Vigenère Cipher Encryption

The Vigenère cipher is one of the oldest and most widely known encryption methods. A similar scheme was first described in 1553 by Giovan Battista Bellaso and later improved in 1586 by Blaise de Vigenère.

The cipher produces one ciphertext letter for each plaintext letter by combining every plaintext letter with the key letter at the corresponding position. When the key is shorter than the plaintext, it is repeated as many times as needed. For a key of length 3 and a plaintext of length 7 the letters are combined as follows (Ki is the key letter, Pi is the plaintext letter, and Ci is the resulting ciphertext letter):

 K1 K2 K3 K1 K2 K3 K1
 P1 P2 P3 P4 P5 P6 P7
----------------------
 C1 C2 C3 C4 C5 C6 C7

Each key letter says how many positions the plaintext letter is shifted forward in the alphabet. Key letter A shifts by one position, B shifts by two, and so on. The alphabet is circular, so shifting past Z wraps back around to A. Note that key A combined with plaintext A yields B, which differs slightly from the classic Vigenère cipher. In other words, the ciphertext letter equals (plaintext + key + 1) modulo 26 (with A = 0). The Vigenère square in the Hint shows every combination of a plaintext letter and a key letter.

Write a program that encrypts messages with the Vigenère cipher for a given key.

The input consists of several test instances. Each instance is given on two lines: the first line is the encryption key and the second line is the plaintext. Both the key and the plaintext consist only of uppercase English letters (A–Z). The key length is between 1 and 1000, and the plaintext length is between 1 and 100000, inclusive.

The input ends with a line containing a single 0.

For each instance, output the ciphertext — the encrypted plaintext — on its own line.

The Vigenère square below maps a plaintext letter (column) together with a key letter (row) to the resulting ciphertext letter.