Basic CryptographyBasic Cryptography

Jenny KammerJenny KammerDepartment of Computer ScienceDepartment of Computer Science

University of Tulsa, Tulsa, OK 74104University of Tulsa, Tulsa, OK 74104

What is What is Cryptography?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 Why do we Need Cryptography?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 Cryptography in a NutshellNutshell

plaintext ciphertext plaintext

Alice wants to send a message to Bob

Message: Hi Bob! Message: Hi Bob!

Message: qks9!h&

Eve

Bob

Secret Algorithm vs. Secret Algorithm vs. Secret KeySecret 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 Examples of Encryption Encryption in Historyin 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. Substitution vs. TranspositionTransposition

• Substitution – exchanging one letter for another• Monoalphabetic vs. Polyalphabetic• Vulnerable to frequency analysis

• Transposition – scrambling the message up• Analyze digraphs and trigraphs

Symmetric vs. Symmetric vs. Asymmetric Asymmetric EncryptionEncryption

• 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 Hashing and ChecksumsChecksums

• 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 Limitations of CryptographyCryptography

• 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 Breaking CryptographyCryptography

• 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!

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

Recent StandardsRecent 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. AESDES vs. AES

DES vs. AESDES 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 sizeKeyspace 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

Questions?Questions?

SourcesSources

• 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.

SourcesSources

• 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