Question about unicity distance

[oshfdk]

So, you need about 140 characters of the ciphertext to identify the key for an English plaintext using this encoding scheme.

I'm not familiar with that measure but this link You are not allowed to view links. Register or Login to view.  says:

Note that just because you have 30 characters of ciphertext for a substitution cipher, this does not mean you will be able to break it. The Unicity distance is a theoretical minimum. In general you may need 4 times more characters (100 or more) to reliably break substitution ciphers. The same problem exists with the other ciphers.

From my understanding this is not exactly correct. Maybe the author talks about statistical methods of breaking a cipher, but then the unicity distance seems to be of little use. Unicity distance looks like a solid theoretical average metric for the brute force exhaustive attack. With a known type of cipher (e.g., simple alphabet substitution) one should be able to reliably identify the key using just a few bits more than the unicity distance amount of ciphertext (albeit it might take prohibitively long time in practice). On the other hand, if one doesn't know the exact type of the cipher or if the attack is not an exhaustive brute force, which is the case in most real life situations, then the unicity distance simply doesn't apply, as far as I understand.

I also have a feeling that this value depends on the type of the cipher used. As we don't know the type of cipher used in VM, then we don't know how to count it. We don't even know the language which is probably a problem as well.

But Voynich Manuscript is really long. Common sense tells us that with this amount of text it should be easily broken. Yet it isn't.

Yes, I agree. I was just answering a specific question about possible ways to compute the unicity distance. The Voynich Manuscript is definitely long enough for all kinds of attacks, but its script is definitely weird and ambiguous enough to prevent computing even simple metrics unambiguously.