Ok, so I find it hard to ignore hard questions. I'm always prepared to try (and fail). In this case, DAMN ! My head really hurts now trying to understand the maths.
The task is really simple to understand. You and wiki describe it well.
I have the [plain text] the [cypher] and the <algorithm> but not the (key).
Anyhow, I don't expect to come up with an answer. I'll continue to think about for a while and re-read it to see if I can understand it. Maybe even write some code.
But don't hold your breath, I think I'm too stupid for this one
The most obvious method to me is brute force. But 64 bits is going to take a while. About 30,000 years at 20 megaflops.
It might be possible to search all known common polys, that should reduce it to a few million choices.
I'll have another look at the source coding later tonight.
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