EECS 690 Cryptography and Information Security

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EECS 690 Cryptography and Information Security

• Dr. Weichao Wang
• Jan 23rd, Monday, 2006

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Syllabus

• See attachment
• Homework will usually has 4-5 questions and due
  in two weeks. It is due at the time that the
  class begins. (timestamp)
• Project
• Group project (3 people)
• Proposal, Demo, Final report
• Midterm and final
• Misc lunch time

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Before class

• An interesting question
• A bank has a safe and 7 VPs. How can we give each
  VP a code, and at least 4 out of 7 are required
  to open the safe?
• Secret sharing
• Can two rich ladies/gentlemen determine who is
  richer without disclosing the numbers?
• Blind computation

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Examples in real life

• Merging of two large companies
• Sharp increase in traffic
• Buy both stocks
• Company merge, sell stock, make money
• Go to prison

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Examples in real life

• Code during the war
• Navajo Code in WWII
• http//www.imdb.com/gallery/ss/0245562/W-266R.jpg

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Information security

• Encryption
• Symmetric and public key encryption
• Block and stream encryption
• Authentication
• You are who you claim you are
• Authorization
• The role and the right

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Information security

• Information integrity
• The packet has never be changed
• Non-repudiation
• Cannot deny your words
• Privacy
• Who should know, how much, how to use the
  information
• Your cell phone or medical records
• RFID

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Chap 1 Introduction

• Two kinds of cryptographic algorithms
• Keep the method secret
• Good safe for low security requirement
• Bad update, proof of correctness, how to
  communicate with outsider
• Make the algorithm public but keep the key secret
• Safety depends on the key only
• Good safety analysis can be conducted

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Introduction (cntd)

• Symmetric algorithms
• The encryption and decryption key can be
  calculated from each other easily (most of the
  time the same).
• Block algorithms and stream algorithms
• Good efficient and fast, easy to deploy
• Bad key distribution, scalability, broadcast or
  multicast

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Introduction (cntd)

• Public-key encryption
• First appear in 1970s
• Two keys public key and private key
• Private key cannot be derived from public key
• Everyone can send a packet to Alice
• Only Alice has the private key to recover the
  packet
• If Alice uses the private key to encrypt a
  message, can be viewed as digital signature
• Strong, scalable, easy for broadcast and
  multicast, but very slow

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Introduction (cntd)

• Attack to encryption system
• Cipher-text only attack
• The amount of traffic matters
• Known plaintext attack
• Chosen plaintext attack
• Key point
• Keep the cost to break the system higher than the
  gain of the information

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Introduction (cntd)

• Several old fashion encryption algorithms
• Substitution ciphers
• Replace a character in the plaintext with another
  character
• Example Caesar cipher
• Transposition ciphers
• Shuffle the order of characters
• The frequency of characters does not change
• XOR and one-time pad
• If the random bits repeat in cycle, it is bad
• Synchronization at both side is always a problem

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Introduction (cntd)

• Can you always break an encryption system?
• One time pad
• Brute-force attack Try every possible key

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Chap 2 Building blocks

• There are hundreds of security related protocols,
  fortunately, only a limited number of building
  blocks are involved
• How to organize these blocks shows the skills of
  the researchers
• Security protocols
• Prevent eavesdroppers
• Prevent or detect cheaters

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Building blocks (cntd)

• Classification of protocols
• Arbitrated Protocols
• Example Buying a used car from a stranger
• Good simple, everyone feel safe
• Bad how to find the Trusted Third Party (TTP),
  who pay the cost, bottleneck at TTP, single point
  of failure

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Building blocks (cntd)

• Classification of protocols
• Judge Involved Protocols
• TTP get in only when disagreement arise
• Good avoid the bottleneck
• Bad still need to keep the TTP, and need to keep
  evidence now

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Building blocks (cntd)

• Classification of protocols
• Self enforced protocols
• The best group of protocols
• If one party wants to cheat, the other party can
  detect and abort the procedure
• Good avoid the bottleneck
• Bad usually more complicated and more overhead

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Block 1 one way functions

• One way function is easy to calculate in one
  direction, but not the other.
• Given x, easy to get f(x)
• Given f(x), even f() is known, still not easy to
  get a x
• Trap door one way function
• Given x, easy to calculate f(x)
• Given f(x), difficult to get x
• Given f(x) and a secret y, easy to get x

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Block 1 one way hash function

• Map a variable-length input string to a fixed
  length string fingerprint the file
• Easy to get Hash(x) when giving x
• Almost impossible to find a x that satisfies
  Hash(x)
• Almost impossible to find two files x and x to
  have the same hash value
• Minor change in x, large changes in Hash(x)
• Since the hash value is shorter, we have
  conflict
• We can easily rule out files, but not guarantee
  this is the origin file
• Still good enough in courts, like DNA tests

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Block 1 one way hash function

• Usage of hash function
• Timestamp a file and prove that you are the
  creator (can be used to timestamp the homework)
• Verify the integrity of the files in a file
  system
• Security problems how and where to save the hash
  values
• Hash(x, k) to prevent change on the computer

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Block 2 communication using symmetric crypto
algorithms

• Steps
• (1) Alice and Bob agree a key k and an encryption
  algorithm
• (2) Alice calculates E_k (message) and sends the
  cipher text to Bob
• (3) Bob decrypts the message and gets the
  plaintext
• Problems
• How to determine the key must in a secret place
• How to convince other people it is from Alice
  instead of Bob
• Number of keys increases fast, not scalable

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Block 3 communication using asymmetric crypto
algorithms

• First appeared in 1976, proposed by Diffie and
  Hellman
• Two keys public key and private key, it is
  almost impossible to get private key from public
  key.
• A certain kind of trap door one way functions
  private key is the secret
• Steps
• Alice and Bob agree a public key encryption
  algorithm
• Bob sends his public key to Alice
• (3) Alice calculates E_pubB (message) and sends
  the cipher text to Bob
• (4) Bob decrypts the message with the private
  key and gets the plaintext

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Block 3 communication using asymmetric crypto
algorithms

• Solve the problem in symmetric crypto methods
  the key can be transferred in public
• More scalable, easy for multicast
• New problems
• How can we know that is Bobs public key
• Trusted Third Party
• Certificate for the public key
• Some story about public key
• NSA says it is unnecessary
• But claims credit for it