This document serves as an introduction to classical cryptography, covering various historical ciphers such as the Caesar cipher, Vigenère cipher, and Enigma machine. Students will learn about key concepts, encryption and decryption processes, and the vulnerabilities of these methods against modern analysis techniques. While these ciphers were effective in their time, they are now easily cracked using frequency analysis and pattern recognition.

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Intro to Cryptography
Lesson 1: Classic Crypto

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Learning Outcomes
Upon completion of this lesson, students will be able to:
• Describe historical ciphers such as: shift, substitution, Vigenere, enigma
machines and others.

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Basic Terms for this Lesson
• Encrypt – Encode a message
• Decrypt – Decode a message
• Plaintext – The message
• Ciphertext – The encoded message (called a “cipher”)
• Symmetric – Encode and decode a message using the same secret
• Substitution cipher – Substitute one character for another
• Polyalphabetic cipher – Substitute one character for another, but
uses multiple substitution alphabets in one cipher

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Caesar Cipher
• Simply substitute one letter for another
• Goes back to Egyptian times but contributed to Julius Caesar
• Caesar Cipher – substitute letters +3 (c is for Caesar)
• A becomes D
• B becomes E
• C becomes F
• ….
• Z becomes C
Plaintext: cybersecurity is fun
Ciphertext: fbehuvhfxulwb lv ixq

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Substitution Cipher Variations
• Substitute a different offset, +1,+2,+4, +24…
• Atbash – Wrap the Alphabet (or +25)
Plaintext: If you think you can do a thing or think you can’t
do a thing, you are right.
Ciphertext: ???
• ROT13 is +13 (Originally used to encrypt word documents!)
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Z Y X W V U T S R Q P O N M L K J I H G F E D C B A

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Fun way to do substitution ciphers
Build a Caesar Cipher Wheel!
When the inner A aligns with the outer A, this is called position 0. The key
indicates how many letters to shift each letter. For example, +4 means to
rotate the inner wheel counter clockwise such that the inner E aligns to the
outer A.

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Some More Ciphers to Try….
Ciphertext: lwfhj mtuujw nsajsyji htgtq
Plaintext: ???
Ciphertext: wlmgurmwuzfogurmwzivnvwb
Plaintext: ????
• What are some techniques to discover the cipher?
• Why can’t we use these ciphers to secure our personal
information?

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One Way to Break the Substitution Cipher:
Frequency Analysis
• In a long enough English text, the letters are distributed
approximately as shown in this graph:
“English letter frequency (alphabetic)” by Nandhp. Public Domain.

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Polybius Square
• Ancient Greek – Invented by Scholar Polybius
• Convert symbols ( letters into numbers) using an easily created and
memorized grid (Row, Column). M= 32
• Be able to hold up a set of torches in two hands, or knock on a prison
wall
Ciphertext:
52231144 5215 4423243325 5215
121513343215
Plaintext: ???
1 2 3 4 5
1 A B C D E
2 F G H I/J K
3 L M N O P
4 Q R S T U
5 V W X Y Z

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Vigenère Cipher
• A shift cipher where every
character is shifted differently
based on a code word (key)
• In this example, the key is PIE.
Then use the ciphertext
generated from the character
before.
• Row = Key, Column =Plaintext,
Middle = Ciphertext
Plaintext: V I G E N E R E
Key: P I E P I E P I
Ciphertext: K Q K T V I G M
Image From https://commons.wikimedia.org/wiki/File:Vigenere-square.png

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Alberti Cipher Disk
• Created by Italian Leon Battista
Alberti (1404)
• Both parties agree on a priming
key (inner circle) – Lets use k
• Every time you see a capital
letter, you switch to use that as
the priming key
Ft&yxaepQmmepe&vmsGgimhva
ahmyiAakz
Alberti was a true renaissance
man

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Enigma Machine
• Used in Nazi Germany to
communicate during WWII
• Required a daily code book and
the machine to encrypt/decrypt
each day.
• But it is a polyalphabetic cipher,
although very complex!
Image by Alessandro Nassiri
CC BY-SA 4.0

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In Conclusion
• There are many other historical
ciphers to explore:
• Playfair
• Bacon
• Pigpen
• Navajo Code Talkers
• Rail Fence
• Alan Turing
• Read “The Gold Bug” by Edgar
Allan Poe
• Read “The Code Book” by Simon
Singh
• Although fun and may have
worked in their day, they are
easily cracked today:
• Letter Frequency Analysis
• Word length
• Letter patterns
• Finding a crib (a word you know is
being used)