CipherEncoderDecoder

This is a program that implements 4 common ciphers (Atbash, Caesar, Affine, & Viginere) to encode and decode text.

Ciphers Explanations

Atbash Cipher

Encodes a message by reversing the alphabet.

• a simple map was used for quick lookups for each character

Caesar Cipher / Shift Cipher

Encodes a message by replacing each letter with a different letter x units away.

• using the ASCII table you can add x to the current character's value

lowercase letters are in the range (a: 97 - z: 122)

if a value goes below 97 or extends past 122 you have to calculate the new value of the char

asciiNum = 122 - 97 % asciiNum + 1;

asciiNum = asciiNum % 122 + 97 - 1;

Affine Cipher

Affine Cipher Description

Encodes a message by mapping a letter to its numerical equivalent, applying a mathematical function, and then converting it back to a letter.

Encryption Function

E(x) = (ax+b) mod m

'key' consists of two values (a & b)

• a - must be prime to m (not have any factors in common with m)
• a - 1, 3, 5, 7, 9, 11, 15, 17, 19, 21, 23, 25
• m = 26 (the letters of the English alphabet)

My values chosen: a = 17 b = 7

Decryption function

D(x)= a^-1 *(x-b) mod m

a^-1 is the modular multiplicative inverse

Modular multiplicative inverse

if neg res, make pos

• result = (result + m) % m;

Vigenere Cipher

Encodes a message by using a different Caesar cipher on each letter, where a key determines how much it increments.

• choose a key
• that key is repeated until it is as long as the message to encrypt

Encryption Function

E(x) = (x + k) mod 26

• x = num of letter of message to encode from map
• k = num of letter of key

Decryption Function

D(x) = (x - k) mod 26

• x = num of letter of message in cipher to decode from map

FUTURE CHANGES

• Affine: allow user to select a (among possible choices) and b

RESEARCH NOTES

Casting between types in c++

Power function

Algorithms library

• have to include # in order to use std::transform

Transform function

• in c++ you have to use getline(cin, variable_name) to get full string with spaces

• getline won't function correctly unless input buffer is cleared before calling getline

  • cin.clear();
  • cin.sync();

  or

  • cin.ignore();

cin.ignore(256, '\n');

• ignores up to 256 characters before newline delimiter