Substitution is a method of replacing a letter in plaintext with other letters, numbers or symbols. Substitution cipher is to set up a substitution table first. When encrypting, the plaintext that needs to be encrypted is replaced by corresponding characters by looking up the table. After plaintext characters are replaced one by one, a meaningless string is generated, that is, ciphertext; When decrypting, the corresponding reverse substitution table is used to replace the ciphertext to be decrypted with the corresponding characters by looking up the table. The key of the replacement password is its replacement table.
According to how many substitution tables are used in encryption and decryption, substitution passwords can be divided into single table substitution password and multi table substitution password.
- Single table replacement cipher: a fixed replacement table is used in encryption and decryption of cipher algorithm. At this time, the same letter in the plaintext message is replaced by the same fixed letter in encryption, no matter where it appears.
- Multi table replacement cipher: the cipher algorithm uses multiple replacement tables for encryption and decryption. In this way, the same letter appearing in the plaintext message is not completely replaced by the same fixed letter in encryption, but is replaced by different letters according to the position order of its appearance.
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