pgpcrc.c

来自「著名的加密软件的应用于电子邮件中」· C语言 代码 · 共 77 行

/*
* pgpCRC.c - CRC computation routines (wow, how thrilling!)
*
* Copyright (C) 1993-1997 Pretty Good Privacy, Inc. All rights reserved.
*
* Written by Colin Plumb
*
* $Id: pgpCRC.c,v 1.1.2.1 1997/06/07 09:50:54 mhw Exp $
*/

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#include "pgpCRC.h"
#include "pgpUsuals.h"

/* PRZ's magic 24-bit polynomial - (x+1) * (irreducible of degree 23) */
#define POLY24 0x1864cfbu

/*
* Build the CRC-24 table.
*
* A byte-at-a-time CRC table is simply a table of the CRCs
* of the individual bytes.
*
* From basic CRC theory, crc(c^y) = crc(x) ^ crc(y).
* (^ is XOR, which is addition to CRCs.)
*
* Through this code, h holds the CRC of i. (i is a power of 2.)
* The CRCs of the values from i to 2*i-1 are the CRCs of the
* values from 0 to i-1, XORed with h.
*
* Then when you get to 2*i, shift h up one bit, XOR in the polynomail
* if necessary to get the right high 8 bits, and continue for 2*i
* through 4*i-1.
*
* This drastically reduces the amount of bit-twiddling necessary
* to generate a CRC table.
*/
static void
crcInit(word32 table[256])
	{
			unsigned i, j;
			word32 h;	/* CRC of i, where i is a power of 2 */

	table[0] = 0;
	table[1] = h = POLY24;

	for (i = 2; i < 256; i *= 2) {
	if ((h <<= 1) & 0x1000000)		/* h <<= 1 (mod poly) */
		h ^= POLY24;
	for (j = 0; j < i; j++)
	table[i+j] = table[j] ^ h;
}
}


/*
* Update a CRC to reflect the addition of some more bytes.
* It doesn't matter how the bytes are broken up; the CRC is the same.
* I.e. crcUpdate(0, "abcd", 3) == crcUpdate(crcUpdate(0, "ab", 2), "cd", );
*/
word32
crcUpdate(word32 crc, byte const *buf, unsigned len)
	{
			static word32 table[256];		/* Initialized to 0 */

			if (!table[1])
				 crcInit(table);

			while (len--)
				 crc = crc<<8 ^ table[*buf++ ^ (byte)(crc>>16)];

			return crc & 0xffffff;
}