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I need to decrypt a ciphertext which is short, and I have the first word of the plaintext. I thought of doing a probable-word attack, but I already know the position of the known word.

In addition, subtracting word from cipher gives a random sequence, which makes me kind of hopeless. So consider this is the ciphertext:

URRLMBURLQHPIFGETNIAZXTCEAHVLXKOHAHGMTP

And the first word is : somebody

I also know that the cipher is supposedly immune to statistical attacks meaning that the ratio of cipher_length/key_length is not sufficient and also I cannot notice any bigrams/trigrams for that matter.

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  • $\begingroup$ 1. The key need not to be a meaningful word. 2. Just extract the part of the key and slide it to see that there are meaningful word parts there. Rest is completing the word to identify the real key. $\endgroup$
    – kelalaka
    May 6 '20 at 9:37
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So Vigenere is similar to caesar. So you start decrypting it like you do with caesar:

S - +2 --> U
O - +3 --> R
M - +5 --> R
E - +7 --> L
B - +11 -> M
O - +13 -> B
D - +17 -> U
Y - +19 -> R

So you may have noticed, that the text is decrypt via using the next bigger prime. Just subtract the primes from the decrypted text and you get the original message.

I did not decrypt the whole message, but i have a strong guess "Somebodyoncetoldmetheworldisgonnarollme" is the result...

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  • $\begingroup$ I noticed the increasing sequence with terms +1 +2 +4 but they seemed random. Should have thought of primes! Thanks a lot! $\endgroup$
    – Jeff
    May 6 '20 at 10:17
  • $\begingroup$ This is not Vigenère, rather a stream cipher uses only the primes. $\endgroup$
    – kelalaka
    May 6 '20 at 11:52

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