How to decrypt a short ciphertext that is said to be immune to attacks?
So Vigenere is similar to caesar. So you start decrypting it like you do with caesar:

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