diff --git a/sys/crypto/chacha20/chacha.c b/sys/crypto/chacha20/chacha.c
index ac4eef22211..c882078c0bb 100644
--- a/sys/crypto/chacha20/chacha.c
+++ b/sys/crypto/chacha20/chacha.c
@@ -120,8 +120,10 @@ chacha_encrypt_bytes(chacha_ctx *x,const u8 *m,u8 *c,u32 bytes)
 
   for (;;) {
     if (bytes < 64) {
+#ifndef KEYSTREAM_ONLY
       for (i = 0;i < bytes;++i) tmp[i] = m[i];
       m = tmp;
+#endif
       ctarget = c;
       c = tmp;
     }