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Source file src/crypto/rc4/rc4.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	// Package rc4 implements RC4 encryption, as defined in Bruce Schneier's
     6	// Applied Cryptography.
     7	package rc4
     8	
     9	// BUG(agl): RC4 is in common use but has design weaknesses that make
    10	// it a poor choice for new protocols.
    11	
    12	import "strconv"
    13	
    14	// A Cipher is an instance of RC4 using a particular key.
    15	type Cipher struct {
    16		s    [256]uint32
    17		i, j uint8
    18	}
    19	
    20	type KeySizeError int
    21	
    22	func (k KeySizeError) Error() string {
    23		return "crypto/rc4: invalid key size " + strconv.Itoa(int(k))
    24	}
    25	
    26	// NewCipher creates and returns a new Cipher. The key argument should be the
    27	// RC4 key, at least 1 byte and at most 256 bytes.
    28	func NewCipher(key []byte) (*Cipher, error) {
    29		k := len(key)
    30		if k < 1 || k > 256 {
    31			return nil, KeySizeError(k)
    32		}
    33		var c Cipher
    34		for i := 0; i < 256; i++ {
    35			c.s[i] = uint32(i)
    36		}
    37		var j uint8 = 0
    38		for i := 0; i < 256; i++ {
    39			j += uint8(c.s[i]) + key[i%k]
    40			c.s[i], c.s[j] = c.s[j], c.s[i]
    41		}
    42		return &c, nil
    43	}
    44	
    45	// Reset zeros the key data so that it will no longer appear in the
    46	// process's memory.
    47	func (c *Cipher) Reset() {
    48		for i := range c.s {
    49			c.s[i] = 0
    50		}
    51		c.i, c.j = 0, 0
    52	}
    53	
    54	// xorKeyStreamGeneric sets dst to the result of XORing src with the
    55	// key stream. Dst and src may be the same slice but otherwise should
    56	// not overlap.
    57	//
    58	// This is the pure Go version. rc4_{amd64,386,arm}* contain assembly
    59	// implementations. This is here for tests and to prevent bitrot.
    60	func (c *Cipher) xorKeyStreamGeneric(dst, src []byte) {
    61		i, j := c.i, c.j
    62		for k, v := range src {
    63			i += 1
    64			j += uint8(c.s[i])
    65			c.s[i], c.s[j] = c.s[j], c.s[i]
    66			dst[k] = v ^ uint8(c.s[uint8(c.s[i]+c.s[j])])
    67		}
    68		c.i, c.j = i, j
    69	}
    70	

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