实现移位密码的加密，解密过程.移位密码，即向后移k位，k是密钥,是任意的。选用一个英文短语或单词作为密钥，去掉其中重复的字母得到一个无重复字母的字符串，然后将字母表中的其他字母依此写于此字母串之后。找出明文字母在原字母表中位置,则在加入密钥的字母表中该位置上的对应字母就是密文。-To realize the shift encryption and decryption process. Shift the password, that is, move backward k-bit, k is the key, is arbitrary. An English phrase or word used as a key, remove the duplicate letters which have no repeated letters of a string, and then the other letters in the alphabet so after writing this letter string. Expressly identify the alphabet letters in the original location, then add the key position on the alphabet letter that corresponds to ciphertext.