f);
  }
  
  if (buffer)
  {
    char line[128];
    int i = 0;
    int j = 0;
    for (i = 0; buffer[i] != '\0'; i++)
    {
      line[j] = buffer[i];
      j++;
      if (buffer[i] == '\n')
      {
        j = 0;
        decrypt(line);
      }
    }
  }
  printf(" max: %d\n result: %s\n original hex: %s\n", max, result, original_hex);
  return 0;
}