Message Integrity Protocols - Advanced Computer Networks - Lecture Slides, Slides of Computer Networks

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Lecture No. 41

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Message Integrity Protocols

• Digital signature using RSA
  • Special case of a message integrity where the code can only have been generated by one participant
  • Compute signature with private key and verify with public key

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Message Integrity Protocols

• MD5 with RSA signature
  • Sender: m + E(MD5( m ), private )
  • Receiver
    • Decrypts signature with sender’s public key
    • Compares result with MD5 checksum sent with message

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Authentication

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Session Key Communication

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Key

Distribution

Center

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Man-in-the-Middle Attack

in Diffie-Hellman

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Key Distribution

• Certificate
  • Special type of digitally signed document: “I certify that the public key in this document belongs to the entity named in this document, signed X.”
  • The name of the entity being certified
  • The public key of the entity
  • The name of the certification authority
  • A digital signature

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Tree-structured CA Hierarchy

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Key Distribution (cont)

• Chain of Trust
  • If X certifies that a certain public key belongs to Y , and Y certifies that another public key belongs to Z , then there exists a chain of certificates from X to Z
  • Someone that wants to verify Z ’s public key has to know X ’s public key and follow the chain
• Certificate Revocation List

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PGP Message Encryption

Decrypt message using DES with secret key k

Decrypt E( k ) using RSA with my private key k

Convert ASCII message

Encrypt k using RSA with recipient s public key

Encode message + E( k ) in ASCII for transmission

Encrypt message using DES with secret key k

Original message

Transmitted message

Create a random secret key k

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Example (PGP)

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Secure Transport Layer

Application (e.g. HTTP) Secure transport layer TCP IP Subnet

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TLS Handshake Protocol

Client Server