github.com/varialus/godfly@v0.0.0-20130904042352-1934f9f095ab/src/pkg/crypto/cipher/ofb.go (about)

     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  	out     []byte
    12  	outUsed int
    13  }
    14  
    15  // NewOFB returns a Stream that encrypts or decrypts using the block cipher b
    16  // in output feedback mode. The initialization vector iv's length must be equal
    17  // to b's block size.
    18  func NewOFB(b Block, iv []byte) Stream {
    19  	blockSize := b.BlockSize()
    20  	if len(iv) != blockSize {
    21  		return nil
    22  	}
    23  
    24  	x := &ofb{
    25  		b:       b,
    26  		out:     make([]byte, blockSize),
    27  		outUsed: 0,
    28  	}
    29  	b.Encrypt(x.out, iv)
    30  
    31  	return x
    32  }
    33  
    34  func (x *ofb) XORKeyStream(dst, src []byte) {
    35  	for i, s := range src {
    36  		if x.outUsed == len(x.out) {
    37  			x.b.Encrypt(x.out, x.out)
    38  			x.outUsed = 0
    39  		}
    40  
    41  		dst[i] = s ^ x.out[x.outUsed]
    42  		x.outUsed++
    43  	}
    44  }