src/crypto/cipher/cfb.go

// 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/alias"
	"crypto/subtle"
)

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

	decrypt bool
}

func (x *cfb) XORKeyStream(dst, src []byte) {
	if len(dst) < len(src) {
		panic("crypto/cipher: output smaller than input")
	}
	if alias.InexactOverlap(dst[:len(src)], src) {
		panic("crypto/cipher: invalid buffer overlap")
	}
	for len(src) > 0 {
		if x.outUsed == len(x.out) {
			x.b.Encrypt(x.out, x.next)
			x.outUsed = 0
		}

		if x.decrypt {
			// We can precompute a larger segment of the
			// keystream on decryption. This will allow
			// larger batches for xor, and we should be
			// able to match CTR/OFB performance.
			copy(x.next[x.outUsed:], src)
		}
		n := subtle.XORBytes(dst, src, x.out[x.outUsed:])
		if !x.decrypt {
			copy(x.next[x.outUsed:], dst)
		}
		dst = dst[n:]
		src = src[n:]
		x.outUsed += n
	}
}

// NewCFBEncrypter returns a [Stream] which encrypts with cipher feedback mode,
// using the given [Block]. The iv must be the same length as the [Block]'s block
// size.
func NewCFBEncrypter(block Block, iv []byte) Stream {
	return newCFB(block, iv, false)
}

// NewCFBDecrypter returns a [Stream] which decrypts with cipher feedback mode,
// using the given [Block]. The iv must be the same length as the [Block]'s block
// size.
func NewCFBDecrypter(block Block, iv []byte) Stream {
	return newCFB(block, iv, true)
}

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
}