# TLS Sessions and Tickets The TLS protocol offers methods of "resuming" a previous "session". A TLS "session" is a negotiated security context across a connection (which may be via TCP or UDP or other transports.) By "resuming", the TLS protocol means that the security context from before can be fully or partially resurrected when the TLS client presents the proper crypto stuff to the server. This saves on the amount of TLS packets that need to be sent back and forth, reducing amount of data and even latency. In the case of QUIC, resumption may send application data without having seen any reply from the server, hence this is named 0-RTT data. The exact mechanism of session tickets in TLSv1.2 (and earlier) and TLSv1.3 differs. TLSv1.2 tickets have several weaknesses (that can be exploited by attackers) which TLSv1.3 then fixed. See [Session Tickets in the real world](https://words.filippo.io/we-need-to-talk-about-session-tickets/) for an insight into this topic. These difference between TLS protocol versions are reflected in curl's handling of session tickets. More below. ## Curl's `ssl_peer_key` In order to find a ticket from a previous TLS session, curl needs a name for TLS sessions that uniquely identifies the peer it talks to. This name has to reflect also the various TLS parameters that can be configured in curl for a connection. We do not want to use a ticket from an different configuration. Example: when setting the maximum TLS version to 1.2, we do not want to reuse a ticket we got from a TLSv1.3 session, although we are talking to the same host. Internally, we call this name a `ssl_peer_key`. It is a printable string that carries hostname and port and any non-default TLS parameters involved in the connection. Examples: - `curl.se:443:CA-/etc/ssl/cert.pem:IMPL-GnuTLS/3.8.7` is a peer key for a connection to `curl.se:443` using `/etc/ssl/cert.pem` as CA trust anchors and GnuTLS/3.8.7 as TLS backend. - `curl.se:443:TLSVER-6-6:CA-/etc/ssl/cert.pem:IMPL-GnuTLS/3.8.7` is the same as the previous, except it is configured to use TLSv1.2 as min and max versions. Different configurations produce different keys which is just what curl needs when handling SSL session tickets. One important thing: peer keys do not contain confidential information. If you configure a client certificate or SRP authentication with username/password, these will not be part of the peer key. However, peer keys carry the hostnames you use curl for. The *do* leak the privacy of your communication. We recommend to *not* persist peer keys for this reason. **Caveat**: The key may contain file names or paths. It does not reflect the *contents* in the filesystem. If you change `/etc/ssl/cert.pem` and reuse a previous ticket, curl might trust a server which no longer has a root certificate in the file. ## Session Cache Access #### Lookups When a new connection is being established, each SSL connection filter creates its own peer_key and calls into the cache. The cache then looks for a ticket with exactly this peer_key. Peer keys between proxy SSL filters and SSL filters talking through a tunnel will differ, as they talk to different peers. If the connection filter wants to use a client certificate or SRP authentication, the cache will check those as well. If the cache peer carries client cert or SRP auth, the connection filter must have those with the same values (and vice versa). On a match, the connection filter gets the session ticket and feeds that to the TLS implementation which, on accepting it, will try to resume it for a shorter handshake. In addition, the filter gets the ALPN used before and the amount of 0-RTT data that the server announced to be willing to accept. The filter can then decide if it wants to attempt 0-RTT or not. (The ALPN is needed to know if the server speaks the protocol you want to send in 0-RTT. It makes no sense to send HTTP/2 requests to a server that only knows HTTP/1.1.) #### Updates When a new TLS session ticket is received by a filter, it adds it to the cache using its peer_key and SSL configuration. The cache looks for a matching entry and, should it find one, adds the ticket for this peer. ### Put, Take and Return when a filter accesses the session cache, it *takes* a ticket from the cache, meaning a returned ticket is removed. The filter then configures its TLS backend and *returns* the ticket to the cache. The cache needs to treat tickets from TLSv1.2 and 1.3 differently. 1.2 tickets should be reused, but 1.3 tickets SHOULD NOT (RFC 8446). The session cache will simply drop 1.3 tickets when they are returned after use, but keep a 1.2 ticket. When a ticket is *put* into the cache, there is also a difference. There can be several 1.3 tickets at the same time, but only a single 1.2 ticket. TLSv1.2 tickets replace any other. 1.3 tickets accumulate up to a max amount. By having a "put/take/return" we reflect the 1.3 use case nicely. Two concurrent connections will not reuse the same ticket. ## Session Ticket Persistence #### Privacy and Security As mentioned above, ssl peer keys are not intended for storage in a file system. They'll clearly show which hosts the user talked to. This maybe "just" privacy relevant, but has security implications as an attacker might find worthy targets among your peer keys. Also, we do not recommend to persist TLSv1.2 tickets. ### Salted Hashes The TLS session cache offers an alternative to storing peer keys: it provides a salted SHA256 hash of the peer key for import and export. #### Export The salt is generated randomly for each peer key on export. The SHA256 makes sure that the peer key cannot be reversed and that a slightly different key still produces a very different result. This means an attacker cannot just "grep" a session file for a particular entry, e.g. if they want to know if you accessed a specific host. They *can* however compute the SHA256 hashes for all salts in the file and find a specific entry. But they *cannot* find a hostname they do not know. They'd have to brute force by guessing. #### Import When session tickets are imported from a file, curl only gets the salted hashes. The tickets imported will belong to an *unknown* peer key. When a connection filter tries to *take* a session ticket, it will pass its peer key. This peer key will initially not match any tickets in the cache. The cache then checks all entries with unknown peer keys if the passed key matches their salted hash. If it does, the peer key is recovered and remembered at the cache entry. This is a performance penalty in the order of "unknown" peer keys which will diminish over time when keys are rediscovered. Note that this also works for putting a new ticket into the cache: when no present entry matches, a new one with peer key is created. This peer key will then no longer bear the cost of hash computes.