Overview
Perfect forward secrecy (PFS) is a security feature in cryptographic systems that ensures session keys used to encrypt data are not compromised even if the server's long-term private key is. This is achieved by generating unique session keys for each communication session, which are not derived from the server's private key. As a result, even if an attacker gains access to the server's private key, they cannot decrypt past communications.
How can PFS enhance security?
From a security perspective, PFS provides the two following benefits:
- Private Key Compromise: In the instance that the private key on the server is compromised, then it cannot be used to derive session keys for any communication and thus prevents an attacker from using it to decrypt communication. However, if an attacker has access to the server, depending on permissions they may be able to modify the way the sessions are generated or steal a signing key to impersonate the server itself. In other words, with such a level of access, other attack paths may be exploited instead to achieve confidentiality compromise.
- Session Key Compromise: In the instance that a session key is compromised, then it cannot be used to decrypt neither past nor future communication sessions. The session key may still be used to decrypt the specific communication that the key was used for. This may require the attacker to have saved the whole communication data to then decrypt with the compromised session key. For this reason, PFS is a good security addition, as it limits the impact of the compromised session keys.
Simplified overview of how FPS works:
- Client and server initiate a secure communication session.
- They use an ephemeral key exchange algorithm like Diffie-Hellman Ephemeral (DHE) or Elliptic Curve Diffie-Hellman Ephemeral (ECDHE).
- A unique session key is generated for this specific session.
- The session key is used to encrypt the data exchanged during this session.
- Once the session ends, the session key is discarded.
- For any future sessions, new unique session keys are generated, ensuring no reuse of previous keys.
- If an attacker compromises a session key, they can only decrypt data from that specific session, not past or future sessions.
Attack considerations
Whilst not a weakness of PFS itself, but it is often mentioned when discussing PFS implementation, and that is the cryptographic attacks against the ciphers themselves. Therefore, this is not inherently a weakness of PFS, yet if weak ciphers are used these are more likely to be compromised. Likewise, as with any discussions in cryptography, even with a strong cipher, given sufficient time and computing power it could still be compromised.
Additionally, in non-interactive protocols, a message suppression attack could be carried out. An attacker who can control the network and selectively prevent messages from reaching their intended recipient whilst saving these for future attacks. Since the recipient never received the messages, it may prevent a key from being generated properly, with potential for then the private key being used to decrypt saved data.
Exhaustions attacks may also be possible, where large number of requests can lead to denial of service or the downgrade in the key exchange method.
Conclusion
As with any security measures, given enough resources they may be circumvented. However, PFS adds a lot of benefits to security and if these do not pose an exceptional weight on performance, then implementation of PFS is a great addition to the overall security of communication.
CSA Cyber
