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<tr><td width="100%" colspan="2" height="100" align="center" valign="top"><br><p align="center">[<a href="index.htm">回到开始</a>][<a href="184.htm">上一层</a>][<a href="338.htm">下一篇</a>]
<hr><p align="left"><small>发信人: xh (强吻), 信区: Socket <br>

标  题: CRC <br>

发信站: 华南网木棉站 (Mon Nov  8 20:35:09 1999), 转信 <br>

  <br>

    记得有位师弟问过要CRC的东西，今天逛去清华，就转了这篇 <br>

东西过来。也讲两句吧,CRC是用来验证数据完整性的，如链路层在发 <br>

送数据帧的时候计算出CRC值，并保存在帧末尾。接受方收到帧时 <br>

再算一次，比较判断数据是否完整。一些废话是，在网络层，IP <br>

包用校验和判断。而一些安全要求比较高的报文，用报文摘要 <br>

判断报文有没有出错和被外人恶意改动。 <br>

  <br>

发信人: nomore (小苏苏), 信区: Networking <br>

标  题: Re: 有谁知道Ethernet 2的CRC怎么算？ <br>

发信站: BBS 水木清华站 (Tue Dec 30 12:20:38 1997) <br>

  <br>

As its name implies (Cyclic Redundancy Check) a crc adds redundancy <br>

whereas the topic of this group is to remove it. But since this <br>

question comes up often, here is some code (by Rob Warnock <rpw3@sgi.com>). <br>

  <br>

The following C code does CRC-32 in BigEndian/BigEndian byte/bit order. <br>

That is, the data is sent most significant byte first, and each of the bits <br>

within a byte is sent most significant bit first, as in FDDI. You will need <br>

to twiddle with it to do Ethernet CRC, i.e., BigEndian/LittleEndian byte/bit <br>



order. [Left as an exercise for the reader.] <br>

  <br>

The CRCs this code generates agree with the vendor-supplied Verilog models <br>

of several of the popular FDDI "MAC" chips. <br>

  <br>

u_long crc32_table[256]; <br>

/* Initialized first time "crc32()" is called. If you prefer, you can <br>

 * statically initialize it at compile time. [Another exercise.] <br>

 */ <br>

  <br>

u_long crc32(u_char *buf, int len) <br>

{ <br>

        u_char *p; <br>

        u_long  crc; <br>

  <br>

        if (!crc32_table[1])    /* if not already done, */ <br>

                init_crc32();   /* build table */ <br>

        crc = 0xffffffff;       /* preload shift register, per CRC-32 spec */ <br>

        for (p = buf; len > 0; ++p, --len) <br>

                crc = (crc << 8) ^ crc32_table[(crc >> 24) ^ *p]; <br>

        return ~crc;            /* transmit complement, per CRC-32 spec */ <br>

} <br>

} <br>

  <br>

/* <br>

 * Build auxiliary table for parallel byte-at-a-time CRC-32. <br>

 */ <br>

#define CRC32_POLY 0x04c11db7     /* AUTODIN II, Ethernet, & FDDI */ <br>

  <br>

init_crc32() <br>

{ <br>

        int i, j; <br>

        u_long c; <br>

  <br>

        for (i = 0; i < 256; ++i) { <br>

                for (c = i << 24, j = 8; j > 0; --j) <br>

                        c = c & 0x80000000 ? (c << 1) ^ CRC32_POLY : (c << 1); <br>

                crc32_table[i] = c; <br>

        } <br>

} <br>

  <br>

  <br>

  <br>

  <br>

-- <br>

-- <br>

  <br>

  <br>

-- <br>

  <br>

  <br>

  <br>

  <br>

                                                   V2 Studio Xh2000 <br>

  <br>

  <br>

  <br>

  <br>

※ 修改:．xh 于 Nov  8 20:37:04 修改本文．[FROM: 202.38.248.62] <br>

发信人: scz (小四), 信区: Socket <br>

标  题: Re: CRC <br>

发信站: 华南网木棉站 (Tue Nov  9 12:59:45 1999), 转信 <br>

【 在 xh (强吻) 的大作中提到: 】 <br>

:     记得有位师弟问过要CRC的东西，今天逛去清华，就转了这篇 <br>

可以去freesoft.cei.gov.cn下载tripwire的源代码包 <br>

里头有个目录下是八种加密校验算法的源代码， <br>

包括crc/crc32/md5/snefru等等，不用做太大修改就 <br>

可以单独提取出来使用。我在华中站系统安全版给出 <br>



过一个在VC下单独使用md5算法的例子，现在在精华区 <br>

里。crc的那些可以自己简单修改得到。 <br>

BTW，看到水木转载过NetGuy写的一篇关于Base64的文章， <br>

真是唏嘘不已。 <br>

</small><hr>
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