public class PRZ24
extends java.lang.Object
implements java.util.zip.Checksum
Notes on making a good 24-bit CRC: [FH]
The primitive irreducible polynomial of degree 23 over GF(2), 040435651 (octal), comes from Appendix C of "Error Correcting Codes, 2nd edition" by Peterson and Weldon, page 490. This polynomial was chosen for its uniform density of ones and zeros, which has better error detection properties than polynomials with a minimal number of nonzero terms. Multiplying this primitive degree-23 polynomial by the polynomial x + 1 yields the additional property of detecting any odd number of bits in error, which means it adds parity. This approach was recommended by Neal Glover.
To multiply the polynomial 040435651 by (x + 1), shift it left 1 bit and bitwise add (xor) the unshifted version back in. Dropping the unused upper bit (bit 24) produces a CRC-24 generator bitmask of 041446373 octal, or 0x864CFB (hex).
You can detect spurious leading zeros or framing errors in the message by initializing the CRC accumulator to some agreed-upon nonzero "random- like" value, but this is a bit nonstandard.
General notes:
These CRC functions are derived from code in chapter 19 of the book "C Programmer's Guide to Serial Communications", by Joe Campbell. Generalized to any CRC width by Philip Zimmermann.
Adapted from PGP, by Philip Zimmermann, source file armor.c.
Copyright © 1997
Systemics Ltd on behalf of the
Cryptix Development Team.
All rights reserved.
$Revision: 1.1.1.1 $
| Constructor and Description |
|---|
PRZ24() |
| Modifier and Type | Method and Description |
|---|---|
long |
getValue()
Returns the value in the CRC accumulator.
|
void |
reset()
Resets to start processing a new CRC.
|
void |
update(byte[] buffer,
int offset,
int length)
Processes a buffer of bytes into the CRC accumulator.
|
void |
update(int n)
Accumulates 1 byte into the CRC accumulator.
|
public void reset()
reset in interface java.util.zip.Checksumpublic void update(byte[] buffer,
int offset,
int length)
update in interface java.util.zip.Checksumbuffer - buffer data.offset - offset into buffer.length - number of bytes to consider.public void update(int n)
update in interface java.util.zip.Checksumpublic long getValue()
getValue in interface java.util.zip.Checksum