# Assignment #2- Cryptography

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Assignment #2
Assignment Overview
In this assignment you will practice with strings, functions, conditionals and loops (for, while) to perform
specialized string manipulation operations, namely encrypting and decrypting plain-text messages.
Background
Cryptography is “is the practice and study of techniques for secure communication in the presence of third
The companion field of cryptanalysis focuses on “finding some weakness orinsecurity in a cryptographic
scheme, thus permitting its subversion or evasion.”[Wikipedia]
The main classical cryptography methods are “transposition ciphers, which rearrange the order of letters in a
message and substitution ciphers, which systematically replace letters or groups of letters with other letters
or groups of letters. An early substitution cipher was the Caesar cipher, in which each letter in the plaintext
was replaced by a letter some fixed number of positions further down the alphabet. Suetonius reports
thatJulius Caesar used it with a shift of three to communicate with his generals.”[Wikipedia]
Project Specification
In this assignment you will implement one of the earliest (and simplest) encryption/decryption algorithms: the
Caesar cypher, where each letter in the plaintext is replaced by a letter some fixed number of positions further
down the alphabet. The number of positions is the ‘key’ and can be chosen by the user (rather than hardcoded
1. Print a brief (2-5 lines) message explaining the purpose of this “app”.
2. Prompt for:
 The input phrase (upper and lower case letters only, no spaces, punctuation symbols, or other
characters);
 The key;
 The choice of operation (E/D, for encrypt/decrypt).
3. Process the phrase and key according to the encryption/decryption algorithm.
4. Print the output phrase and ask if the user wants to submit another phrase/key/option or quit.
Deliverables
You must submit:
o This is your source code solution; be sure to include your name, date, assignment number and
– A README.md file with “project notes” (describing what my TA and I cannot see by looking at your
source code and/or running your program).
o Examples: design decisions, documented limitations,future improvements, etc.
– A screenshot of the results produced by your code (make sure to show input and output for at least
one run).
Notes and Hints:
 Follow the “cardinal rules” of programming in Python (as perthe textbook).
 Start by breaking the program down into parts and solve smaller problems before producing the final
solution.
 Try to handle special cases and prevent runtime errors to the best of your knowledge.
 Don’t overdo it! You might want to try some of the bonus points options below but should refrain
from going beyond the scope of the course so far.
o Things to avoid (as cool as they might be) include:reading from / writing to files; making the
solution web-accessible; implementing complex error-checking schemes; using classes and
other OOP aspects (polymorphism, inheritance, etc.).
Bonus opportunities:
If you successfully implement one or more of the bonus options below without breaking the baseline
solution (*) you will earn extra points (max 10% each):
1. Brute force attack
 As you can tell, the baseline solution is very easy to break via brute force.
 If you implement an elegant solution for breaking the code using brute force, you will earn up to
extra 10 points.
2. Encoding spaces, other characters, and punctuation symbols
 One of the tell-tale signs of simple encryption schemes is that they preserve spaces, punctuation
symbols, etc.
 If you implement an improved solution that takes those into account and encrypt/decrypt them as
well, you will earn up to extra 10 points.
3. More complex key
 An elegant way to increase the robustness of the baseline algorithm to brute force attacks is to
allow an alphabetical phrase to be used as a key, traverse the key in a circular fashion, and take
each character in the string as the numerical key for the shift operation.
 If you implement an improved solution that allows the user to choose a second phrase as key, you
will earn up to extra 10 points.
(*) A common mistake students make is to attempt bonus items before producing a “perfect 100” baseline
solution.
Make no mistake: if your baseline solution doesn’t meet all grading criteria (see rubric on Canvas), you are not
eligible for bonus points. Period.

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