#828Deciphering Messages

The plaintext messages are written in the standard 26–letter English alphabet, A–Z. The encryption is done with the help of a cipher key. This cipher key is composed of an ordered set L of English alphabet letters, which is called the alphabetical key, and an integer N in the interval [1..25], which is called the numerical key. Each word of k characters is encrypted into a word of length between k and k ∗ 3 characters. The encryption rules are as follows:

• Rule 1: Each word is encrypted separately; the space between words is preserved. Each word is encrypted from the leftmost to the rightmost letters.
• Rule 2: A plaintext message letter p that does not belong to L is enciphered as a ciphertext letter c = ciph(p) where ciph is a function defined below.
• Rule 3: A plaintext message letter p that belongs to L is enciphered as a string of three letters: the m-th letter of L, followed by a ciphertext letter c = ciph(p), followed by the (m + 1)-th letter of L. The value of m is incremented by one each time this rule is applied. Arithmetic is performed as if L were circular.

For each plaintext message the initial value for m is one.

The function ciph translates each letter to the letter N letters after it in the alphabet and arithmetic is performed as if the alphabet were circular (for example, if N is 2, then ciph(A) = C, ciph(B) = D, …, ciph(X) = Z; ciph(Y ) = A; ciph(Z) = B).

NOTE: The keys used in the processes of ciphering and deciphering are such that the ciphered text can always be uniquely deciphered (that is, the keys do not contain images of function ciph for none of its elements).

1

RSAEIO
2
5
RTSSKAEAGE
GRSCAV
RGSSCAV
RUSIQO
RUSSGAACEV JEGIITOOGR

RICE
error in encryption
EAT
error in encryption
SEAT HERE