RC4 加解密问题

问题

• 需要注意RC4连续加解密的状态问题（下述实现中的key的x、y值）。例如CS两端数据交互的加解密场景。

官方实现

// https://github.com/openssl/openssl/blob/master/crypto/rc4/rc4_enc.c

/*
 * Copyright 1995-2020 The OpenSSL Project Authors. All Rights Reserved.
 *
 * Licensed under the Apache License 2.0 (the "License").  You may not use
 * this file except in compliance with the License.  You can obtain a copy
 * in the file LICENSE in the source distribution or at
 * https://www.openssl.org/source/license.html
 */

/*
 * RC4 low level APIs are deprecated for public use, but still ok for internal
 * use.
 */
#include "internal/deprecated.h"

#include <openssl/rc4.h>
#include "rc4_local.h"

/*-
 * RC4 as implemented from a posting from
 * Newsgroups: sci.crypt
 * Subject: RC4 Algorithm revealed.
 * Message-ID: <sternCvKL4B.Hyy@netcom.com>
 * Date: Wed, 14 Sep 1994 06:35:31 GMT
 */

void RC4(RC4_KEY *key, size_t len, const unsigned char *indata,
         unsigned char *outdata)
{
    register RC4_INT *d;
    register RC4_INT x, y, tx, ty;
    size_t i;

    x = key->x;
    y = key->y;
    d = key->data;

#define LOOP(in,out) \
                x=((x+1)&0xff); \
                tx=d[x]; \
                y=(tx+y)&0xff; \
                d[x]=ty=d[y]; \
                d[y]=tx; \
                (out) = d[(tx+ty)&0xff]^ (in);

    i = len >> 3;
    if (i) {
        for (;;) {
            LOOP(indata[0], outdata[0]);
            LOOP(indata[1], outdata[1]);
            LOOP(indata[2], outdata[2]);
            LOOP(indata[3], outdata[3]);
            LOOP(indata[4], outdata[4]);
            LOOP(indata[5], outdata[5]);
            LOOP(indata[6], outdata[6]);
            LOOP(indata[7], outdata[7]);
            indata += 8;
            outdata += 8;
            if (--i == 0)
                break;
        }
    }
    i = len & 0x07;
    if (i) {
        for (;;) {
            LOOP(indata[0], outdata[0]);
            if (--i == 0)
                break;
            LOOP(indata[1], outdata[1]);
            if (--i == 0)
                break;
            LOOP(indata[2], outdata[2]);
            if (--i == 0)
                break;
            LOOP(indata[3], outdata[3]);
            if (--i == 0)
                break;
            LOOP(indata[4], outdata[4]);
            if (--i == 0)
                break;
            LOOP(indata[5], outdata[5]);
            if (--i == 0)
                break;
            LOOP(indata[6], outdata[6]);
            if (--i == 0)
                break;
        }
    }
    key->x = x;
    key->y = y;
}