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+--- Thread: Question about unicity distance (/thread-4715.html)
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RE: Question about unicity distance - oshfdk - 23-05-2025
(21-05-2025, 11:54 PM)kckluge Wrote: You are not allowed to view links. Register or Login to view.Unicity distance "is the length of an original ciphertext needed to break the cipher by reducing the number of possible spurious keys to zero in a brute force attack" (You are not allowed to view links. Register or Login to view.). It's a potential way of trying to fomalize the "too many degrees of freedom" critique of proposed Voynich solutions as well as other possible prunings (i.e., if you can't get a coherent stretch of plaintext from a 28+ character long stretch of the ciphertext you haven't proposed a credible solution of the text as a simple monoalphabetic cipher).
I think a better metric to formalize "too many degrees of freedom" is not the unicity distance, but the expected number of spurious plaintext solutions given the cipher and the ciphertext length.
The advantage of this approach is that this can be computed explicitly for a specific key with lossy ciphers like Brumbaugh's. Given a key (mapping of Latin letters to digits) it should be possible to compute the expected number of spurious plaintexts for a given ciphertext length.
Also, one needs to consider how exactly was the ciphertext obtained. For example, if a solver offers a decoding of 140 continuous plaintext characters from the start of the first line of the first folio of the MS, this will work directly for the metrics above (the unicity and the expected number of spurious solutions). On the other hand, if these plaintext characters are obtained from a random place in the MS or, even worse, from multiple snippets of various parts of the MS, then the result should be adjusted by the amount of entropy added, but this can be computed combinatorially as well.