libgo/go/crypto/cipher/cfb.go

   1 // Copyright 2010 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 // CFB (Cipher Feedback) Mode.
   6
   7 package cipher
   8
   9 type cfb struct {
  10         b       Block
  11         next    []byte
  12         out     []byte
  13         outUsed int
  14
  15         decrypt bool
  16 }
  17
  18 func (x *cfb) XORKeyStream(dst, src []byte) {
  19         for len(src) > 0 {
  20                 if x.outUsed == len(x.out) {
  21                         x.b.Encrypt(x.out, x.next)
  22                         x.outUsed = 0
  23                 }
  24
  25                 if x.decrypt {
  26                         // We can precompute a larger segment of the
  27                         // keystream on decryption. This will allow
  28                         // larger batches for xor, and we should be
  29                         // able to match CTR/OFB performance.
  30                         copy(x.next[x.outUsed:], src)
  31                 }
  32                 n := xorBytes(dst, src, x.out[x.outUsed:])
  33                 if !x.decrypt {
  34                         copy(x.next[x.outUsed:], dst)
  35                 }
  36                 dst = dst[n:]
  37                 src = src[n:]
  38                 x.outUsed += n
  39         }
  40 }
  41
  42 // NewCFBEncrypter returns a Stream which encrypts with cipher feedback mode,
  43 // using the given Block. The iv must be the same length as the Block's block
  44 // size.
  45 func NewCFBEncrypter(block Block, iv []byte) Stream {
  46         return newCFB(block, iv, false)
  47 }
  48
  49 // NewCFBDecrypter returns a Stream which decrypts with cipher feedback mode,
  50 // using the given Block. The iv must be the same length as the Block's block
  51 // size.
  52 func NewCFBDecrypter(block Block, iv []byte) Stream {
  53         return newCFB(block, iv, true)
  54 }
  55
  56 func newCFB(block Block, iv []byte, decrypt bool) Stream {
  57         blockSize := block.BlockSize()
  58         if len(iv) != blockSize {
  59                 // stack trace will indicate whether it was de or encryption
  60                 panic("cipher.newCFB: IV length must equal block size")
  61         }
  62         x := &cfb{
  63                 b:       block,
  64                 out:     make([]byte, blockSize),
  65                 next:    make([]byte, blockSize),
  66                 outUsed: blockSize,
  67                 decrypt: decrypt,
  68         }
  69         copy(x.next, iv)
  70
  71         return x
  72 }