diff --git a/common/bitutil/bitutil.go b/common/bitutil/bitutil.go
index 59b9562329..c68733cb0b 100644
--- a/common/bitutil/bitutil.go
+++ b/common/bitutil/bitutil.go
@@ -18,6 +18,12 @@ const supportsUnaligned = runtime.GOARCH == "386" || runtime.GOARCH == "amd64" |
 
 // XORBytes xors the bytes in a and b. The destination is assumed to have enough
 // space. Returns the number of bytes xor'd.
+//
+// If dst does not have length at least n,
+// XORBytes panics without writing anything to dst.
+//
+// dst and x or y may overlap exactly or not at all,
+// otherwise XORBytes may panic.
 func XORBytes(dst, a, b []byte) int {
 	return subtle.XORBytes(dst, a, b)
 }