为了提高密码的破译的难度，人们有发明一种多表置换的密码，即一个明文字母可以表示为多个密文字母，多表密码加密算法结果将使得对单表置换用的简单频率分析方法失效，其中维吉尼亚密码就是一种典型的加密方法。维吉尼亚密码是使用一个词组（语句）作为密钥，词组中每一个字母都作为移位替换密码密钥确定一个替换表，维吉尼亚密码循环的使用每一个替换表完成明文字母到密文字母的变换，最后所得到的密文字母序列即为加密得到的密文(In order to improve the difficulty of deciphering the password, people have invented a multi-table replacement password, that is, a plaintext can be expressed as multiple ciphertext letters. The result of the multi-table cipher encryption algorithm will make the simple frequency analysis method Failure, of which the Virginia password is a typical method of encryption. The Virginia password uses a phrase (statement) as the key, and each letter in the phrase replaces the cryptographic key with a replacement key. The Virginia password loop uses each substitution table to complete the plaintext to Ciphertext transformation, the final ciphertext letter sequence is encrypted ciphertext)