Wednesday, August 21, 2013

Essence of a secure socket protocol

This evening I went back and read RFC 5246 Transport Layer Security Protocol 1.2 just for the heck of it. It is pretty complex. Pages 7 through 14 define how to serialize messages. Pages 15 through 63 define the structure of different message types passed between server and client and their meaning. The remaining few pages define how client and server may use a shared secret to exchanging data.

A secure socket protocol really boils down to this. Alice and Bob wish to talk to each other, so they send each other their public keys. The public key is used to encrypt a session key for the recipient which is subsequently used to encrypt the actual message from sender. After receiving each other's public keys, they send the encrypted session key, and encrypted data would follow.
Phase
Alice
Bob
1
Sends public key to Bob.Sends public key to Alice.
2
On receipt of Bob's public key, tells Bob about how subsequent messages will be encrypted (session key). On receipt of Alice's public key, tells Alice about how subsequent messages will be encrypted (session key).
3
Starts sending ciphertext to Bob.Starts sending ciphertext to Alice.
Since TLS is layered on top of TCP, phase one begins after the TCP 3-way handshake, and TLS handshake is at least 6 ways. In total this takes 9 one-way trips or 4.5 round trips.

Phase 1 can be used to advertise the capabilities of the owner of the public key, so that the peer can choose an appropriate encryption and/or compression method. TLS also defines a mechanism to resume the use of a previous session key. This can be folded into Phase 1 as an option. The additional information sent for Phase 1 and 2 must be signed by the sender's public key. Authenticity and integrity of phase 3 communication is done using HMAC with the session key.

Here are some more ideas if I were to design my secure socket protocol from scratch.

A lower level secure socket protocol can embed Phase one with the SYN and Phase 2 with the ACK. If Alice is the client and Bob is the server, Alice would send her public key in SYN, Bob would respond with his public key as well as session key for alice in SYN-ACK, and then Alice will send session key for Bob in ACK. Then both can start communicating in ciphertext.

Notice that Phase 3 does not have to wait for Phase 2. Sender can immediately start sending ciphertext as soon as the session key is sent. This allows us to achieve just 1 round-trip. However, potential for network packet loss means that the sender has to be ready to resend the session key. It is probably easier implementation wise for Phase 3 to wait.

Rather than sending the public key all the times, a variation is to send a key-digest instead. If the recipient does not know the sender's key, then an additional round trip is required to obtain the key from sender. For a 4096-bit public key, this can save 512 bytes for the SYN. This is not a lot given today's high speed network.

Either party may terminate the connection early if:
  • It does not trust the peer's public key.
  • It does not understand how subsequent messages could be decrypted.
To ensure ciphertext authenticity and integrity, append HMAC to plaintext to be encrypted together. This makes it harder for a third-party to verify if they decrypted the message successfully. The receiver will decrypt first and then verify the HMAC.

The new secure socket would require extending the BSD socket API as follows:

  • connect_ex() is like connect() but allows passing arbitrary data, limited by single packet size, as part of the SYN. Upon return, the caller also gets the SYN data from the peer.
  • accept_ex() is like accept() but allows retrieving SYN data from peer as well as send its own, again limited by single packet size. This function might also take a callback which decides whether to accept or reject the peer's SYN data before sending our own.
The application layer implements its own encryption and compression so it can decide when symmetric key exchange ends and ciphertext communication starts.
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