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If you haven't seen my response here , it may be worth a read.

Interesting. What your guy seems to trying to do reminds of 3 months wasted work a few years ago.

The specification for a proprietary transmission protocol (over RS232) called for each block to carry a checksum. But the bright spark that specified it decided tha the checksum should be calculated such that it included its own bytes. Thus packet:

# payload checksum xx xx xx xx xx xx xx xx xx xx ... xx xx xx cc cc cc cc

The idea was that when verifying the checksum upon receipt, the entire packet (including checksum) could be passed to the checksum algorithm and the result compared against the unsigned integer in the last four bytes.

Problem: The very thing that makes cryptographic digests of plain text messages work -- the insane combinatorial explosion of work required to find the appropriate padding to make the fake message match the digest -- makes this almost impossible to calculate.

I suspect your guys problem suffers similarly.


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Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
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In reply to Re^3: Dynamically Updating Frequency Analysis by BrowserUk
in thread Dynamically Updating Frequency Analysis by Limbic~Region

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