[Day 5] 6. Message Authentication Code

Why is MAC Used?

Imagine you're sending a box of chocolates to a friend through a delivery service. Along with the box, you attach a unique lock (the MAC) that only you and your friend can open using the same key (symmetric key).

• The lock ensures:
  • Integrity: The chocolates haven't been replaced or tampered with during delivery.
  • Authentication: Only your friend, who has the same key, can confirm it came from you.

If someone else tries to replace the chocolates, they can't re-lock the box without your unique key.

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Technical Explanation

• A MAC (Message Authentication Code) is like the lock in the example. It is a code generated by combining a secret key and the message (data).
• The receiver uses the same secret key to verify that the code matches the message.
• If the code doesn't match, the message has been altered or doesn't come from the expected sender.

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**Types of MAC **

1. Hash MAC (HMAC)

• Imagine This: Instead of a physical lock, you write a secret "signature" (hash value) on the delivery note. Only your friend knows how to recreate the signature using their secret formula (hash function + key).

• How It Works (Technically):

  1. The sender combines the message with a key and calculates a hash value (a fixed-size digital fingerprint).
  2. The hash value (HMAC) is sent along with the message.
  3. The receiver:

     • Combines their copy of the key with the received message.

     • Calculates the hash value again and compares it to the received HMAC.

  • If it matches: The message is authentic and hasn't been changed.
  • Key Points: HMAC ensures data integrity but relies on the shared key for trust.

2. Cipher Block Chaining MAC (CBC-MAC)

• Imagine This: Instead of a signature, you use a high-security lock (encryption algorithm) to secure the box. This lock is stronger and uses more complex techniques.

• How It Works (Technically):

  • It uses block ciphers like AES in Cipher Block Chaining mode:

    1. The message is split into blocks.
    2. Each block is processed (encrypted) using the key.
    3. The result of the last block is the MAC.

  • Key Points:

    • Provides stronger security for authentication.
    • Useful for systems that require high levels of protection.

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Digital Signature

What is a Digital Signature?

• A digital signature ensures both data integrity and authentication using public-key cryptography.
• The hash value of the message is encrypted using the sender’s private key, and the receiver verifies it using the sender’s public key.

Example: Tinku and Pinki Communicating

1. Keys Setup:

   • Tinku has a private key and a public key.
   • Pinki has her own private key and public key.
   • Both share their public keys with each other.

2. Tinku Sends a Message:

   • Tinku creates a message M1.
   • Calculates the hash of the message (H1).
   • Encrypts H1 using his private key to generate the digital signature.
   • Sends the M1 (message) and the digital signature to Pinki.

3. Pinki Verifies the Message:

   • Pinki decrypts the digital signature using Tinku’s public key, obtaining the original hash value (H1).
   • She recalculates the hash value for the received message M1.
   • If both hash values match:
     • The message is authentic (ensured by Tinku’s private key).
     • The message integrity is verified (not tampered in transit).

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Comparison: MAC vs. Digital Signature

|Feature|MAC|Digital Signature| |---|---|---| |Key Type|Symmetric Key|Asymmetric Key (Private/Public)| |Authentication|Assumes shared key provides authentication|Ensures authentication using private key| |Integrity|Ensures integrity|Ensures integrity| |Use Case|Faster, simpler for trusted parties|More secure for untrusted environments|

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