text: // Copyright 2009 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  // Package rc4 implements RC4 encryption, as defined in Bruce Schneier's
  // Applied Cryptography.
  package rc4
  
  // BUG(agl): RC4 is in common use but has design weaknesses that make
  // it a poor choice for new protocols.
  
  import "strconv"
  
  // A Cipher is an instance of RC4 using a particular key.
  type Cipher struct {
  	s    [256]uint32
  	i, j uint8
  }
  
  type KeySizeError int
  
  func (k KeySizeError) Error() string {
  	return "crypto/rc4: invalid key size " + strconv.Itoa(int(k))
  }
  
  // NewCipher creates and returns a new Cipher.  The key argument should be the
  // RC4 key, at least 1 byte and at most 256 bytes.
  func NewCipher(key []byte) (*Cipher, error) {
  	k := len(key)
  	if k < 1 || k > 256 {
  		return nil, KeySizeError(k)
  	}
  	var c Cipher
  	for i := 0; i < 256; i++ {
  		c.s[i] = uint32(i)
  	}
  	var j uint8 = 0
  	for i := 0; i < 256; i++ {
  		j += uint8(c.s[i]) + key[i%k]
  		c.s[i], c.s[j] = c.s[j], c.s[i]
  	}
  	return &c, nil
  }
  
  // Reset zeros the key data so that it will no longer appear in the
  // process's memory.
  func (c *Cipher) Reset() {
  	for i := range c.s {
  		c.s[i] = 0
  	}
  	c.i, c.j = 0, 0
  }
  
  // xorKeyStreamGeneric sets dst to the result of XORing src with the
  // key stream.  Dst and src may be the same slice but otherwise should
  // not overlap.
  //
  // This is the pure Go version. rc4_{amd64,386,arm}* contain assembly
  // implementations. This is here for tests and to prevent bitrot.
  func (c *Cipher) xorKeyStreamGeneric(dst, src []byte) {
  	i, j := c.i, c.j
  	for k, v := range src {
  		i += 1
  		j += uint8(c.s[i])
  		c.s[i], c.s[j] = c.s[j], c.s[i]
  		dst[k] = v ^ uint8(c.s[uint8(c.s[i]+c.s[j])])
  	}
  	c.i, c.j = i, j
  }
  