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// 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++
}
}
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