blake2b: implement hash.XOF

Fixes golang/go#69518

Change-Id: Id9989ac9b28262df77017e97f985f67c1571c3ce
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/644255
Reviewed-by: Austin Clements <austin@google.com>
Auto-Submit: Austin Clements <austin@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

blake2b/blake2x.go

@@ -12,6 +12,8 @@ import (
 
 // XOF defines the interface to hash functions that
 // support arbitrary-length output.
+//
+// New callers should prefer the standard library [hash.XOF].
 type XOF interface {
 	// Write absorbs more data into the hash's state. It panics if called
 	// after Read.
@@ -47,6 +49,8 @@ const maxOutputLength = (1 << 32) * 64
 //
 // A non-nil key turns the hash into a MAC. The key must between
 // zero and 32 bytes long.
+//
+// The result can be safely interface-upgraded to [hash.XOF].
 func NewXOF(size uint32, key []byte) (XOF, error) {
 	if len(key) > Size {
 		return nil, errKeySize
@@ -93,6 +97,10 @@ func (x *xof) Clone() XOF {
 	return &clone
 }
 
+func (x *xof) BlockSize() int {
+	return x.d.BlockSize()
+}
+
 func (x *xof) Reset() {
 	x.cfg[0] = byte(Size)
 	binary.LittleEndian.PutUint32(x.cfg[4:], uint32(Size)) // leaf length

blake2b/go125.go

@@ -0,0 +1,11 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.25
+
+package blake2b
+
+import "hash"
+
+var _ hash.XOF = (*xof)(nil)