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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

}