enhance hex_decode

docs/crypto.md

@@ -2,6 +2,9 @@
 
 Cryptographic tools
 
+Any functions that return string representations of bytes in hexadecimal format will
+always be formatted with no leading `0x` and lower case letters
+
 ## Functions
 
 ### b64_encode
@@ -26,22 +29,25 @@ unsigned char *b64_decode(const char *s, size_t sz, size_t *decode_sz);
 ### hex_encode
 
 Encodes an array of bytes into a string in hex representation. For example, converts 
-`[0xDE, 0xAD, 0xBE, 0xEF]` into `"DEADBEEF"`. The returned string will always be in capital
-letters with no leading `0x`. User is responsible for the freeing of the returned string
+`[0xDE, 0xAD, 0xBE, 0xEF]` into `"deadbeef"`. User is responsible for freeing the returned string
 
 ```c
 const char *hex_encode(const unsigned char *hex, size_t sz);
 
 // Example
+unsigned char h[4] = {
+    0xde, 0xad, 0xbe, 0xef
+};
 const char *s = hex_encode(h, 4);
-assert(strcmp(s, "DEADBEEF") == 0);
+assert(strcmp(s, "deadbeef") == 0);
 free(s);
 ```
 
 ### hex_decode
 
 Decodes a string of characters representing hexadecimal bytes into an array of `unsigned char`.
-For example, converts `"DEADBEEF"` into `[0xDE, 0xAD, 0xBE, 0xEF]`. User is reponsible for the
+Characters can be in upper or lower case, and can start with a leading `0x` or not.
+For example, converts `"DEADBEEF"` into `[0xde, 0xad, 0xbe, 0xef]`. User is reponsible for the
 freeing of the returned byte array
 
 ```c

src/crypto.c

@@ -204,16 +204,22 @@ unsigned char *b64_decode(const char *s, size_t sz, size_t *decode_sz) {
 
 unsigned char *hex_decode(const char *orig, size_t *sz) {
     size_t buf_sz = strlen(orig) + 1;
+    char *sptr = orig;
+    if (strncmp(orig, "0x", 2) == 0) {
+        buf_sz -= 2;
+        sptr += 2;
+    }
+
     if (strlen(orig) % 2 != 0) {
         buf_sz += 1;
     }
 
     char *buf = malloc(sizeof(char) * buf_sz);
-    if (strlen(orig) % 2 != 0) {
-        strcpy(buf + 1, orig);
+    if (strlen(sptr) % 2 != 0) {
+        strcpy(buf+ 1, sptr);
         buf[0] = '0';
     } else {
-        strcpy(buf, orig);
+        strcpy(buf, sptr);
     }
     buf[buf_sz - 1] = '\0';
 

tests/tests.c

@@ -349,6 +349,13 @@ void test_crypto() {
     }
     free(s);
 
+    // leading 0x
+    s = hex_decode("0xf00f5", &s_sz);
+    for (size_t i = 0; i < 3; ++i) {
+        assert(s[i] == h2[i]);
+    }
+    free(s);
+
     s = hex_encode(h, 4);
     assert(strcmp(s, "deadbeef") == 0);
     free(s);