You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: cfb.go
    path of file: ./repos/gccrs/libgo/go/crypto/cipher
    
    the code of the file until where you have to start completion: // Copyright 2010 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.

// CFB (Cipher Feedback) Mode.

package cipher

import "crypto/internal/subtle"

type cfb struct {
	b       Block
	next    []byte
	out     []byte
	outUsed int

	decrypt bool
}

func (x *cfb) XORKeyStream(dst, src []byte) {
	if le
    
func newCFB(block Block, iv []byte, decrypt bool) Stream {
	blockSize := block.BlockSize()
	if len(iv) != blockSize {
		// stack trace will indicate whether it was de or encryption
		panic("cipher.newCFB: IV length must equal block size")
	}
	x := &cfb{
		b:       block,
		out:     make([]byte, blockSize),
		next:    make([]byte, blockSize),
		outUsed: blockSize,
		decrypt: decrypt,
	}
	copy(x.next, iv)

	return x
}