How do you update or rotate TLS session resumption keys and tickets without breaking existing sessions?

1 What are session resumption keys and tickets?

Session resumption keys and tickets are mechanisms that enable clients and servers to resume a TLS session without performing a full handshake. A session resumption key is a symmetric key derived from the master secret of a previous session, and stored in the server's cache. A session resumption ticket is a blob of encrypted data that contains the session state, and sent by the server to the client. The client can present the ticket to the server in a subsequent connection to resume the session.

2 Why update or rotate session resumption keys and tickets?

Updating or rotating session resumption keys and tickets is important for security and compliance reasons. If a session resumption key or ticket is compromised, an attacker can use it to decrypt or hijack the resumed session. Therefore, it is recommended to limit the lifetime and reuse of these mechanisms, and to replace them with fresh ones periodically. Additionally, some regulations and standards, such as PCI DSS and NIST, require session resumption keys and tickets to be rotated at least every 24 hours.

3 How to update or rotate session resumption keys and tickets?

There are different ways to update or rotate session resumption keys and tickets, depending on the type of session resumption mechanism used. For session resumption with session IDs, the server can simply generate a new random session ID for each new session, and delete the old ones from its cache. For session resumption with session tickets, the server can use a key rotation algorithm, such as the one described in RFC 5077, to encrypt and decrypt the tickets with different keys over time. The server can also send new tickets to the client after each successful resumption.

4 How to manage session resumption cache size and lifetime?

Managing session resumption cache size and lifetime is important for performance and scalability reasons. If the cache is too large or too long-lived, it can consume more memory and CPU resources, and increase the risk of cache collisions and stale entries. If the cache is too small or too short-lived, it can reduce the benefits of session resumption, and cause more full handshakes and network latency. Therefore, it is advisable to tune the cache size and lifetime according to the expected traffic patterns and resumption rates of the server.

6 How to test and monitor session resumption?

Testing and monitoring session resumption is essential for verifying its functionality and effectiveness. There are various tools and methods that can be used for this purpose, such as openssl s_client, curl, sslyze, or Wireshark. These tools can help to check if the server supports session resumption, what type of mechanism it uses, how long the keys and tickets are valid, how often they are reused, and how much time they save compared to a full handshake. Additionally, these tools can help to identify any errors or anomalies that may affect the session resumption process.