# What is Cryptography? - PowerPoint PPT Presentation

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Basic Cryptography Jenny Kammer Department of Computer Science University of Tulsa, Tulsa, OK 74104. What is Cryptography?. Cryptography – process of designing systems to communicate over non-secure channels Encryption – making a message unreadable except to the intended recipient

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What is Cryptography?

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### What is Cryptography?

• Cryptography – process of designing systems to communicate over non-secure channels

• Encryption – making a message unreadable except to the intended recipient

• Decryption – making an encrypted message readable to the intended recipient

• Cryptanalysis – Examining cryptosystems in an attempt to break encryption techniques, allowing unintended recipients to view the message.

### Why do we Need Cryptography?

• Want to transmit or send a message securely over an insecure medium

• Ensures confidentiality – making sure data is secret from all except authorized persons

### Cryptography in a Nutshell

Alice wants to send a message to Bob

Bob

Message: Hi Bob!

Message: Hi Bob!

ciphertext

plaintext

plaintext

Message: qks9!h&

Eve

### Secret Algorithm vs. Secret Key

• Secret Algorithm – Only the two parties communicating know how to encrypt/decrypt

• Secret Key – Everyone knows how to encrypt and decrypt, but you need a secret key to do it, and only the two parties communicating have the key(s)

• Better if we want to communicate with large numbers of people

### Examples of Encryption in History

• 1900 BC – Egyptian scribe uses non-standard hieroglyphs (1st documented example written cryptography)

• Caesar used simple substitution cipher (Decoder rings)

• German Enigma Machines

### Substitution vs. Transposition

• Substitution – exchanging one letter for another

• Monoalphabetic vs. Polyalphabetic

• Vulnerable to frequency analysis

• Transposition – scrambling the message up

• Analyze digraphs and trigraphs

### Symmetric vs. Asymmetric Encryption

• Symmetric – Same key used to encrypt and to decrypt message

• How do we share the key?

• Lots of keys to keep (n*(n-1)/2 )

• Asymmetric – Uses key pairs. Key pair is a set of a public and private key where public key is used to encrypt a message and private key is used to decrypt a message.

• Don’t have to share secret keys

• Fewer keys – (2n)

### Hashing and Checksums

• Hashing and checksums are similar to encryption, but they are NOT the same

• Encryption can be decrypted; hashes cannot be decrypted (hashes are one-way functions)

• Hashes are used to verify the integrity of message, not ensure the confidentiality of a message

### Limitations of Cryptography

• Flaws in cryptosystems

• Start to finish problem

• If data is encrypted during transport but stored on a server in plaintext, it is still vulnerable

• Moore’s Law

• Human component

### Breaking Cryptography

• Cryptanalysis

• Try to find weaknesses in encryption algorithms

• Gives weight to older algorithms – they have stood the test of time

• Brute Force – trying every possible password

• Will find password on average in n/2 time

• This is why longer passwords are “safer”

• Dictionary – trying common passwords/English words first

• This is why strong password rules are important!

• Cryptosystems don’t have to be impossible to break, just computationally infeasible.

### Recent Standards

• DES was standard from 1976 until 2002

• 1977 – Diffe and Hellman propose a parallel attack, which required 10^6 chips, each testing 1 key per microsecond would require 20 hrs and cost \$20,000/solution

• 1997 – An attack on DES cracked it in 120 days

• 1998 – EFF broke DES in 56 hours

• 1999 – EFF’s Deep Crack and a distributed net break DES in 22 hours

• 2001 – AES is published

• 2002 – AES is adopted as new standard

### DES vs. AES

• “Assuming that one could build a machine that could recover a DES key in a second (i.e., try 255 keys per second), then it would take that machine approximately 149 thousand-billion (149 trillion) years to crack a 128-bit AES key. To put that into perspective, the universe is believed to be less than 20 billion years old.”

### Keyspace size

• Assume alphanumeric keyspace (A-Z, a-z, 0-9)

### Sources

• Trappe, Wade and Washington, Lawrence. Introduction to Cryptography with Coding Theory. Pearson Prentice Hall. New Jersey. 2006.

• Loehr, Nick. Class Lecture. Cryptography I. Virginia Tech, Blacksburg, VA. 2008.

• Shenoi, Sujeet. Class Lecture. Computer and Network Security. University of Tulsa, Tulsa, OK. Feb 2010.

### Sources

• www.Cryptographyworld.com

• http://www.ciphersbyritter.com/LEARNING.HTM#WhatCryptCanNotDo