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Basic CryptographyJenny KammerDepartment of Computer ScienceUniversity 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
- 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

- Weak passwords
- 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

- Password Cracking
- 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!

- Brute Force – trying every possible password
- 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)
- 5 character password: 916,132,832
- 6 character password: 56,800,235,584
- 7 character password: 3,521,614,606,208
- 8 character password: 218,340,105,584,896
- 9 character password: 13,537,086,546,263,552

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
- http://www.sans.org/reading_room/whitepapers/vpns/history_of_encryption_730
- http://xkcd.com/
- http://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_4-2/goodbye_des.html
- http://www.nist.gov/public_affairs/releases/aesq&a.htm
- http://en.wikipedia.org/wiki/Data_Encryption_Standard

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