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POChecksum Algorithm Producing Unpredictable Results
 

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  1. POChecksum Algorithm Producing Unpredictable Results
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    <p>I'm working on a checksum algorithm, and I'm having some issues. The kicker is, when I hand craft a "fake" message, that is substantially smaller than the "real" data I'm receiving, I get a correct checksum. However, against the real data - the checksum does not work properly.</p> <p>Here's some information on the incoming data/environment:</p> <ul> <li>This is a groovy project (see code below)</li> <li>All bytes are to be treated as unsigned integers for the purpose of checksum calculation <ul> <li>You'll notice some finagling with shorts and longs in order to make that work.</li> </ul></li> <li>The size of the real data is 491 bytes.</li> <li>The size of my sample data (which appears to add correctly) is 26 bytes</li> <li>None of my hex-to-decimal conversions are producing a negative number, as best I can tell</li> <li>Some bytes in the file are not added to the checksum. I've verified that the switch for these is working properly, and when it is supposed to - so that's not the issue.</li> <li>My calculated checksum, and the checksum packaged with the real transmission always differ by the same amount.</li> <li>I have manually verified that the checksum packaged with the real data is correct.</li> </ul> <p>Here is the code:</p> <pre><code>// add bytes to checksum public void addToChecksum( byte[] bytes) { //if the checksum isn't enabled, don't add if(!checksumEnabled) { return; } long previouschecksum = this.checksum; for(int i = 0; i &lt; bytes.length; i++) { byte[] tmpBytes = new byte[2]; tmpBytes[0] = 0x00; tmpBytes[1] = bytes[i]; ByteBuffer tmpBuf = ByteBuffer.wrap(tmpBytes); long computedBytes = tmpBuf.getShort(); logger.info(getHex(bytes[i]) + " = " + computedBytes); this.checksum += computedBytes; } if(this.checksum &lt; previouschecksum) { logger.error("Checksum DECREASED: " + this.checksum); } //logger.info("Checksum: " + this.checksum); } </code></pre> <p>If anyone can find anything in this algorithm that could be causing drift from the expected result, I would greatly appreciate your help in tracking this down.</p>
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    1. USMatt Farmer
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      1. VTUpMod
    1. COQuick question... Why not use md5, sha or some other proven hashing algorithm? Ie: http://groovyconsole.appspot.com/script/256001
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      1. POChecksum Algorithm Producing Unpredictable Results
      1. UStim_yates
    2. COWithout diving into details of your implementation, why aren't you using any well-known, established and good enough checksum algorithms like CRC, MD5 or SHA (from trivial to close to ideal)?
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      1. POChecksum Algorithm Producing Unpredictable Results
      1. USTomasz Nurkiewicz
    3. COWell, first let me state that I don't get to determine which checksum algoritm is used. So, that's a moot point. I have to conform to protocol - no option about that. Second, it was done to conserve bandwidth. With the amount of data we're moving - using this method translates to real dollar signs.
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      1. USMatt Farmer
 

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