[core] add comment to ck_memeq_const_time()
add comment to ck_memeq_const_time() with some implementation details
@@ -270,6 +270,11 @@ ck_memeq_const_time (const void *a, const size_t alen, const void *b, const size
/* rounds to next multiple of 64 to avoid potentially leaking exact
* string lengths when subject to high precision timing attacks
*/
+ /* Note: implementation detail
+ * each string is expected to have a valid char one byte after len,
+ * i.e. a[alen] and b[blen], and which must match if the strings match.
+ * (In most use cases, this char is end of string '\0').
+ */
/* Note: some libs provide similar funcs but might not obscure length, e.g.
* OpenSSL:
* int CRYPTO_memcmp(const void * in_a, const void * in_b, size_t len)
