diff --git a/include/openssl/des.h b/include/openssl/des.h
index af1c822..539b2c5 100644
--- a/include/openssl/des.h
+++ b/include/openssl/des.h
@@ -65,6 +65,12 @@
 
 
 // DES.
+//
+// This module is deprecated and retained for legacy reasons only. It is slow
+// and may leak key material with timing or cache side channels. Moreover,
+// single-keyed DES is broken and can be brute-forced in under a day.
+//
+// Use a modern cipher, such as AES-GCM or ChaCha20-Poly1305, instead.
 
 
 typedef struct DES_cblock_st {