ocfb.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.

// OpenPGP CFB Mode. http://tools.ietf.org/html/rfc4880#section-13.9

package packet

import (
	"crypto/cipher"
)

type ocfbEncrypter struct {
	b       cipher.Block
	fre     []byte
	outUsed int
}

// An OCFBResyncOption determines if the "resynchronization step" of OCFB is
// performed.
type OCFBResyncOption bool

const (
	OCFBResync   OCFBResyncOption = true
	OCFBNoResync OCFBResyncOption = false
)

// NewOCFBEncrypter returns a cipher.Stream which encrypts data with OpenPGP's
// cipher feedback mode using the given cipher.Block, and an initial amount of
// ciphertext.  randData must be random bytes and be the same length as the
// cipher.Block's block size. Resync determines if the "resynchronization step"
// from RFC 4880, 13.9 step 7 is performed. Different parts of OpenPGP vary on
// this point.
func NewOCFBEncrypter(block cipher.Block, randData []byte, resync OCFBResyncOption) (cipher.Stream, []byte) {
	blockSize := block.BlockSize()
	if len(randData) != blockSize {
		return nil, nil
	}

	x := &ocfbEncrypter{
		b:       block,
		fre:     make([]byte, blockSize),
		outUsed: 0,
	}
	prefix := make([]byte, blockSize+2)

	block.Encrypt(x.fre, x.fre)
	for i := 0; i < blockSize; i++ {
		prefix[i] = randData[i] ^ x.fre[i]
	}

	block.Encrypt(x.fre, prefix[:blockSize])
	prefix[blockSize] = x.fre[0] ^ randData[blockSize-2]
	prefix[blockSize+1] = x.fre[1] ^ randData[blockSize-1]

	if resync {
		block.Encrypt(x.fre, prefix[2:])
	} else {
		x.fre[0] = prefix[blockSize]
		x.fre[1] = prefix[blockSize+1]
		x.outUsed = 2
	}
	return x, prefix
}

func (x *ocfbEncrypter) XORKeyStream(dst, src []byte) {
	for i := 0; i < len(src); i++ {
		if x.outUsed == len(x.fre) {
			x.b.Encrypt(x.fre, x.fre)
			x.outUsed = 0
		}

		x.fre[x.outUsed] ^= src[i]
		dst[i] = x.fre[x.outUsed]
		x.outUsed++
	}
}

type ocfbDecrypter struct {
	b       cipher.Block
	fre     []byte
	outUsed int
}

// NewOCFBDecrypter returns a cipher.Stream which decrypts data with OpenPGP's
// cipher feedback mode using the given cipher.Block. Prefix must be the first
// blockSize + 2 bytes of the ciphertext, where blockSize is the cipher.Block's
// block size. If an incorrect key is detected then nil is returned. On
// successful exit, blockSize+2 bytes of decrypted data are written into
// prefix. Resync determines if the "resynchronization step" from RFC 4880,
// 13.9 step 7 is performed. Different parts of OpenPGP vary on this point.
func NewOCFBDecrypter(block cipher.Block, prefix []byte, resync OCFBResyncOption) cipher.Stream {
	blockSize := block.BlockSize()
	if len(prefix) != blockSize+2 {
		return nil
	}

	x := &ocfbDecrypter{
		b:       block,
		fre:     make([]byte, blockSize),
		outUsed: 0,
	}
	prefixCopy := make([]byte, len(prefix))
	copy(prefixCopy, prefix)

	block.Encrypt(x.fre, x.fre)
	for i := 0; i < blockSize; i++ {
		prefixCopy[i] ^= x.fre[i]
	}

	block.Encrypt(x.fre, prefix[:blockSize])
	prefixCopy[blockSize] ^= x.fre[0]
	prefixCopy[blockSize+1] ^= x.fre[1]

	if prefixCopy[blockSize-2] != prefixCopy[blockSize] ||
		prefixCopy[blockSize-1] != prefixCopy[blockSize+1] {
		return nil
	}

	if resync {
		block.Encrypt(x.fre, prefix[2:])
	} else {
		x.fre[0] = prefix[blockSize]
		x.fre[1] = prefix[blockSize+1]
		x.outUsed = 2
	}
	copy(prefix, prefixCopy)
	return x
}

func (x *ocfbDecrypter) XORKeyStream(dst, src []byte) {
	for i := 0; i < len(src); i++ {
		if x.outUsed == len(x.fre) {
			x.b.Encrypt(x.fre, x.fre)
			x.outUsed = 0
		}

		c := src[i]
		dst[i] = x.fre[x.outUsed] ^ src[i]
		x.fre[x.outUsed] = c
		x.outUsed++
	}
}