rc4.go

Documentation: crypto/rc4

		 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  //
		 8  // RC4 is cryptographically broken and should not be used for secure
		 9  // applications.
		10  package rc4
		11  
		12  import (
		13  	"crypto/internal/subtle"
		14  	"strconv"
		15  )
		16  
		17  // A Cipher is an instance of RC4 using a particular key.
		18  type Cipher struct {
		19  	s		[256]uint32
		20  	i, j uint8
		21  }
		22  
		23  type KeySizeError int
		24  
		25  func (k KeySizeError) Error() string {
		26  	return "crypto/rc4: invalid key size " + strconv.Itoa(int(k))
		27  }
		28  
		29  // NewCipher creates and returns a new Cipher. The key argument should be the
		30  // RC4 key, at least 1 byte and at most 256 bytes.
		31  func NewCipher(key []byte) (*Cipher, error) {
		32  	k := len(key)
		33  	if k < 1 || k > 256 {
		34  		return nil, KeySizeError(k)
		35  	}
		36  	var c Cipher
		37  	for i := 0; i < 256; i++ {
		38  		c.s[i] = uint32(i)
		39  	}
		40  	var j uint8 = 0
		41  	for i := 0; i < 256; i++ {
		42  		j += uint8(c.s[i]) + key[i%k]
		43  		c.s[i], c.s[j] = c.s[j], c.s[i]
		44  	}
		45  	return &c, nil
		46  }
		47  
		48  // Reset zeros the key data and makes the Cipher unusable.
		49  //
		50  // Deprecated: Reset can't guarantee that the key will be entirely removed from
		51  // the process's memory.
		52  func (c *Cipher) Reset() {
		53  	for i := range c.s {
		54  		c.s[i] = 0
		55  	}
		56  	c.i, c.j = 0, 0
		57  }
		58  
		59  // XORKeyStream sets dst to the result of XORing src with the key stream.
		60  // Dst and src must overlap entirely or not at all.
		61  func (c *Cipher) XORKeyStream(dst, src []byte) {
		62  	if len(src) == 0 {
		63  		return
		64  	}
		65  	if subtle.InexactOverlap(dst[:len(src)], src) {
		66  		panic("crypto/rc4: invalid buffer overlap")
		67  	}
		68  	i, j := c.i, c.j
		69  	_ = dst[len(src)-1]
		70  	dst = dst[:len(src)] // eliminate bounds check from loop
		71  	for k, v := range src {
		72  		i += 1
		73  		x := c.s[i]
		74  		j += uint8(x)
		75  		y := c.s[j]
		76  		c.s[i], c.s[j] = y, x
		77  		dst[k] = v ^ uint8(c.s[uint8(x+y)])
		78  	}
		79  	c.i, c.j = i, j
		80  }
		81
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