Solve challenge 3

ex03/main.c

@@ -0,0 +1,63 @@
+#include "utils.h"
+#include <ctype.h>
+#include <math.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+double frequency_score(unsigned char *buf, size_t len) {
+  static const double english_freqs[26] = {
+      0.08167, 0.01492, 0.02782, 0.04253, 0.12702, 0.02228, 0.02015, // A-G
+      0.06094, 0.06966, 0.00153, 0.00772, 0.04025, 0.02406, 0.06749, // H-N
+      0.07507, 0.01929, 0.00095, 0.05987, 0.06327, 0.09056, 0.02758, // O-U
+      0.00978, 0.02360, 0.00150, 0.01974, 0.00074                    // V-Z
+  };
+
+  unsigned int counts[26] = {0};
+  for (size_t i = 0; i < len; ++i) {
+    unsigned char c = tolower(buf[i]) - 'a';
+    if (c < 26) {
+      counts[c]++;
+    }
+  }
+
+  double chi2 = 0;
+  for (size_t i = 0; i < 26; ++i) {
+    double expected = len * english_freqs[i];
+    chi2 += pow(counts[i] - expected, 2) / expected;
+  }
+  return chi2;
+}
+
+int main(int argc, char *argv[]) {
+  if (argc < 2) {
+    printf("Usage: %s <hex1>\n", argv[0]);
+    return EXIT_FAILURE;
+  }
+
+  unsigned char buf[512] = {0};
+  size_t len = hex_to_bytes(buf, argv[1]);
+
+  unsigned char cur[512];
+  double min_score = INFINITY;
+  unsigned char key;
+  unsigned char best[512];
+  // Test for keys from A to z
+  for (char c = 65; c < 127; ++c) {
+    for (size_t i = 0; i < len; ++i) {
+      cur[i] = buf[i] ^ c;
+    }
+    double score = frequency_score(cur, len);
+    if (score < min_score) {
+      min_score = score;
+      key = c;
+      memcpy(best, cur, len);
+    }
+  }
+  for (size_t i = 0; i < len; ++i) {
+    printf("%c", best[i]);
+  }
+  printf(" (key = %c, score = %.2f)\n", key, min_score);
+
+  return EXIT_SUCCESS;
+}