diff --git a/include/openssl/chacha.h b/include/openssl/chacha.h
index 61deb8e..684fc5b 100644
--- a/include/openssl/chacha.h
+++ b/include/openssl/chacha.h
@@ -17,10 +17,14 @@
 
 #include <openssl/base.h>
 
-#ifdef  __cplusplus
+#if defined(__cplusplus)
 extern "C" {
 #endif
 
+// ChaCha20.
+//
+// ChaCha20 is a stream cipher. See https://tools.ietf.org/html/rfc7539.
+
 
 // CRYPTO_chacha_20 encrypts |in_len| bytes from |in| with the given key and
 // nonce and writes the result to |out|. If |in| and |out| alias, they must be