Practical Cryptographic Systems  Assignment 1

650.445/600.454: Practical Cryptographic Systems 
Assignment 1

Name:
The assignment must be completed individually. You are permitted to use the
Internet and any printed references, though your code must be your own!
Please submit the completed assignment via Blackboard.
Problem 1: Classical encipherment and decipherment (20 points)
For this problem you will implement software that programmatically enciphers and deciphers messages using the Bellaso (Vigen`ere) cipher. Your programs must be written in Go.
In either case, your programs should run at the command line using the command line
syntax given below.
vigenere-encrypt <encipherment key <plaintext filename
vigenere-decrypt <decipherment key <ciphertext filename
The key field should consist of a string of uppercase letters up to 32 characters in length.
The input file field should contain the name of a text file (of size up to a maximum of
100KB) containing plaintext (resp. ciphertext). Your program should strip out and ignore
all characters in the file that are not letters ([a-zA-Z]). The program output should consist
only of uppercase letters with no white space, unless you wish to add optional newline
characters for formatting. Example usage on a Unix system is:
vigenere-encrypt PLEBISCITE plaintext.txt ciphertext.txt
vigenere-decrypt PLEBISCITE ciphertext.txt recovered plaintext.txt
Problem 2: Finding key lengths (30 points)
Now that you’ve written a tool to compute Vigen`ere ciphertexts, write a second tool that
uses the index of coincidence method to find the length of the key used in a Vigen`ere
ciphertext. Your tool should take an enciphered file with an unknown key, and output a
guess for the length of the key. Your tool should be reliable for keys of up to 20 characters in
length, when run on English language texts of at least 20,000 characters. Your tool should
output a number that represents the most likely guess for the key length. The required
command-line syntax is:
vigenere-keylength <ciphertext filename
Problem 3: Full cryptanalysis (45 points)
Assignment 1-1
Armed with a guess for the key length used in a given ciphertext, write a tool that attempts to complete decrypt a Vigen`ere ciphertext using frequency analysis, or make a best
guess for the decryption key using ciphertext only. Your tool should output the recovered
decipherment key for the analyzed ciphertext (or a reasonably-sized list of keys if multiple
candidates are discovered).
vigenere-cryptanalyze <ciphertext filename <key length
You may combine the answers to Part 2 and Part 3 into a single program that omits the
key length argument. Remember that you do not need to perfectly decrypt every ciphertext,
but you should be able to at least partially decrypt some English-language messages.
Problem 4: LOPs (5 points)
Imagine that we modify the Vigen`ere encipherment to use a single key that is at least as
long as the message. Explain why this cipher might be more secure, and what conditions
you would place on the keys to ensure it.
Assignment 1-2