isalpha (): raise ValueError ( "Text must be ASCII and contain no numbers." ) lowercase = string. Import string def caesar_cipher ( text, shift, decrypt = False ): if not text. Both the encryption and decryption processes can be described with the following expressions, where char_index is the index of the character in the alphabet: Below you can see the encryption process for the text REALPYTHON with a shift of 5:ĭecrypting the cipher is done by reversing the shift. This same shift value is applied to all characters in the string.įor example, if the shift were 5, then A would shift up five letters to become F, B would become G, and so on. Whichever letter is in that position is used as the encrypted character. The Caesar cipher works by taking a letter to be encrypted and shifting it a certain number of positions to the left or right in the alphabet. It’s a substitution cipher that uses letter substitution to encrypt a string of text. The first cipher that you’ll look at is the Caesar cipher, named after Julius Caesar, who used it to secretly communicate messages. In this section, you’ll look at two ciphers, the Caesar cipher and the Vigenère cipher. A cipher is a type of algorithm for performing encryption and decryption on an input, usually text. The Python modulo operator can be used to create ciphers. You need to take the result, 17, and use mod to get its equivalent value in a twelve-hour context: On a twelve-hour clock, you can’t simply add 9 to 8 because you would get 17. The modulo operator is used when you want to compare a number with the modulus and get the equivalent number constrained to the range of the modulus.įor example, say you want to determine what time it would be nine hours after 8:00 a.m. A twelve-hour clock can be classified as “modulo 12,” sometimes shortened to “mod 12.” When counting on a twelve-hour clock, you count up to the modulus 12 and then wrap back to 1. A twelve-hour clock has a fixed set of values, from 1 to 12. All arithmetic operations performed on this number line will wrap around when they reach a certain number called the modulus.Ī classic example of modulo in modular arithmetic is the twelve-hour clock. Modular arithmetic deals with integer arithmetic on a circular number line that has a fixed set of numbers. The term modulo comes from a branch of mathematics called modular arithmetic.
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