TLS HMAC Verification: Can it be done?

TL;DR

No, a TLS HMAC (Hash-based Message Authentication Code) cannot reliably be used after the fact to verify the authenticity of a message once the TLS connection has ended. The HMAC is tied to the specific session keys and parameters negotiated during that connection. Reusing it without those details is insecure.

Why HMACs are Session-Specific

TLS uses HMACs (typically based on SHA-256 or similar) as part of its record layer security. These HMACs aren’t like a general-purpose signature you can apply and verify at any time. They depend on several things that change with each TLS session:

• Session Keys: The master secret, which is used to derive the encryption and MAC keys, is unique for each connection.
• Initialization Vectors (IVs): These are random values used in conjunction with the keys to ensure different messages produce different HMACs even if they’re identical.
• Sequence Numbers: TLS includes sequence numbers to prevent replay attacks. The HMAC calculation incorporates these numbers.

Without knowing *all* of these details from the original connection, you can’t accurately verify an HMAC.

Step-by-step Explanation

1. HMAC Generation During TLS: When a message is sent over a secure TLS connection, the sending side calculates an HMAC using:
   • The message itself.
   • A session key derived from the master secret.
   • An IV (initialization vector).
   • Sequence numbers.

   The HMAC is then appended to the message and sent.

2. HMAC Verification During TLS: The receiving side performs the *same* calculation using:
   • The received message.
   • The same session key (shared secret).
   • The same IV.
   • The same sequence numbers.

   If the calculated HMAC matches the received HMAC, the message is considered authentic.

3. Why After-the-Fact Verification Fails: If you try to verify an HMAC after the TLS connection has closed, you won’t have access to:
   • The session key. It’s destroyed when the connection ends.
   • The IV used for that specific message.
   • The correct sequence number.

What if I have the Session Key?

Even *with* the session key, after-the-fact verification is generally unsafe. While you could recalculate the HMAC, you still need to know the IV and sequence number used for that specific message. Reconstructing these accurately is extremely difficult and prone to errors.

Alternatives for Message Authentication

If you need to verify message authenticity outside of a TLS connection, use standard digital signature schemes:

• Digital Signatures (e.g., RSA, ECDSA): These involve using private/public key pairs. The sender signs the message with their private key, and the receiver verifies it with the corresponding public key.

  # Example using OpenSSL to sign a message
  openssl dgst -sha256 -sign private_key.pem -out signature.bin message.txt
  

• HMACs with Shared Secrets (e.g., HMAC-SHA256): If you have a pre-shared secret key, you can use an HMAC algorithm to generate and verify MACs.

  import hmac
  hash = hmac.new(b'your_secret_key', msg.encode('utf-8'), hashlib.sha256)
  digested = hash.digest()
  

These methods are designed for independent verification and don’t rely on the ephemeral nature of TLS session keys.