Message Authentication Overview

Message Authentication

• Message authentication protects against active attacks (falsification of data and transactions)
• Two main objectives:
  • Verify the contents have not been altered
  • Verify the source of the message
• Other concerns:
  • Timeliness verification (prevent message delay and replay)
  • Sequencing of messages between parties

Authentication Using Conventional Encryption

• Uses symmetric encryption (sender and receiver use the same key)
• Assumes the sender can encrypt a message successfully, and the receiver can recognize a valid message.
• If the message includes an error-detection code, a sequence number and a timestap, the receiver can confirm:
  • No alterations have been made
  • Sequencing is proper
  • The message hasn't been delayed beyond expected network transit

Message Authentication without Message Encryption

• Does not rely on encryption
• Appends an authentication tag to each message
• Allows the message to be read at the destination without decryption
• Three situations when message authentication without confidentiality is preferred:
  • Broadcasting messages: Cheaper and more efficient to have a single destination monitor authentication
  • Exchange with heavy load: Selective authentication on random messages
  • Authenticating a computer program in plaintext: Avoids costly decryption and allows program execution

Message Authentication Code (MAC)

• Uses a secret key to generate a small block of data (called a MAC) appended to the message
• Both sender and receiver share a secret key (KAB)
• MAC is calculated as a function of the message (M) and the key: MACM = F(KAB, M)
• The message plus MAC are transmitted to the receiver
• Receiver performs the same calculation using the same key to verify the received MAC
• Assumes only the receiver and sender know the secret key
• Benefits:
  • Assures the message has not been altered: An attacker cannot modify the message without changing the code
  • Assures the message is from the alleged sender: No one else knows the secret key
  • If the message includes a sequence number, it ensures the correct order

One-Way Hash Function

• Alternative to MAC
• Uses a hash function that takes a variable-size message M as input and outputs a fixed-size message digest H(M)
• Does not use a secret key
• To authenticate a message, the message digest is sent with the message
• Three authentication approaches:
  • Encrypt the message digest using conventional encryption (shared key)
  • Encrypt the message digest using public-key encryption
  • Avoid encryption altogether

Advantages of One-Way Hash Function over encryption

• Less computation
• Encryption software is slow
• Encryption hardware can be expensive