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	import "crypto/internal/subtle"
    10	
    11	type cfb struct {
    12		b       Block
    13		next    []byte
    14		out     []byte
    15		outUsed int
    16	
    17		decrypt bool
    18	}
    19	
    20	func (x *cfb) XORKeyStream(dst, src []byte) {
    21		if len(dst) < len(src) {
    22			panic("crypto/cipher: output smaller than input")
    23		}
    24		if subtle.InexactOverlap(dst[:len(src)], src) {
    25			panic("crypto/cipher: invalid buffer overlap")
    26		}
    27		for len(src) > 0 {
    28			if x.outUsed == len(x.out) {
    29				x.b.Encrypt(x.out, x.next)
    30				x.outUsed = 0
    31			}
    32	
    33			if x.decrypt {
    34				// We can precompute a larger segment of the
    35				// keystream on decryption. This will allow
    36				// larger batches for xor, and we should be
    37				// able to match CTR/OFB performance.
    38				copy(x.next[x.outUsed:], src)
    39			}
    40			n := xorBytes(dst, src, x.out[x.outUsed:])
    41			if !x.decrypt {
    42				copy(x.next[x.outUsed:], dst)
    43			}
    44			dst = dst[n:]
    45			src = src[n:]
    46			x.outUsed += n
    47		}
    48	}
    49	
    50	// NewCFBEncrypter returns a Stream which encrypts with cipher feedback mode,
    51	// using the given Block. The iv must be the same length as the Block's block
    52	// size.
    53	func NewCFBEncrypter(block Block, iv []byte) Stream {
    54		return newCFB(block, iv, false)
    55	}
    56	
    57	// NewCFBDecrypter returns a Stream which decrypts with cipher feedback mode,
    58	// using the given Block. The iv must be the same length as the Block's block
    59	// size.
    60	func NewCFBDecrypter(block Block, iv []byte) Stream {
    61		return newCFB(block, iv, true)
    62	}
    63	
    64	func newCFB(block Block, iv []byte, decrypt bool) Stream {
    65		blockSize := block.BlockSize()
    66		if len(iv) != blockSize {
    67			// stack trace will indicate whether it was de or encryption
    68			panic("cipher.newCFB: IV length must equal block size")
    69		}
    70		x := &cfb{
    71			b:       block,
    72			out:     make([]byte, blockSize),
    73			next:    make([]byte, blockSize),
    74			outUsed: blockSize,
    75			decrypt: decrypt,
    76		}
    77		copy(x.next, iv)
    78	
    79		return x
    80	}
    81	

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