Message Authentication and Encryption Concepts

Message Authentication

• Purpose: Protect against active attacks (falsification of data and transactions) by ensuring message authenticity.
• Authentication: Verifying a message's genuineness and source.
• Focus: Ensuring message content hasn't been altered, verifying the source's authenticity, confirming message timeliness (no artificial delays), and checking message sequence.

Authentication Using Conventional Encryption

• Key Concept: Utilize symmetric encryption to ensure message authenticity.
• Shared Key: Only the sender and receiver share a key, guaranteeing that only the legitimate sender can encrypt a message.
• Error Detection: A message's error-detection code, sequence number, and timestamp ensure data integrity and timeliness.

Message Authentication without Message Encryption

• Authentication Tag: An authentication tag is generated and appended to a message, but the message remains unencrypted.
• Benefits:
  • Suitable for scenarios where a message is broadcast to multiple recipients.
  • Allows selective authentication of messages.
  • Enables authentication of computer programs in plaintext, saving processing resources.

Message Authentication Code (MAC)

• Concept: A shared secret key (KAB) between sender (A) and receiver (B) is used to create a small data block (MAC).
• MAC Calculation: MACM = F(KAB, M), where M is the message.
• Verification: The recipient calculates the MAC using the same key and compares it to the received MAC.
• Security:
  • Ensures message integrity since any alteration will result in a different calculated MAC.
  • Authenticates the sender because only parties with the shared key can generate a valid MAC.
  • Protects message sequence if a sequence number is included.

One-Way Hash Function

• Concept: A function that takes a variable-size message as input and produces a fixed-size message digest, H(M).
• Hash Function Properties: No secret key is involved.
• Authentication Methods:
  • Message digest encrypted with conventional encryption (shared key).
  • Message digest encrypted with public-key encryption.
• Benefits of Hash Function:
  • Less computational effort than encrypting the entire message.
  • Provides digital signature and message authentication (in the public-key approach).
  • No need for key distribution.

Reasons for avoiding encryption altogether:

• Performance: The speed of encryption software can be slow.
• Cost: Hardware costs for encryption can be a significant factor.

This quiz covers the essentials of message authentication, focusing on methods to ensure authenticity, integrity, and timeliness of messages. Explore concepts such as conventional encryption, shared keys, and authentication tags to understand how messages can be securely transmitted. Test your knowledge on the techniques that protect data against active attacks.