Security Engineering. Ross Anderson

, and uses this to encrypt the message to Bob. On receiving it, Bob looks up Alice's public key and forms which is also equal to , so he can decrypt her message.

Alternatively, Alice and Bob can use transient keys, and get a mechanism for providing forward security. As before, let the prime and generator be common to all users. Alice chooses a random number , calculates and sends it to Bob; Bob does the same, choosing a random number and sending to Alice; they then both form , which they use as a session key (see Figure 5.18).

      Alice and Bob can now use the session key to encrypt a conversation. If they used transient keys, rather than long-lived ones, they have managed to create a shared secret ‘out of nothing’. Even if an opponent had inspected both their machines before this protocol was started, and knew all their stored private keys, then provided some basic conditions were met (e.g., that their random number generators were not predictable and no malware was left behind) the opponent could still not eavesdrop on their traffic. This is the strong version of the forward security property to which I referred in section 5.6.2. The opponent can't work forward from knowledge of previous keys, however it was obtained. Provided that Alice and Bob both destroy the shared secret after use, they will also have backward security: an opponent who gets access to their equipment later cannot work backward to break their old traffic. In what follows, we may write the Diffie-Hellman key derived from and as when we don't have to be explicit about which group we're working in, and don't need to write out explicitly which is the private key and which is the public key .

Figure 5.18: The Diffie-Hellman key exchange protocol

      Slightly more work is needed to provide a full solution. Some care is needed when choosing the parameters and ; we can infer from the Snowden disclosures, for example, that the NSA can solve the discrete logarithm problem for commonly-used 1024-bit prime numbers⁶. And there are several other details which depend on whether we want properties such as forward security.

      But this protocol has a small problem: although Alice and Bob end up with a session key, neither of them has any real idea who they share it with.

      Suppose that in our padlock protocol Caesar had just ordered his slave to bring the box to him instead, and placed his own padlock on it next to Anthony's. The slave takes the box back to Anthony, who removes his padlock, and brings the box back to Caesar who opens it. Caesar can even run two instances of the protocol, pretending to Anthony that he's Brutus and to Brutus that he's Anthony. One fix is for Anthony and Brutus to apply their seals to their locks.

With the Diffie-Hellman protocol, the same idea leads to a middleperson attack. Charlie intercepts Alice's message to Bob and replies to it; at the same time, he initiates a key exchange with Bob, pretending to be Alice. He ends up with a key which he shares with Alice, and another key which he shares with Bob. So long as he continues to sit in the middle of the network and translate the messages between them, they may have a hard time detecting that their communications are compromised. The usual solution is to authenticate transient keys, and there are various possibilities.

In the STU-2 telephone, which is now obsolete but which you can see in the NSA museum at Fort Meade, the two principals would read out an eight-digit hash of the key they had generated and check that they had the same value before starting to discuss classified matters. Something similar is implemented in Bluetooth versions 4 and later, but is complicated by the many versions that the protocol has evolved to support devices with different user interfaces. The protocol has suffered from multiple attacks, most recently the Key Negotiation of Bluetooth (KNOB) attack, which allows a middleperson to force one-byte keys that are easily brute forced; all devices produced before 2018 are vulnerable [125]. The standard allows for key lengths between one and sixteen bytes; as the keylength negotiation is performed in the clear, an attacker can force the length to the lower limit. All standards-compliant chips are vulnerable; this may be yet more of the toxic waste from the Crypto Wars, which I discuss in section 26.2.7. Earlier versions of Bluetooth are more like the ‘just-works’ mode of the HomePlug protocol