CMSC 414 Computer and Network Security Lecture 14

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CMSC 414Computer and Network SecurityLecture 14

• Jonathan Katz

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Password-based protocols

• Any password-based protocol is potentially
  vulnerable to an on-line dictionary attack
• On-line attacks can be detected and limited
• How?
• Off-line attacks can never be prevented, but
  protocols can be made secure against such attacks
• Any password-based protocol is vulnerable to
  off-line attack if the server is compromised
• Once the server is compromised, why do we care?

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Password-based protocols

• Best Use a password-based protocol which is
  secure against off-line attacks when server is
  not compromised
• Unfortunately, this has not been the case in
  practice (e.g., telnet, cell phones, etc.)
• This is a difficult problem!

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Password storage

• In the clear
• Hash of password (done correctly)
• Doesnt always achieve anything!
• Makes adversarys job harder
• Potentially protects users who choose good
  passwords
• Salt-ed hash of password
• Makes bulk dictionary attacks harder, but no
  harder to attack a particular password
• Encrypted passwords? (What attack is this
  defending against?)
• Centralized server stores password
• Threshold password storage

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Centralized password storage

• Authentication storage node
• Central server stores password servers request
  the password to authenticate user
• Auth. facilitator node
• Central server stores password servers send
  information from user to be authenticated by the
  central server
• Note that communication with the central server
  must be authenticated!

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Authentication tokens

• RSA SecureID
• PIN-protected memory card
• Cryptographic smartcards
• Aladdin eTokens
• Still need a secure protocol!

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Biometrics

• How much entropy is there?
• How private are these?
• How reliable are they?
• Revocation?

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Biometrics

• Difficult to use securely
• Errors
• Non-uniform
• Still need a secure protocol

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Biometric authentication

• How can you securely authenticate yourself to a
  remote server using your fingerprint?
• Trivial solution

Server
User
close?
Completely vulnerable to eavesdropping!
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Better(?) solution
Server
User
nonce
h
A single-bit difference in the scanned
fingerprint results in a failed authentication!