Signing vs MAC Authentication

TL;DR

Choosing between signing (using digital signatures) and Message Authentication Codes (MACs) depends on your security needs. Signing offers stronger authentication and non-repudiation but is slower and requires more infrastructure (PKI). MACs are faster and simpler, but rely on shared secrets and don’t provide non-repudiation.

1. Understanding the Basics

Both signing and MACs verify that a message hasn’t been tampered with and comes from a trusted source. However, they work differently:

• Signing (Digital Signatures): Uses asymmetric cryptography (public/private key pairs). The sender uses their private key to create a signature. Anyone can verify the signature using the sender’s public key.
• MACs: Use symmetric cryptography (shared secret keys). Both the sender and receiver have the same secret key. The sender creates a MAC based on the message and the shared secret, and the receiver verifies it using the same secret.

2. Key Differences – A Table

3. When to Use Signing

Use signing when:

• Non-repudiation is critical: You need proof of origin and integrity that the sender can’t deny later (e.g., legal documents, financial transactions).
• You don’t have a pre-existing shared secret: Establishing a secure shared secret channel might be difficult or impractical.
• Scalability is important: Public key infrastructure allows for many verifiers without sharing secrets with each one.

Example (Python using RSA):

from Crypto.PublicKey import RSA
from Crypto.Signature import PKCS1_v1_5
from Crypto.Hash import SHA256

# Generate a key pair (in reality, load from secure storage)
key = RSA.generate(2048)
privkey = key.export_key()
pubkey = key.publickey().export_key()

message = b'This is the message to sign.'
hash_object = SHA256.new(message)

signer = PKCS1_v1_5.new(privkey)
hash_value = signer.sign(hash_object)

verifier = PKCS1_v1_5.new(pubkey)
try:
    verifier.verify(hash_object, hash_value)
    print('Signature verified!')
except ValueError:
    print('Signature verification failed!')

4. When to Use MACs

Use MACs when:

• Speed is paramount: You need fast authentication (e.g., network packets, internal system communication).
• You have a secure channel for sharing secrets: Establishing and maintaining a shared secret key is feasible.
• Non-repudiation isn’t required: It’s acceptable if either party could theoretically generate the MAC.

Example (Python using HMAC):

import hmac
hash_key = b'YourSecretKey'
message = b'This is the message to authenticate.'

mac = hmac.new(hash_key, message, hash_key).hexdigest()
print(f'MAC: {mac}')

# Verification (on the receiving end)
received_mac = '...' # The MAC received from the sender
calculated_mac = hmac.new(hash_key, message, hash_key).hexdigest()
if hmac.compare_digest(received_mac, calculated_mac):
    print('MAC verified!')
else:
    print('MAC verification failed!')

5. Important Considerations for cyber security

• Key Length: Use sufficiently long keys for both signing and MACs (e.g., 2048-bit RSA key, 256-bit HMAC key).
• Secure Key Storage: Protect private keys and shared secrets from unauthorized access. Hardware Security Modules (HSMs) are recommended for high security requirements.
• Algorithm Choice: Use strong cryptographic algorithms (e.g., SHA-256 or SHA-3 for hashing, RSA with appropriate padding schemes). Avoid outdated or weak algorithms.
• Salt/Nonce: When using MACs, consider adding a salt or nonce to prevent replay attacks and other vulnerabilities.