How to decrypt a short ciphertext that is said to be immune to attacks? 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.
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...