diff --git a/src/crypto/cipher/cipher.go b/src/crypto/cipher/cipher.go
index 67afdb1e057a5637c4cf35452e11a108c089de19..7d27fde61d8fac2a0fd63548d35d0b8bf02a7493 100644
--- a/src/crypto/cipher/cipher.go
+++ b/src/crypto/cipher/cipher.go
@@ -29,6 +29,9 @@ type Block interface {
 type Stream interface {
 	// XORKeyStream XORs each byte in the given slice with a byte from the
 	// cipher's key stream. Dst and src may point to the same memory.
+	// If len(dst) < len(src), XORKeyStream should panic. It is acceptable
+	// to pass a dst bigger than src, and in that case, XORKeyStream will
+	// only update dst[:len(src)] and will not touch the rest of dst.
 	XORKeyStream(dst, src []byte)
 }