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I'm learning about Vigenere cipher,and I came across an exercise asking to encrypt a plain text without a key.

How would that work exactly?

DS_UNI
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  • That sounds strange. Could you quote the exact text of the exercise? – hmakholm left over Monica Dec 20 '16 at 09:18
  • the exercise exactly says:

    (Inform yourself about the Vigenère procedure. Encrypt the mentioed text with such a procedure without key and with the key "goedel".)

     – DS_UNI Dec 20 '16 at 09:20
  • That makes no sense unless your textbook uses a strangely non-standard sense of "Vigenère". – hmakholm left over Monica Dec 20 '16 at 10:33
  • The problem is I have no text book on this everything I'm learning is based on an online research – DS_UNI Dec 20 '16 at 10:35
  • If the text is $M=M_1 M_2 \dots M_n$ would it make sense to get the cypher like this: $$ C_i = (M_i + M_{i+1}) mod 26$$ s.t. $C_n = (M_n + M_1) mod 26 – DS_UNI Dec 20 '16 at 10:35
  • Okay, then that makes no sense unless whatever place you found the exercise uses a strangely non-standard sense of "Vigenère". – hmakholm left over Monica Dec 20 '16 at 10:36
  • The course does not mention this procedure at all, which is why the question starts with "Inform yourself about Vigenere procedure" – DS_UNI Dec 20 '16 at 10:43
  • Which is why I'm pretty sure that we were ment to learn the standard version of the procedure – DS_UNI Dec 20 '16 at 10:44
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    @Mathematiker the procedure you suggest is not invertible. How would a recipient "decrypt" it, without knowing the message (which would be pointless)? – Henno Brandsma Dec 20 '16 at 21:43
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    The standard procedure is just "add a cylic keystream to the plain text" for a suitable variant of add. It makes no sense at all without a key, even an autokey variant needs some key to get started. MAybe a 1-shifted autokey is meant with 0-key. This is what you suggested with $C_0 = M_0$ to make it invertible. – Henno Brandsma Dec 20 '16 at 21:45
  • aha ok, that makes sense – DS_UNI Dec 21 '16 at 10:03

2 Answers2

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Guess what! There is actually a way to encrypt Vigenére without a key, assuming that you are using the Vigenére Square. Simple use the english alphabet as the key (repeat as necessary).

Example:

MYSECRETMESSAGEWHICHYOUPROBABLYCANFINDANYWAY ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQR

Now just use the Vigenére Square to replace the message letters with whatever corresponds to row/column.

Hope this helps.

Friend
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There is no such thing as Vigenère encryption "without a key".

Whoever asked you to do such a thing is either deeply confused or is using an utterly nonstandard and idiosyncratic meaning of the word "Vigenère". In the latter case you have to ask them what they mean by it. Random people on the internet will not be able to read their mind remotely for you.

hmakholm left over Monica
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  • I have no idea why you are so angry and offended by this question, feel free not to give an answer or an opinion, I simply thought I was asking people who know more than I do – DS_UNI Dec 20 '16 at 10:53
  • x @Mathematiker: The only person who knows more than you do is the person who set you the nonsense exercise. If you are not going to listen to answers to your question, why do you even ask here? Just because you don't like the answer to the question (namely: the exercise you're asking about is nonsense) is not going to magically make this answer stop being true. – hmakholm left over Monica Dec 20 '16 at 10:57
  • I'm still trying to figure out why you are so angry, have I offended you or something? no one said anything about your answer – DS_UNI Dec 20 '16 at 11:08
  • x @Mathematiker: I'm still trying to figure out why you refuse to listen to the answer you get. You appear to consider my answer to be "not an answer", because you have convinced yourself -- on no reasonable evidence at all, as far as I can see -- that the exercise cannot possibly be the nonsense it is. – hmakholm left over Monica Dec 20 '16 at 11:17