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Source file src/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	const streamBufferSize = 512
    23	
    24	// ctrAble is an interface implemented by ciphers that have a specific optimized
    25	// implementation of CTR, like crypto/aes. NewCTR will check for this interface
    26	// and return the specific Stream if found.
    27	type ctrAble interface {
    28		NewCTR(iv []byte) Stream
    29	}
    30	
    31	// NewCTR returns a Stream which encrypts/decrypts using the given Block in
    32	// counter mode. The length of iv must be the same as the Block's block size.
    33	func NewCTR(block Block, iv []byte) Stream {
    34		if ctr, ok := block.(ctrAble); ok {
    35			return ctr.NewCTR(iv)
    36		}
    37		if len(iv) != block.BlockSize() {
    38			panic("cipher.NewCTR: IV length must equal block size")
    39		}
    40		bufSize := streamBufferSize
    41		if bufSize < block.BlockSize() {
    42			bufSize = block.BlockSize()
    43		}
    44		return &ctr{
    45			b:       block,
    46			ctr:     dup(iv),
    47			out:     make([]byte, 0, bufSize),
    48			outUsed: 0,
    49		}
    50	}
    51	
    52	func (x *ctr) refill() {
    53		remain := len(x.out) - x.outUsed
    54		copy(x.out, x.out[x.outUsed:])
    55		x.out = x.out[:cap(x.out)]
    56		bs := x.b.BlockSize()
    57		for remain <= len(x.out)-bs {
    58			x.b.Encrypt(x.out[remain:], x.ctr)
    59			remain += bs
    60	
    61			// Increment counter
    62			for i := len(x.ctr) - 1; i >= 0; i-- {
    63				x.ctr[i]++
    64				if x.ctr[i] != 0 {
    65					break
    66				}
    67			}
    68		}
    69		x.out = x.out[:remain]
    70		x.outUsed = 0
    71	}
    72	
    73	func (x *ctr) XORKeyStream(dst, src []byte) {
    74		for len(src) > 0 {
    75			if x.outUsed >= len(x.out)-x.b.BlockSize() {
    76				x.refill()
    77			}
    78			n := xorBytes(dst, src, x.out[x.outUsed:])
    79			dst = dst[n:]
    80			src = src[n:]
    81			x.outUsed += n
    82		}
    83	}
    84	

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