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// Copyright 2009 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.
// Cipher feedback (CFB) mode.
// CFB provides confidentiality by feeding a fraction of
// the previous ciphertext in as the plaintext for the next
// block operation.
// See NIST SP 800-38A, pp 11-13
package block
import (
"io"
)
type cfbCipher struct {
c Cipher
blockSize int // our block size (s/8)
cipherSize int // underlying cipher block size
iv []byte
tmp []byte
}
func newCFB(c Cipher, s int, iv []byte) *cfbCipher {
if s == 0 || s%8 != 0 {
panic("crypto/block: invalid CFB mode")
}
b := c.BlockSize()
x := new(cfbCipher)
x.c = c
x.blockSize = s / 8
x.cipherSize = b
x.iv = dup(iv)
x.tmp = make([]byte, b)
return x
}
func (x *cfbCipher) BlockSize() int { return x.blockSize }
func (x *cfbCipher) Encrypt(dst, src []byte) {
// Encrypt old IV and xor prefix with src to make dst.
x.c.Encrypt(x.tmp, x.iv)
for i := 0; i < x.blockSize; i++ {
dst[i] = src[i] ^ x.tmp[i]
}
// Slide unused IV pieces down and insert dst at end.
for i := 0; i < x.cipherSize-x.blockSize; i++ {
x.iv[i] = x.iv[i+x.blockSize]
}
off := x.cipherSize - x.blockSize
for i := off; i < x.cipherSize; i++ {
x.iv[i] = dst[i-off]
}
}
func (x *cfbCipher) Decrypt(dst, src []byte) {
// Encrypt [sic] old IV and xor prefix with src to make dst.
x.c.Encrypt(x.tmp, x.iv)
for i := 0; i < x.blockSize; i++ {
dst[i] = src[i] ^ x.tmp[i]
}
// Slide unused IV pieces down and insert src at top.
for i := 0; i < x.cipherSize-x.blockSize; i++ {
x.iv[i] = x.iv[i+x.blockSize]
}
off := x.cipherSize - x.blockSize
for i := off; i < x.cipherSize; i++ {
// Reconstruct src = dst ^ x.tmp
// in case we overwrote src (src == dst).
x.iv[i] = dst[i-off] ^ x.tmp[i-off]
}
}
// NewCFBDecrypter returns a reader that reads data from r and decrypts it using c
// in s-bit cipher feedback (CFB) mode with the initialization vector iv.
// The returned Reader does not buffer or read ahead except
// as required by the cipher's block size.
// Modes for s not a multiple of 8 are unimplemented.
func NewCFBDecrypter(c Cipher, s int, iv []byte, r io.Reader) io.Reader {
return NewECBDecrypter(newCFB(c, s, iv), r)
}
// NewCFBEncrypter returns a writer that encrypts data using c
// in s-bit cipher feedback (CFB) mode with the initialization vector iv
// and writes the encrypted data to w.
// The returned Writer does no buffering except as required
// by the cipher's block size, so there is no need for a Flush method.
// Modes for s not a multiple of 8 are unimplemented.
func NewCFBEncrypter(c Cipher, s int, iv []byte, w io.Writer) io.Writer {
return NewECBEncrypter(newCFB(c, s, iv), w)
}
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