Source file src/pkg/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		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	}