Block 2: communication using symmetric crypto algorithms

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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

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Hybrid crypto systems

• Symmetric methods are fast, easy to implement,
  but require special attention during key
  distribution
• Asymmetric methods are slow, but more secure
• Hybrid
• Using asymmetric method to distribute key
• Using symmetric method to encrypt data

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Communication using hybrid crypto systems

• Steps
• Bob sends Alice his public key
• Alice encrypts the session key with this public
  key and sends to Bob
• Both Alice and Bob know the session key and can
  use it for data traffic

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Dual encryption

• Lets assume that everyone in the network has a
  public-private key pair. Alice wants to send a
  message to Bob and convince that it is from
  Alice.
• Two possible format which is better??

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Dual encryption

• Answer
• Alice should first sign the message, then use
  Bobs public key to encrypt the packet
• Reason 1 If Alices private key is the outer
  layer, everyone with Alices public key will be
  able to decrypt it.
• Reason 2 never sign a random string that you do
  not know what it is (Example of RSA)
• Two pair of public-private key for everyone one
  for encryption, one for digital signature

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Dual encryption

• Dual encryption is too slow, how can we improve
  the efficiency??
• Alice sign the hash result of the message, then
  use Bobs public key to encrypt the whole packet

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Merkles puzzle

• Make the life of an eavesdropper difficult

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Block 4 Digital signature

• Signature by symmetric encryption
• TTP will get involved
• How to send to a third party through TTP again
• Signature with asymmetric encryption
• Using the private key for signature
• If the signature is like This is 100 check to
  Bob, a timestamp should also be included in the
  signature.
• Application of digital signature nuclear test
  monitor

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Resend attacks

• Lets assume that the same public private key
  pair is used for both encryption and signature.
  And if every time after receiving the message, a
  receipt is given, a resend attack can be
  conducted.
• Defending against such attacks using different
  methods, using different keys, or do timestamps

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Block 5 Random number

• Pseudo random numbers
• Usually will repeat after a long sequence
• Must long enough
• Real random numbers
• Lava lamp
• Earthquake strength or interval

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Chap 3 Key exchange protocols

• Protocols 1 using symmetric cryptography
• We assume that every node shares a key with KDC
• Steps
• Alice requests a key from KDC
• KDC encrypts the key with Alice and Bobs key
  respectively and sends both to Alice
• Alice forward Bobs copy to Bob

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• Problems of protocol 1
• Alice knows both the plaintext and cipher text
  for Bob. Bad for Bob.
• When Alice sends a copy to Bob, how can Bob makes
  sure this is a fresh key?

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• Protocol 2 Using public key
• We assume that everyone has a public private key
  pair
• Steps
• Alice generates a session key and encrypts it
  with Bobs public key
• Bob decrypts the message and gets the session key
• Now they can talk safely

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• If the public keys of the nodes do not have
  certificate with them, they can be fake keys. And
  a min-in-the-middle attack can be conducted.
• How the min-in-the-middle attacks are conducted.
• How can the Interlock protocol mitigate the
  attack
• Wormhole attack in wireless networks and the
  prevention mechanisms
• Packet leash, directional antenna, multi-round
  response, visualization

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• Improvement to protocol 2
• Public keys for Alice and Bob should be protected
  by the certificate from a TTP

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Authentication

• Authentication prove that you are who you claim
  you are
• Method 1
• The system stores your password, and compares it
  with the word you type in every time you login
• Problem if the attacker gets access to the file,
  you are done.

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• Method 2
• The system stores the hash result of the
  password, now if the attacker sees the hash
  value, it cannot recover the plaintext.
• Problem
• It is still not safe under dictionary attacks
• The system can add a random number after the
  password, which is called salt
• Public salt and private salt.

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• Method 3 Using public-private key
• The system knows the public key and the user
  keeps the private key
• During login, the system sends a random number to
  user and user encrypts it with the private key
• System decrypts with public key and verifies the
  user
• Problems
• Blind signature
• Chosen plaintext attack

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Authentication (cntd)

• Method 4 one key a time protocol
• Hash chain
• Unlimited one key a time system (2 possible
  solutions)
• Both sides know a secret k
• A knows R1, and B knows hash(k, R1)
• During first login, A sends R1 and hash(k, R2)

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• Method 5 Mutual authentication with symmetric
  key SKID 3
• A and B share a secret k
• A sends a random number to B and B calculates the
  keyed hash value
• Same way for A
• Use random number to replace timestamp avoid
  synchronization
• Man-in-the-middle attack
• Compromise the authentication but cannot crack
  the key

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• Method 6
• Typing the key and moving your mouse
• User re-authentication via mouse movements (ACM
  VisSec)
• User authentication through keystroke dynamics
  (ACM Trans. Inf. Syst. Secur. 2002)