Cryptography

1
Cryptography

• Basic crypto techniques
• Protocols
• Cryptanalysis

2
Basic crypto techniques

• Some principles
• Only the key is secret
• The algorithm is well-known
• The goal is not deception, it is complexity of
  cryptanalysis
• Private key ciphers
• Key known only to intended recipients
• Most algorithms are symmetric
• Usually much faster encryption than public key
• One problem is key distribution

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Basic crypto techniques

• Public key ciphers
• Only owner can feasibly generate private key
• Owner distributes public key
• Much slower algorithms
• Usually used for private key trasmission
• Bulk of encryption is then done in private key
• Vulnerable to chosen plaintext attacks
• One-way Hash functions
• Computatinally unfeasible to invert
• Key encryption in Unix is an example
• This is the classical np-complete problem

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Protocols

• Signature protocols
• Signature must be
• Verifiable
• Attached to document
• Non-repudiation
• Cash transmission
• No duplication
• Anonymity - Law enforcement fights this
• Reliable

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Protocols

• Cast of characters
• Alice, Bob - parties to the transaction
• Carol, Dave - additional parties
• Eve - Passive eavesdropper
• Mallory - Malicious and active
• Trent - a trusted arbitrator
• Walter - Warden - guards Bob and Alice
• Peggy - Prover
• Victor - Verifier

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Public Key Protocol

• Basic idea
• Alice and Bob agree on system
• Bob sends Alice his public key
• Alice encrypts and sends message
• Bob receives and decrypts using his private key
• Modification
• Alice gets Bobs key from the database
• Alice encrypts and sends
• Bob receives and decrypts
• Hybrid
• Alice encrypts and sends a private session key
• Bob and Alice communicate with that key in a
  private-key cipher
• Problem
• Mallory encrypts and sends - claims he is Alice
• Now Bob tells all his secrets to Mallory

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Some Signature Protocols

• With symmetric cryptosystems and arbitrator
• Trent is a powerful and trusted arbitrator
• He shares a private key with Alice and a
  different one with Bob
• Alice sends her message to Trent using Ka
• Trent reencrypts and retransmits to Bob using Kb
  with his certification
• With public key
• Alice encrypts document with her private key -
  this act is signature
• Alice sends the document to Bob
• Bob decrypts with Alices public key
• Comparison
• Trents services and fee arent required

8
How to Cheat and Counteract