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// 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]byte
        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] = uint8(i)
        }
        var j uint8 = 0
        for i := 0; i < 256; i++ {
                j += c.s[i] + key[i%k]
                c.s[i], c.s[j] = c.s[j], c.s[i]
        }
        return &c, nil
}

// XORKeyStream 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.
func (c *Cipher) XORKeyStream(dst, src []byte) {
        for i := range src {
                c.i += 1
                c.j += c.s[c.i]
                c.s[c.i], c.s[c.j] = c.s[c.j], c.s[c.i]
                dst[i] = src[i] ^ c.s[c.s[c.i]+c.s[c.j]]
        }
}

// 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
}