diff --git a/lib/rand.h b/lib/rand.h
index c6fae35537..5deb041613 100644
--- a/lib/rand.h
+++ b/lib/rand.h
@@ -39,8 +39,11 @@
  */
 CURLcode Curl_rand(struct Curl_easy *data, unsigned char *rnd, size_t num);
 
-/* Same as above but outputs only random lowercase hex characters.
-   Does NOT terminate.*/
+/*
+ * Curl_rand_hex() fills the 'rnd' buffer with a given 'num' size with random
+ * hexadecimal digits PLUS a zero terminating byte. It must be an odd number
+ * size.
+ */
 CURLcode Curl_rand_hex(struct Curl_easy *data, unsigned char *rnd,
                        size_t num);