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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Basic cryptography jenny kammer department of computer science university of tulsa tulsa ok 74104
Basic CryptographyJenny KammerDepartment of Computer ScienceUniversity of Tulsa, Tulsa, OK 74104

What is cryptography
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
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
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 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
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 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 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

  • 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
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
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
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 aes1

  • “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
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.