Basic cryptography jenny kammer department of computer science university of tulsa tulsa ok 74104
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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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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


Message: Hi Bob!

Message: Hi Bob!




Message: qks9!h&


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!

  • 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



  • “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



  • 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.









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