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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.
// 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
}