ofb.go

Documentation: crypto/cipher

		 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  import "crypto/internal/subtle"
		10  
		11  type ofb struct {
		12  	b			 Block
		13  	cipher	[]byte
		14  	out		 []byte
		15  	outUsed int
		16  }
		17  
		18  // NewOFB returns a Stream that encrypts or decrypts using the block cipher b
		19  // in output feedback mode. The initialization vector iv's length must be equal
		20  // to b's block size.
		21  func NewOFB(b Block, iv []byte) Stream {
		22  	blockSize := b.BlockSize()
		23  	if len(iv) != blockSize {
		24  		panic("cipher.NewOFB: IV length must equal block size")
		25  	}
		26  	bufSize := streamBufferSize
		27  	if bufSize < blockSize {
		28  		bufSize = blockSize
		29  	}
		30  	x := &ofb{
		31  		b:			 b,
		32  		cipher:	make([]byte, blockSize),
		33  		out:		 make([]byte, 0, bufSize),
		34  		outUsed: 0,
		35  	}
		36  
		37  	copy(x.cipher, iv)
		38  	return x
		39  }
		40  
		41  func (x *ofb) refill() {
		42  	bs := x.b.BlockSize()
		43  	remain := len(x.out) - x.outUsed
		44  	if remain > x.outUsed {
		45  		return
		46  	}
		47  	copy(x.out, x.out[x.outUsed:])
		48  	x.out = x.out[:cap(x.out)]
		49  	for remain < len(x.out)-bs {
		50  		x.b.Encrypt(x.cipher, x.cipher)
		51  		copy(x.out[remain:], x.cipher)
		52  		remain += bs
		53  	}
		54  	x.out = x.out[:remain]
		55  	x.outUsed = 0
		56  }
		57  
		58  func (x *ofb) XORKeyStream(dst, src []byte) {
		59  	if len(dst) < len(src) {
		60  		panic("crypto/cipher: output smaller than input")
		61  	}
		62  	if subtle.InexactOverlap(dst[:len(src)], src) {
		63  		panic("crypto/cipher: invalid buffer overlap")
		64  	}
		65  	for len(src) > 0 {
		66  		if x.outUsed >= len(x.out)-x.b.BlockSize() {
		67  			x.refill()
		68  		}
		69  		n := xorBytes(dst, src, x.out[x.outUsed:])
		70  		dst = dst[n:]
		71  		src = src[n:]
		72  		x.outUsed += n
		73  	}
		74  }
		75
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