Break the ciphertext in blocks and attempt to decipher it

src/ex06/main.c

@@ -48,6 +48,11 @@ int main(int argc, char *argv[]) {
     return EXIT_FAILURE;
   }
 
+  if (fclose(fp)) {
+    printf("Error closing file %s\n", filename);
+    return EXIT_FAILURE;
+  }
+
   unsigned char buf[BUF_SIZE] = {0};
   size_t len = base64_to_bytes(buf, input);
   if (len == 0) {
@@ -67,10 +72,23 @@ int main(int argc, char *argv[]) {
 
   printf("keysize = %u, min edit distance = %f\n", keysize, min_edit_dist);
 
-  if (fclose(fp)) {
-    printf("Error closing file %s\n", filename);
-    return EXIT_FAILURE;
+  size_t blocks_count = 1 + len / keysize;
+  unsigned char blocks[keysize][blocks_count];
+  memset(blocks, 0, blocks_count * keysize);
+
+  for (size_t i = 0; i < blocks_count; ++i) {
+    for (size_t j = 0; j < keysize; ++j) {
+      blocks[j][i] = buf[i * keysize + j];
+    }
   }
 
+  char key[keysize];
+  for (size_t j = 0; j < keysize; ++j) {
+    printf("Guessing key character %2zu: ", j);
+    key[j] = best_single_char_xor_key(blocks_count, blocks[j]);
+    printf("%c\n", key[j]);
+  }
+  printf("key: %s\n", key);
+
   return EXIT_SUCCESS;
 }

src/utils.c

@@ -2,6 +2,7 @@
 #include <ctype.h>
 #include <math.h>
 #include <string.h>
+#include <stdio.h>
 
 #define min(a, b)                                                              \
   ({                                                                           \
@@ -39,7 +40,8 @@ size_t hex_to_bytes(unsigned char out[static 1], const char hex[static 1]) {
 static const char base64_table[65] =
     "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
 
-char *bytes_to_base64(char out[static 4], const unsigned char in[static 3], size_t len) {
+char *bytes_to_base64(char out[static 4], const unsigned char in[static 3],
+                      size_t len) {
   for (size_t i = 0; i < len - 2; i += 3) {
     size_t j = i / 3 * 4;
     out[j] = base64_table[in[i] >> 2];
@@ -133,3 +135,26 @@ unsigned int hamming(const char s1[static 1], const char s2[static 1]) {
   }
   return res;
 }
+
+char best_single_char_xor_key(size_t len, unsigned char buf[static len]) {
+  char cur[len];
+  double min_score = INFINITY;
+  char key = 'X';
+
+  for (char c = 32; c < 127; ++c) {
+    for (size_t i = 0; i < len; ++i) {
+      cur[i] = buf[i] ^ c;
+    }
+    double score = frequency_score(len, cur);
+    //printf("current character: %c, current score: %f\n", c, score);
+    if (score < min_score) {
+      min_score = score;
+      key = c;
+    }
+    if (isfinite(min_score)) {
+      printf("current best character: %c, current best score: %f\n", key, min_score);
+    }
+  }
+
+  return key;
+}

src/utils.h

@@ -9,5 +9,6 @@ char *bytes_to_base64(char out[static 4], const unsigned char in[static 3], size
 size_t base64_to_bytes(unsigned char out[static 3], const char in[static 4]);
 double frequency_score(size_t len, const char buf[static len]);
 unsigned int hamming(const char s1[static 1], const char s2[static 1]);
+char best_single_char_xor_key(size_t len, unsigned char buf[static len]);
 
 #endif /* UTILS_H */