P1079 [NOIP 2012 Senior] Vigenère Cipher

In the 16th century, the French diplomat Blaise de Vigenère designed a polyalphabetic cipher known as the Vigenère cipher. The Vigenère cipher’s encryption and decryption algorithms are simple and easy to use, yet relatively hard to break, and it was widely used by the Confederate Army during the American Civil War. In cryptography, the information to be encrypted is called plaintext, denoted by $M$; the encrypted information is called ciphertext, denoted by $C$; and the key is a parameter, the data input to the algorithm that converts plaintext to ciphertext or ciphertext to plaintext, denoted by $k$. In the Vigenère cipher, the key $k$ is a string of letters, $k = k_1, k_2, \dots, k_n$. When the plaintext is $M = m_1, m_2, \dots, m_n$, the resulting ciphertext is $C = c_1, c_2, \dots, c_n$, where $c_i = m_i \operatorname{\circledR} k_i$, and the rules of the operation $\circledR$ are shown in the table below. $$\begin{array}{c|cccccccccccccccccccccccccc} \circledR &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} \\\hline \tt A &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} \\ \tt B &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} \\ \tt C &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} \\ \tt D &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} \\ \tt E &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} \\ \tt F &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} \\ \tt G &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} \\ \tt H &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} \\ \tt I &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} \\ \tt J &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} \\ \tt K &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} \\ \tt L &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} \\ \tt M &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} \\ \tt N &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} \\ \tt O &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} \\ \tt P &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} \\ \tt Q &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} \\ \tt R &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} \\ \tt S &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} \\ \tt T &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} \\ \tt U &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} \\ \tt V &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} \\ \tt W &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} \\ \tt X &\tt X \kern{-5pt} &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} \\ \tt Y &\tt Y \kern{-5pt} &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} \\ \tt Z &\tt Z \kern{-5pt} &\tt A \kern{-5pt} &\tt B \kern{-5pt} &\tt C \kern{-5pt} &\tt D \kern{-5pt} &\tt E \kern{-5pt} &\tt F \kern{-5pt} &\tt G \kern{-5pt} &\tt H \kern{-5pt} &\tt I \kern{-5pt} &\tt J \kern{-5pt} &\tt K \kern{-5pt} &\tt L \kern{-5pt} &\tt M \kern{-5pt} &\tt N \kern{-5pt} &\tt O \kern{-5pt} &\tt P \kern{-5pt} &\tt Q \kern{-5pt} &\tt R \kern{-5pt} &\tt S \kern{-5pt} &\tt T \kern{-5pt} &\tt U \kern{-5pt} &\tt V \kern{-5pt} &\tt W \kern{-5pt} &\tt X \kern{-5pt} &\tt Y \kern{-5pt} \\ \end{array} $$ When performing Vigenère encryption, note the following: 1. The $\circledR$ operation ignores the case of the letters involved, and preserves the case of each letter as it appears in the plaintext $M$. 2. When the length of the plaintext $M$ exceeds the length of the key $k$, the key $k$ is reused cyclically. For example, when the plaintext is $M=\texttt{Helloworld}$ and the key is $k=\texttt{abc}$, the ciphertext is $C=\texttt{Hfnlpyosnd}$. $$ \def\arraystretch{1.5} \begin{array}{|c|c|c|c|c|c|c|c|c|c|c|} \hline \textsf{明文} & \tt H & \tt e & \tt l & \tt l & \tt o & \tt w & \tt o & \tt r & \tt l & \tt d \\ \hline \textsf{密钥} & \tt a & \tt b & \tt c & \tt a & \tt b & \tt c & \tt a & \tt b & \tt c & \tt a \\ \hline \textsf{密文} & \tt H & \tt f & \tt n & \tt l & \tt p & \tt y & \tt o & \tt s & \tt n & \tt d \\ \hline \end{array} $$

There are 2 lines in total. - The first line contains a string representing the key $k$, with length not exceeding 100, containing only upper- and lowercase letters. - The second line contains a string representing the encrypted ciphertext, with length not exceeding 1000, containing only upper- and lowercase letters.

A single string representing the plaintext corresponding to the given key and ciphertext.

For 100% of the testdata, the length of the input key does not exceed 100, the length of the input ciphertext does not exceed 1000, and both contain only English letters. NOIP 2012 Senior Day 1 Problem 1. Translated by ChatGPT 5