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CS363

Week 4 - Friday. CS363. Last time. What did we talk about last time? Snow day But you should have read about Key management. Questions?. Project 1. Assignment 2. Security Presentation. Claire Chambless. Hash Function Motivation. Where Do Passwords Go?.

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CS363

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  1. Week 4 - Friday CS363

  2. Last time • What did we talk about last time? • Snow day • But you should have read about • Key management

  3. Questions?

  4. Project 1

  5. Assignment 2

  6. Security Presentation Claire Chambless

  7. Hash Function Motivation

  8. Where Do Passwords Go? • What magic happens when you type your password into… • Windows or Unix to log on? • Amazon.com to make a purchase? • A Twilight fan site so that you can post on the forums? • A genie from the 8th dimension travels back in time and checks to see what password you originally created

  9. In Reality… • The password is checked against a file on a computer • But, how safe is the whole process? • Twilight fan site may not be safe at all • Amazon.com is complicated, much depends on the implementation of public key cryptography • What about your Windows or Unix computer?

  10. Catch-22 • Your computer needs to be able read the password file to check passwords • But, even an administrator shouldn’t be able to read everyone’s passwords • Hash functions to the rescue!

  11. Hash Functions Defined

  12. Definition • A cryptographic (or one-way) hash function(called a cryptographic checksum in the book) takes a variable sized message M and produces a fixed-size hash code H(M) • Not the same as hash functions from data structures • The hash code produced is also called a digest • It can be used to provide authentication of both the integrity and the sender of a message • It allows us to store some information about a message that an attacker cannot use to recover the message

  13. Pigeonhole principle • The pigeonhole principle says that if you try to put m items into n categories, where m > n, then at least 2 things will be in the same category • Imagine that you have a 40,000 byte message and a 256-bit hash digest • How does the pigeonhole principle apply?

  14. Collisions • When two messages hash to the same value, this is called a collision • Because of the pigeonhole principle, collisions are unavoidable • The key feature we want from our hash functions is that collisions are difficult to predict

  15. Crucial properties

  16. Additional properties

  17. Avalanching in action • Example (from Wikipedia) of significant changes in hash output with small input changes • SHA-1

  18. Password dilemma resolved • Instead of storing the actual passwords, Windows and Unix machines store the hash of the passwords • When someone logs on, the operating system hashes the password and compares it to the stored version • No one gets to see your original password!

  19. Password File • Below is an example of what a password file might be look like.

  20. Any Problems? • What’s the probability that Ahmad has the same password (or a password that hashes to the same value) as Bai Li? • Very small! • What’s the probability that anyone has the same password (or a password that hashes to the same value) as anyone else? • Not nearly as small!

  21. Salt • If you are the administrator of a large system, you might notice that two people have the same password hash • With people's password habits, the odds are very high that their passwords are the same • To add to the semantic security of such schemes random salt (often 8 bits or so) is added to the end of a password • When checking a password against the hash, the system tries all 28 possible values for the salt

  22. Common Hash Functions

  23. MD5 • Message Digest Algorithm 5 • Very popular hashing algorithm • Designed by Ron Rivest (of RSA fame) • Digest size: 128 bits • Security • Completely broken • Reasonable size attacks (232) exist to create two messages with the same hash value • MD5 hashes are still commonly used to check to see if a download finished without error

  24. SHA family • Secure Hash Algorithm • Created by NIST • SHA-0 was published in 1993, but it was replaced in 1995 by SHA-1 • The difference between the two is only a single bitwise rotation, but the NSA said it was important • Digest size: 160 bits • Security • Mostly broken • Attacks running in 251 - 257 time exist • SHA-2 is a successor family of hash functions • 224, 256, 384, 512 bit digests • Better security, but not as widely used • Designed by the NSA

  25. The future of hash functions • NIST just had the contest for SHA-3 • When I last taught this class, it was down to five finalists: • BLAKE • Grøstl • JH • Keccak • Skein • Keccak was announced as the winner in 2012 • As with AES, Keccak beat out its competitors partly because it's so fast • Joan Daemen (of Rijndael fame) was also one of its designers

  26. Keccak (SHA-3) • Keccak uses a completely different form of hashing than SHA-0, SHA-1, and SHA-2 • Although there are only theoretical attacks on SHA-1 and no real attacks on SHA-2, the attacks on SHA-0 made people nervous about hash functions following the same design • Keccak also allows for variable size digests, for added security • 224, 256, 384, and 512 are standard for SHA-3, but it is possible to go arbitrarily high in Keccak

  27. Quiz

  28. Upcoming

  29. Next time… • Attacks on cryptographic hash functions

  30. Reminders • Read Section 2.8 • Keep working on Project 1 • Keep working on Assignment 2

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