As far as I can tell (from reading the Wikipedia article about it) decrypting a message encrypted with the vigenère cipher is possible on paper.
But what would happen if the message is encrypted multiple times with a set of different keywords?
"Thwaites challenged Babbage to break his cipher encoded twice, with keys of different length. Babbage succeeded in decrypting a sample, which turned out to be the poem "The Vision of Sin", by Alfred Tennyson, encrypted according to the keyword "Emily", the first name of Tennyson's wife. Babbage never explained the method he used. Studies of Babbage's notes reveal that he had used the method later published by Kasiski, and suggest that he had been using the method as early as 1846." Link to the Kasiski examination
(I must admit, I don't quite understand how to apply the method mentioned above effectively).
An example message:
First time (keyword: "ASIMPLEPASSWORD") This is a secret message that was encrypted multiple times with a vigenere cipher ASIM PL E PASSWO RDASIMP LEPA SSW ORDASIMPL EPASSWOR DASIM PLEP A SSWORDAS IMPLEP Second time (keyword: "RANDOM") TZQEXDEHEUJAHDHSKISTELPTOSOSEFRQXFTOQJLLALZVWIEMELTXWANACSEHRWKUESIG RANDOMRANDOMRANDOMRANDOMRANDOMRANDOMRANDOMRANDOMRANDOMRANDOMRANDOMRA Third time (keyword: "UNKNOWNTOYOU") KZDHLPVHRXXMYDUVYUJTRODFFSBVSRIQKIHAHJYOOXQVJLSYELTXWANACSEHRWKUESIG UNKNOWNTOYOUUNKNOWNTOYOUUNKNOWNTOYOUUNKNOWNTOYOUUNKNOWNTOYOUUNKNOWNT Result EMNUZLIAFVLGSQEIMQWMFMRZZFLIGNVJYGVUBWIBCTDOXJGSYYDKKWATQQSBLJUHSOVZ
The reason I'm asking is simple research (and curiosity). I'm writing a story involving encrypted messages in a fantasy setting (so, no computers!). In-depth knowledge about the cipher (including methods and formulas known today) can be used, though.
I'ld like to see if this method would be impossible to crack by "classic means/hand", or if it has it's own (perhaps obvious to experts) weakness.
How would an expert in this field approach the problem?
At what point would that expert either realize the message was encoded multiple times or mistakingly believe it's not a vigenère cipher?
What additional hint or information would that expert need to decrypt the message?