An Introduction to Cryptography

What is cryptography?

noun \krip-ˈtä-grə-fē\ : the process of writing or reading secret messages or codes

• “Encryption”: to put into a format that is unreadable to unauthorized individuals

• “Cipher”: a method of encryption, or a code

Basics

History

History

Substitution Cipher

• Substitute one value for another

Substitution Cipher

Common example:

Cryptograms in newspapers

Substitution Cipher

• Original message: “plaintext”:

• Encrypted message: “ciphertext”

Solve this Substitution Cipher

XKGTJDHI

Solution

SECURITY

Ciphers in Popular Culture“National Treasure” (2004)

Keys

• They are what we use to secure the messages. (And to decrypt them!)

• They are shared

to decrypt the

message.

Keys

• “Asymmetric” cryptography has two keys:– Public key: used to encrypt the message– Private key: used to decrypt the message. (This one

is secret)– Mathematically matched to protect the message

Vigenere Square

• Polyalphabetic cipher

• Column letter: plaintext

• Row Letter: key

Example:

Vigenere Square

Encrypt this word:

Solution

VDOZF

Ciphers in Popular Culture“A Beautiful Mind” (2001)

Transposition Cipher

Rearranges values within a block to create ciphertext

•Example:– “Caesar Block Cipher”– Uses 5X5 square– Message contains

<25 characters

Famously created by Julius Caesar!

Caesar Block Cipher

Plaintext: LEARN ABOUT ENCRYPTION

Ciphertext: L TROEA YNABEP EONT NUCI

read across

fill d

own

Caesar Block Cipher

• Try writing and encrypting your own message!

• Remember: <25 characters (including spaces) read across

fill d

own

• Pass it to a friend to decrypt

Advanced Cryptography

• Using the types of techniques we learned, computers can encrypt files, emails, etc.– Makes communication secure– But we still need to protect our keys and

passwords!

Advanced Cryptography

Technology Example

Real World Application

• Cryptanalysts: study information systems to find hidden aspects

• It’s their job to decrypt messages

Why does this matter to you?

• When you put your personal information on the internet, make it hard for thieves to steal

Check your passwords: http://www.passwordmeter.com/

Create a strong password

• Start with a weak password:Linda!Marie1207

• Come up with a secret key (4-6 characters)

puppy

• Encrypt text using the vigenere square:Accsy!Bugxc1207

Strong Password

Jeopardy!

What four aspects should your

password include?

In what profession do people break

codes?

Where does Gabe work?

What is an encrypted

message called?

What cipher replaces one letter

with another?

What type of cryptography uses public and private

keys?

Who encoded messages on clay

tablets?

What is an unencrypted

message called?

What machine was used to encrypt

messages in World War II?

Questions