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
| Feature | Signing (Digital Signatures) | MACs |
|---|---|---|
| Cryptography | Asymmetric (Public/Private Key) | Symmetric (Shared Secret Key) |
| Speed | Slower | Faster |
| Key Management | Complex (PKI, key rotation) | Simpler (Securely share secret keys) |
| Non-Repudiation | Yes – Sender can’t deny signing the message. | No – Both parties have the same key, so either could create a valid MAC. |
| Trust Model | Requires trust in a Certificate Authority (CA) or other PKI infrastructure. | Requires trust that the shared secret remains confidential. |
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.