Message Authentication Code(MAC)

Minghong Gao

4 min readSep 15, 2019

Purpose:

Integrity
Validation

Level of functionality:

Lower level: the function that produces an authenticator
Higher-level: a higher level protocol that enables a receiver to verify the authenticity of a message

Functions:

Hash functions: A function that maps a message of any length into a fixed-length hash value which serves as the authenticator
Message encryption: The ciphertext of the entire message serves as its authenticator
Message authentication code (MAC): A function of the message and a secret key that produces a fixed-length value that serves as the authenticator

Message Security Requirements:

disclosure
traffic analysis
masquerade
content modification
sequence modification
timing modification
source repudiation
destination repudiation

Message Encryption

Note: Using Private key(no matter it is symmetric or asymmetric key) for encryption ensure that the sender is indeed sender, which provides authentication. Using Private key for decryption ensure that only receiver can decrypt message, which provides confidentiality.

While Private key(used only in public-cryptography), has the ability to provide signature. That’s the only way to ensure non-repudiation.

That’s why for (a), both sender and receiver use private key to encrypt/decrypt ensures both authentication and confidentiality.

That’s why for (b), the sender use other’s public key does not provide authentication. But the receiver use private key to provide confidentiality.

Message Authentication Code(MAC)

Generated by an algorithm that creates a small fixed-sized block
Provides assurance that message is unaltered and comes from sender
Receiver performs same computation on message and checks it matches the MAC

Error Control

Properties

a cryptographic checksum MAC = Cₖ (M)
many-to-one function

Attacks

Message replacement attacks

Brute force attacks:

Requires known message-tag pairs, A brute-force method of finding a collision is to pick a random bit string y and check if H(y) = H(x)

Two lines of attack:

Attack the key space: If an attacker can determine the MAC key then it is possible to generate a valid MAC value for any input x
Attack the MAC value: Objective is to generate a valid tag for a given message or to find a message that matches a given tag

Crypt-analysis: Being weaker with respect to certain parts:

much more variety in the structure of MACs than in hash functions, , so it is difficult to generalize about the crypt-analysis of MACs

HMAC

Keyed Hash Functions as MACs(Proposal) -> MACs Based on Hash Functions: HMAC

It uses hash function on the message: HMAC K (M)= Hash[(K⁺ XOR opad) || Hash[(K⁺ XOR ipad) || M)] ] where K⁺ is the key padded out to size, and opad, ipad are specified padding constants

Can be used for any hash functions

Security is depending on the hash function

Using Symmetric Ciphers for MACs

can use any block cipher chaining mode and use final block as a MAC
Data Authentication Algorithm (DAA) is a widely used MAC based on DES-CBC.(send just the final block as the MAC)

but final MAC is now too small for security

Cipher-Based Message Authentication Code (CMAC)

Authenticated Encryption

Definition: A term used to describe encryption systems that simultaneously protect confidentiality and authenticity of communications

Approaches:
Hash-then-encrypt: E(K, (M || h))
MAC-then-encrypt: T = MAC(K₁ , M), E(K₂ , [M || T])
Encrypt-then-MAC: C = E(K₂ , M), T = MAC(K₁ , C)
Encrypt-and-MAC: C = E(K₂ , M), T = MAC(K₁ , M)

Counter with Cipher Block Chaining- Message Authentication Code (CCM)