hamming distance

docs/crypto.md

@@ -89,3 +89,11 @@ responsible for freeing the returned array
 ```c
 const unsigned char *repeating_key_xor_s(const char* s, const char* key);
 ```
+
+### hamming_distance
+
+Calculates the Hamming Distance (the number of differing bits) between two strings
+
+```c
+unsigned int hamming_distance(const char *a, const char *b);
+```

include/lfcrypto.h

@@ -13,4 +13,6 @@ const char *hex_to_str(const unsigned char *hex, size_t sz);
 const unsigned char* repeating_key_xor(const unsigned char* s, size_t s_sz, const unsigned char* key, size_t k_sz);
 const unsigned char *repeating_key_xor_s(const char* s, const char* key);
 
+unsigned int hamming_distance(const char *a, const char *b);
+
 #endif // LIBFLINT_CRYPTO_H

src/crypto.c

@@ -271,3 +271,25 @@ const unsigned char* repeating_key_xor(const unsigned char* s, size_t s_sz, cons
 const unsigned char *repeating_key_xor_s(const char* s, const char* key) {
     return repeating_key_xor((unsigned char*)s, strlen(s), (unsigned char*)key, strlen(key));
 }
+
+unsigned int hamming_distance(const char *a, const char *b) {
+    size_t sz = strlen(a);
+    if (sz != strlen(b)) {
+        return -1;
+    }
+
+    unsigned int hamming = 0;
+    for (size_t i = 0; i < sz; ++i) {
+        if (a[i] == b[i]) {
+            continue;
+        }
+        unsigned char c = a[i] ^ b[i];
+        unsigned int count = 0;
+        for (; c; count++) {
+            c &= c - 1;
+        }
+        hamming += count;
+    }
+
+    return hamming;
+}

tests/tests.c

@@ -374,6 +374,9 @@ void test_crypto() {
     free(enc);
     free(s);
 
+    unsigned int hamming = hamming_distance("this is a test", "wokka wokka!!!");
+    assert(hamming == 37);
+
     printf("Passes all crypto tests\n");
 }