Caesar cipher decoder c++

CS150 Assignment 7 Cryptography

Date assigned: Monday, November 23, 2015 Date Due: Tuesday, December 8, 2015, 9:40 am (40 points) There is no late grace period for this last assignment!!!!! Cryptography is an exciting area of Computer Science concerned with hiding and protecting information. This is the branch of Computer Science that allows you to send your credit card securely to a web site or to decrypt or encrypt secret messages. For this project, you will be building a program that will take a plain text file and produce an encrypted file (or vice versa). The encryption scheme you need to implement is inspired by the Enigma Machine, a German encryption machine from WWII (http://en.wikipedia.org/wiki/Enigma_machine). The Enigma machine code was broken by Alan Turing and his team at Bletchley Park during World War II. Encryption Scheme: Simple substitution A simple substitution cipher (sometimes called a Caesar cipher) maps each letter of the alphabet to another letter of the alphabet as shown below:

To encrypt a letter, find the letter on the top and use the letter below it as the encrypted output. Using the above table, the encrypted output for C would be Z. To decrypt a letter, find the letter on the bottom and use the letter above it as the decrypted output. To make things more interesting, a secret key is used to specify how the input characters are mapped to output characters. The key, which is a single character, specifies which letter the output row starts on. The key in the above example is X. (The input row always starts with A). N-Way Substitution You will need to implement an N-Way substitution encryption scheme. This means you will have N substitution mappings. The character to be encrypted will be used as the input to the first mapping; the output of the first mapping will be the input to the second mapping. The output of the second mapping will be the input to the third mapping, etc. The output of the N-1 mapping is the input to the N mapping, and the output of the N mapping is the encrypted character. Each mapping has its own key, so you will have a total of N secret keys for this scheme. The value N, which is specified by the user, may be between 2 and 25. You must use a single, multidimensional array to represent all of these mappings.

Input A B C D E F G H I J K L M N O P Q R S T U V W X Y Z X Y Z A B C D E F G H I J K L M N O P Q R S T U V W Output

Encryption example (N = 2, keys of X and R): The initial input character of F gets mapped to C by the first mapping, and this becomes the input of the second mapping. Under the second mapping, this input character gets mapped to T which is the encrypted output character. Mapping 1: Input character: F (decrypted)

partial mapping array

Mapping 2: partial mapping array Output character: T (encrypted)

Decryption example (N = 2, keys of X and R): Decryption works in the opposite direction with the inputs and outputs reversed in each mapping. The input character of T gets mapped to C under the second mapping, and this becomes the input of the first mapping. Under the first mapping, this input character C gets mapped back to F which is the decrypted output character. Mapping 1: Output character: F (decrypted)

partial mapping array