Repeating-key XOR, Hamming distance, and a simple English-likelihood scorer. These are encoding and puzzle helpers, not cryptographic primitives.

Functions

repeating_key_xor

Performs a repeating-key XOR on an array of bytes. Returns NULL if the key size is 0. The caller frees the returned array.

unsigned char *repeating_key_xor(const unsigned char *s, size_t s_sz,
                                 const unsigned char *key, size_t k_sz);

/* Usage */
unsigned char data[] = {0x01, 0x02, 0x03};
unsigned char key[] = {0xFF};
unsigned char *out = repeating_key_xor(data, 3, key, 1);
/* out: {0xFE, 0xFD, 0xFC} */
free(out);

repeating_key_xor_s

Same operation on C strings.

unsigned char *repeating_key_xor_s(const char *s, const char *key);

hamming_distance

Number of differing bits between two byte arrays of length sz.

int hamming_distance(const unsigned char *a, const unsigned char *b, size_t sz);

hamming_distance_s

Hamming distance between two strings. Returns -1 if the lengths differ.

int hamming_distance_s(const char *a, const char *b);

simple_english_scoring

Scores a string on a simple ETAOIN SHRDLU scale. A cheap way to guess whether a buffer looks like English (used with XOR ciphertext). Returns -1 if input is NULL.

int simple_english_scoring(const char *s);