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Source file src/crypto/cipher/cfb.go

     1	// Copyright 2010 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	// CFB (Cipher Feedback) Mode.
     6	
     7	package cipher
     8	
     9	type cfb struct {
    10		b       Block
    11		next    []byte
    12		out     []byte
    13		outUsed int
    14	
    15		decrypt bool
    16	}
    17	
    18	func (x *cfb) XORKeyStream(dst, src []byte) {
    19		for len(src) > 0 {
    20			if x.outUsed == len(x.out) {
    21				x.b.Encrypt(x.out, x.next)
    22				x.outUsed = 0
    23			}
    24	
    25			if x.decrypt {
    26				// We can precompute a larger segment of the
    27				// keystream on decryption. This will allow
    28				// larger batches for xor, and we should be
    29				// able to match CTR/OFB performance.
    30				copy(x.next[x.outUsed:], src)
    31			}
    32			n := xorBytes(dst, src, x.out[x.outUsed:])
    33			if !x.decrypt {
    34				copy(x.next[x.outUsed:], dst)
    35			}
    36			dst = dst[n:]
    37			src = src[n:]
    38			x.outUsed += n
    39		}
    40	}
    41	
    42	// NewCFBEncrypter returns a Stream which encrypts with cipher feedback mode,
    43	// using the given Block. The iv must be the same length as the Block's block
    44	// size.
    45	func NewCFBEncrypter(block Block, iv []byte) Stream {
    46		return newCFB(block, iv, false)
    47	}
    48	
    49	// NewCFBDecrypter returns a Stream which decrypts with cipher feedback mode,
    50	// using the given Block. The iv must be the same length as the Block's block
    51	// size.
    52	func NewCFBDecrypter(block Block, iv []byte) Stream {
    53		return newCFB(block, iv, true)
    54	}
    55	
    56	func newCFB(block Block, iv []byte, decrypt bool) Stream {
    57		blockSize := block.BlockSize()
    58		if len(iv) != blockSize {
    59			// stack trace will indicate whether it was de or encryption
    60			panic("cipher.newCFB: IV length must equal block size")
    61		}
    62		x := &cfb{
    63			b:       block,
    64			out:     make([]byte, blockSize),
    65			next:    make([]byte, blockSize),
    66			outUsed: blockSize,
    67			decrypt: decrypt,
    68		}
    69		copy(x.next, iv)
    70	
    71		return x
    72	}
    73	

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