Message Authentication Codes and Digital Signatures Explained

Message Authentication Code (MAC)

A Message Authentication Code (MAC) is a small block of data generated using a secret key and the original message. It is used to ensure data integrity and authenticity.

• The sender computes the MAC using a secret key and sends it along with the message.
• The receiver re-computes the MAC using the same key and compares it with the received MAC.
• If they match, the message is considered authentic and unaltered.

MAC vs. Hash Function

MAC uses a key, while a regular hash function does not.

Common MAC Algorithms

• HMAC – Based on cryptographic hash functions (e.g., SHA).
• CBC-MAC – Uses block cipher in CBC mode.

Digital Signatures: Integrity and Non-Repudiation

A Digital Signature is a cryptographic technique used to ensure the authenticity, integrity, and non-repudiation of digital data. It serves as a virtual equivalent of a handwritten signature or a stamped seal but is significantly more secure due to the use of encryption.

Objectives of Digital Signatures

1. Authentication: To verify the identity of the sender.
2. Integrity: To ensure that the data has not been altered during transmission.
3. Non-repudiation: To prevent the sender from denying their involvement in the transaction.

Working Process of Digital Signatures

The process of creating and verifying a digital signature involves the following steps:

1. Hashing the Message: The sender applies a cryptographic hash function (e.g., SHA-256) to the original message, generating a fixed-length hash value (message digest).
2. Encrypting the Hash with Private Key: The sender encrypts the hash value using their private key. The resulting encrypted hash is called the Digital Signature.
3. Sending the Message and the Signature: The sender transmits both the original message and the digital signature to the receiver.
4. Verifying the Signature: The receiver decrypts the digital signature using the sender's public key to retrieve the original hash value and then generates a new hash from the received message using the same hash function.
5. Comparing the Hashes: If the hash obtained from the message matches the decrypted hash, the signature is considered valid. Otherwise, it indicates tampering.

Security Guarantees Provided by Digital Signatures

1. Authentication: Verifies the sender’s identity using their private key.
2. Integrity: Ensures the message has not been modified during transmission.
3. Non-repudiation: Prevents the sender from denying the act of signing.