CS162: Homework #2 - Cipher

From David Lu’s Winter 2017 CS162 course.

Cipher Program

The purpose of the second program is to continue refining our programming skills in C++ and practice using arrays of characters, the subscript operator, the cstring library, and functions with arguments. Our goal is to create programs where main() delegates to a series of small functions that do the real work.

In this programming assignment, you are not allowed to use global variables. Avoid using break (unless working with a switch statement). Limit your functions to less than 30 statements of code (not including comments, blank lines, or variable definitions). You may not use a return in the middle of a loop!

Program Assignment:

A cipher is an algorithm for encoding some information so that people can’t immediately tell what it is without decoding it. For you second programming assignment, you will create a program that can take your name, PSU email address, and a key and, through a cipher that you design, output the encoded results. Additionally, your program should, given an encoded name, PSU email addresses, and key, decode the information and output the result.

Cipher Details

Your cipher should generate a mapping for each plaintext character to a unique two digit number given a key which will also be a two digit number. You may disregard letter case, but you should consider what possible symbols may appear in an email address and encode them as well. (PSU email addresses can contain digits 0-9, @, and .)

Your algorithm for generating this mapping must be deterministic. That is, given the same key, the encoding generated should be the same every time. This will allow your program to encode a message with a key and properly decode the encoding with the same key.

You should consider how to generate unique two digit numbers for each character. That is, you should consider how to tell if a number generated has already been mapped to a character.

You should also consider how to “wrap around” so you only generate two digit numbers.

Program details

Your mapping should be stored in an appropriately sized array of ints. For instance, if your array is called cipherMap, then perhaps cipherMap[0] is the two digit int that maps to the letter a, cipherMap[25] is the two digit int that maps to the letter z, and cipherMap[26] is the two digit int that maps to the digit 0. You may or may not want to encode a space. It is up to you what conventions you pick for your mapping, but you must use an array.

The digits that characters map to must be generated by your cipher algorithm and key. They may not be hardcoded.

The mappings must be unique. That is, each character should map to a different two digit number.

Given different keys, your cipher algorithm should produce different mappings.

Ciphertext input shall be of the form dd dd dd dd ... where d is a digit.

Sample Execution

Encoding a name and email

> Menu:
(1) Encode a name and email
(2) Decode a name and email
> 1
> Enter a name:
> John Doe
> Enter an email:
> jdoe3@pdx.edu
> Enter a key:
> 42
> Encoded results:
> 81 24 54 13 55 75 24 83
> 81 75 83 21 33 61 79 75 94 10 83 75 15

Decoding a name and email

> Menu:
(1) Encode a name and email
(2) Decode a name and email
> 2
> Enter an encoded name:
> 81 24 54 13 55 75 24 83
> Enter an encoded email:
> 81 75 83 21 33 61 79 75 94 10 83 75 15
> Enter a key:
> 42
> Decoded results:
> john doe
> jdoe3@pdx.edu

(Note uppercase is not preserved after decoding.)