1 Assignment Overview
This project focuses on using arrays. You will write functions that encrypt strings.
2 Assignment Deliverables
You must turn in completed versions of the following les:
proj05/cipher.cpp
Be sure to use the speci ed le name and to submit your les for grading via Mimir before the project
deadline.
3 Background
In Project 4, we used a variation on the Caesar cipher to encrypt one letter at a time. Unfortunately, this
leaves it somewhat open to frequency analysis as in practice di erent letters will appear at di erent rates.
To make this type of analysis more di cult we will encrypt two characters at once.
The key for our method is a 5 5 square that contains each letter (except for ‘j’) exactly once (‘i’ and
‘j’ are treated as the same letter). This square is constructed from a keyword or phrase by placing each
letter in the rst open space (going left to right then top to bottom), skipping letters that have already been
seen. Any letters not in the original keyword are then placed in alphabetical order. For example, the key
\squarecipher" will generate the following square:
s q u a r
e c i p h
b d f g k
l m n o t
v w x y z
It seems like we would need to use a 2D vector but we don’t. We can represent the above table with
the string \squarecipherdfgklmnotvwxyz". If we look at the letter ‘d’, we see that it is at index 11 in the
string (remember the rst index is 0) or at row 2, column 1 in the square. We can convert between the two
by using the conversions
row = index=width
col = index%width
index = row width + col
To encrypt a message we break it into pairs of letters (known as bigrams) and encrypt each bigram. If
the message has an odd number of characters, we append an extra ‘x’ to then end. Thus, the plaintext
\runfast" is represented by the bigrams ru nf as tx.
1
There are four cases for a particular bigram.
1. Both letters are the same: replace the second letter with an ‘x’ and move to the appropriate other
case.
2. Both letters appear in the same row: each letter is represented by the letter directly to its right
(wrapping around to the left as necessary)
3. Both letters appear in the same column: each letter is represented by the letter directly below it
(wrapping around to the top as necessary)
4. The letters are not in the same row or column: each letter is represented by the letter in the same
column but in the same row as the other letter.
For example, ru appears in one row so these are shifted rightward to sa. Similarly, nf appears in one
column so these are shifted downward to xn. tx doesn’t appear in a single row or column so it instead forms
two corners of a smaller square n tx z We thus use zn by selecting the other letter in the same column of
smaller square.
To decrypt a message we perform. the steps in reverse: going up or left instead of down or right.
4 Assignment Speci cations
In this and in future projects we will provide exactly our function speci cations: the function name, its
return type, its arguments, and each arguments type. The functions will be tested individually in Mimir
using these exact function speci cations. If you do not follow the function speci cations, these independent
tests of your functions will fail. Do not change the function declarations!
What you test on Mimir is a le that contains only the functions. You do not turn in a main program.
We can test the functions individually on Mimir. However, you should write your own main program to test
your functions separate from Mimir. It is more exible and you can debug more easily.
function CellToIndex
params: x : size t, y : size t
returns: size t
function IndexToCellX
params: index : size t
returns: size t
function IndexToCellY
params: index : size t
returns: size t
function CreateSquare
params: keyword: const string
returns: string
function EncodeString
params: plaintext: const string, keyword: const string
returns: string
function DecodeString
params: ciphertext: const string, keyword: const string
returns: string
5 Assignment Notes
You will receive no points if your solution does not compile on Mimir.
Points will be deducted if your solution has any compiler warnings.
Points will be deducted if you include using namespace std.
The library contains a number of useful methods.
You do not need to check for bad input values for this project. In future projects we will explicitly
indicate the errors we are looking for.
You do need to be able to handle the letter ‘j’ and strings of odd length.
size t is the return type of the size function of collections like string and vector and is the same
as string::size type. It is an unsigned integer.
You may assume that the string is all lowercase.
The encoding of \runfast" with the keyword \squarecipher" should yield the ciphertext \saxnrqzn".
If you decode the string that you encode using the same keyword you may not get the same string back;
strings of odd length, containing double letters, or the letter ‘j’ will be transformed in a consistent and
recognizable way.