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:
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.