diff --git a/docs/crypto.md b/docs/crypto.md
index 624d27f..5aee9fe 100644
--- a/docs/crypto.md
+++ b/docs/crypto.md
@@ -1,6 +1,6 @@
 # crypto
 
-Cryptographic library
+Cryptographic tools
 
 ## Functions
 
@@ -65,4 +65,21 @@ responsible for freeing the returned string.
 
 ```c
 const char *hex_to_str(const unsigned char *hex, size_t sz);
-```
\ No newline at end of file
+```
+
+###  repeating_key_xor
+
+Performs a repeating-key XOR encryption on an array of bytes. User is responsible for freeing the returned array 
+
+```c
+const unsigned char* repeating_key_xor(const unsigned char* s, size_t s_sz, const unsigned char* key, size_t k_sz);
+```
+
+### repeating_key_xor_s
+
+Performs a repeating-key XOR encryption on a string. Returns the encrypted string as an array of bytes. User is 
+responsible for freeing the returned array
+
+```c
+const unsigned char *repeating_key_xor_s(const char* s, const char* key);
+```
diff --git a/include/lfcrypto.h b/include/lfcrypto.h
index 7ba54ec..624e776 100644
--- a/include/lfcrypto.h
+++ b/include/lfcrypto.h
@@ -10,4 +10,7 @@ const char *hex_encode(const unsigned char *hex, size_t sz);
 unsigned char *hex_decode(const char *orig, size_t *sz);
 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);
+
 #endif // LIBFLINT_CRYPTO_H
diff --git a/src/crypto.c b/src/crypto.c
index 7c332bf..46200c3 100644
--- a/src/crypto.c
+++ b/src/crypto.c
@@ -236,7 +236,7 @@ const char *hex_encode(const unsigned char *hex, size_t sz) {
     char *pos = s;
 
     for (size_t i = 0; i < sz; ++i) {
-        snprintf(pos, 3, "%2X", hex[i]);
+        snprintf(pos, 3, "%02X", hex[i]);
         pos += 2;
     }
     s[ssz - 1] = '\0';
@@ -251,4 +251,17 @@ const char *hex_to_str(const unsigned char *hex, size_t sz) {
     }
     s[sz] = '\0';
     return s;
+}
+
+const unsigned char* repeating_key_xor(const unsigned char* s, size_t s_sz, const unsigned char* key, size_t k_sz) {
+    unsigned char* r = malloc(sizeof(unsigned char) * s_sz);
+    for (size_t i = 0, j = 0; i < s_sz; ++i) {
+        r[i] = s[i] ^ key[j];
+        j = (j + 1) % k_sz;
+    }
+    return r;
+}
+
+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));
 }
\ No newline at end of file
diff --git a/tests/tests.c b/tests/tests.c
index bae1c5a..37ef820 100644
--- a/tests/tests.c
+++ b/tests/tests.c
@@ -361,6 +361,12 @@ void test_crypto() {
     assert(strcmp(s, "Sup?") == 0);
     free(s);
 
+    s = repeating_key_xor_s("TEST", "HI");
+    const char *enc = hex_encode(s, 4);
+    assert(strcmp(enc, "1C0C1B1D") == 0);
+    free(enc);
+    free(s);
+
     printf("Passes all crypto tests\n");
 }