CSIS 5857: Encoding and Encryption
Assignment 1: Traditional Ciphers
Due Feb. 1
1. Despite the advice given in this course, Alice and Bob have decided to try to hide the
encryption algorithm they have used. Assume that they send a ciphertext message,
which is intercepted by Darth. How can Darth determine whether a monoalphabetic
substitution cipher or a transposition cipher was used just by examining the
message?
2. Suppose there is a language that has only the letters a and b. The frequency of the
letter a is 10% and the frequency of the letter b is 90%. A message is encrypted using
a Vigenere cipher (with a keyword of size 2). The ciphertext is BABABAAABA.
Determine the most likely key and decrypt the message.
3. Caesar wants to arrange a secret meeting with Marc Antony either at the river or the
arena. He sends the ciphertext EVIRE, created using an additive Caesar cipher.
However, Antony has forgotten the key, so he tries all possibilities. Where will he
meet Ceasar? (Hint: this is a trick question!)
4. The encryption key in a transposition cipher is the following. Give the decryption
key.
3
6
2
4
5
1
Please turn these in using a word processor (such as Word), instead of hand-written form.