I have confirmed that the plaintext is a phrase of words, not random characters.
1.30 We describe another stream cipher, which incorporates one of the ideas from the Enigma system used by Germany in World War II. Suppose that Πis a fixed permutation of Z26. The key is an element K ∈ Z26. For all integers i > 1, the keystream element zi ∈ Z26 is defined according to the rule zi = (K + i - 1) mod 26. Encryption and decryption are performed using the permutations Πand Τ1, respectively, as follows:
ex = (x + z) mod 26
and
dy = (-1y - z) mod 26,
where z ∈ Z26. Suppose that Πis the following permutation of Z26:
0
6
8
9
10
16
22
23
24
25
13
14
15
16
21
22
23
4
10
6
24
25
18
12
20
The following ciphertext has been encrypted using this stream cipher; use exhaustive key search to decrypt it: WRTCNRLDSAFARWKXFTXCZRNHNYPDTZUUKMPLUSOXNEUDOKLXRMCBKGRCCURR