Source file src/pkg/crypto/cipher/ctr.go
1 // Copyright 2009 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 // Counter (CTR) mode. 6 7 // CTR converts a block cipher into a stream cipher by 8 // repeatedly encrypting an incrementing counter and 9 // xoring the resulting stream of data with the input. 10 11 // See NIST SP 800-38A, pp 13-15 12 13 package cipher 14 15 type ctr struct { 16 b Block 17 ctr []byte 18 out []byte 19 outUsed int 20 } 21 22 // NewCTR returns a Stream which encrypts/decrypts using the given Block in 23 // counter mode. The length of iv must be the same as the Block's block size. 24 func NewCTR(block Block, iv []byte) Stream { 25 if len(iv) != block.BlockSize() { 26 panic("cipher.NewCTR: iv length must equal block size") 27 } 28 29 return &ctr{ 30 b: block, 31 ctr: dup(iv), 32 out: make([]byte, len(iv)), 33 outUsed: len(iv), 34 } 35 } 36 37 func (x *ctr) XORKeyStream(dst, src []byte) { 38 for i := 0; i < len(src); i++ { 39 if x.outUsed == len(x.ctr) { 40 x.b.Encrypt(x.out, x.ctr) 41 x.outUsed = 0 42 43 // Increment counter 44 for i := len(x.ctr) - 1; i >= 0; i-- { 45 x.ctr[i]++ 46 if x.ctr[i] != 0 { 47 break 48 } 49 } 50 } 51 52 dst[i] = src[i] ^ x.out[x.outUsed] 53 x.outUsed++ 54 } 55 }