libgo/go/crypto/cipher/ofb.go

   1 // Copyright 2011 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 // OFB (Output Feedback) Mode.
   6
   7 package cipher
   8
   9 type ofb struct {
  10         b       Block
  11         cipher  []byte
  12         out     []byte
  13         outUsed int
  14 }
  15
  16 // NewOFB returns a Stream that encrypts or decrypts using the block cipher b
  17 // in output feedback mode. The initialization vector iv's length must be equal
  18 // to b's block size.
  19 func NewOFB(b Block, iv []byte) Stream {
  20         blockSize := b.BlockSize()
  21         if len(iv) != blockSize {
  22                 return nil
  23         }
  24         bufSize := streamBufferSize
  25         if bufSize < blockSize {
  26                 bufSize = blockSize
  27         }
  28         x := &ofb{
  29                 b:       b,
  30                 cipher:  make([]byte, blockSize),
  31                 out:     make([]byte, 0, bufSize),
  32                 outUsed: 0,
  33         }
  34
  35         copy(x.cipher, iv)
  36         return x
  37 }
  38
  39 func (x *ofb) refill() {
  40         bs := x.b.BlockSize()
  41         remain := len(x.out) - x.outUsed
  42         if remain > x.outUsed {
  43                 return
  44         }
  45         copy(x.out, x.out[x.outUsed:])
  46         x.out = x.out[:cap(x.out)]
  47         for remain < len(x.out)-bs {
  48                 x.b.Encrypt(x.cipher, x.cipher)
  49                 copy(x.out[remain:], x.cipher)
  50                 remain += bs
  51         }
  52         x.out = x.out[:remain]
  53         x.outUsed = 0
  54 }
  55
  56 func (x *ofb) XORKeyStream(dst, src []byte) {
  57         for len(src) > 0 {
  58                 if x.outUsed >= len(x.out)-x.b.BlockSize() {
  59                         x.refill()
  60                 }
  61                 n := xorBytes(dst, src, x.out[x.outUsed:])
  62                 dst = dst[n:]
  63                 src = src[n:]
  64                 x.outUsed += n
  65         }
  66 }