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f25124338c
It's mostly a filler word. I've read through each use of it in the code base and did minor rephrasings when "simply" carried some meaning. The overwhelming majority of cases, removing it improved the text significantly. Inspired by #20793. Closes #20822
162 lines
6.8 KiB
Markdown
162 lines
6.8 KiB
Markdown
<!--
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Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
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SPDX-License-Identifier: curl
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-->
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# TLS Sessions and Tickets
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The TLS protocol offers methods of "resuming" a previous "session". A
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TLS "session" is a negotiated security context across a connection
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(which may be via TCP or UDP or other transports.)
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By "resuming", the TLS protocol means that the security context from
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before can be fully or partially resurrected when the TLS client presents
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the proper crypto stuff to the server. This saves on the amount of
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TLS packets that need to be sent back and forth, reducing amount
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of data and even latency. In the case of QUIC, resumption may send
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application data without having seen any reply from the server, hence
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this is named 0-RTT data.
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The exact mechanism of session tickets in TLSv1.2 (and earlier) and
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TLSv1.3 differs. TLSv1.2 tickets have several weaknesses (that can
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be exploited by attackers) which TLSv1.3 then fixed. See
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[Session Tickets in the real world](https://words.filippo.io/we-need-to-talk-about-session-tickets/)
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for an insight into this topic.
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These difference between TLS protocol versions are reflected in curl's
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handling of session tickets. More below.
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## curl's `ssl_peer_key`
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In order to find a ticket from a previous TLS session, curl
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needs a name for TLS sessions that uniquely identifies the peer
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it talks to.
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This name has to reflect also the various TLS parameters that can
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be configured in curl for a connection. We do not want to use
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a ticket from an different configuration. Example: when setting
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the maximum TLS version to 1.2, we do not want to reuse a ticket
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we got from a TLSv1.3 session, although we are talking to the
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same host.
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Internally, we call this name a `ssl_peer_key`. It is a printable
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string that carries hostname and port and any non-default TLS
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parameters involved in the connection.
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Examples:
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- `curl.se:443:CA-/etc/ssl/cert.pem:IMPL-GnuTLS/3.8.7` is a peer key for
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a connection to `curl.se:443` using `/etc/ssl/cert.pem` as CA
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trust anchors and GnuTLS/3.8.7 as TLS backend.
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- `curl.se:443:TLSVER-6-6:CA-/etc/ssl/cert.pem:IMPL-GnuTLS/3.8.7` is the
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same as the previous, except it is configured to use TLSv1.2 as
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min and max versions.
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Different configurations produce different keys which is what curl needs when
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handling SSL session tickets.
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One important thing: peer keys do not contain confidential information. If you
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configure a client certificate or SRP authentication with username/password,
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these are not part of the peer key.
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Peer keys carry the hostnames you use curl for. They *do* leak the privacy of
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your communication. We recommend to *not* persist peer keys for this reason.
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**Caveat**: The key may contain filenames or paths. It does not reflect the
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*contents* in the file system. If you change `/etc/ssl/cert.pem` and reuse
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a previous ticket, curl might trust a server which no longer has a root
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certificate in the file.
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## Session Cache Access
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#### Lookups
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When a new connection is being established, each SSL connection filter creates
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its own peer_key and calls into the cache. The cache then looks for a ticket
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with exactly this peer_key. Peer keys between proxy SSL filters and SSL
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filters talking through a tunnel differ, as they talk to different peers.
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If the connection filter wants to use a client certificate or SRP
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authentication, the cache checks those as well. If the cache peer carries
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client cert or SRP auth, the connection filter must have those with the same
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values (and vice versa).
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On a match, the connection filter gets the session ticket and feeds that to
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the TLS implementation which, on accepting it, tries to resume it for a
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shorter handshake. In addition, the filter gets the ALPN used before and the
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amount of 0-RTT data that the server announced to be willing to accept. The
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filter can then decide if it wants to attempt 0-RTT or not. (The ALPN is
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needed to know if the server speaks the protocol you want to send in 0-RTT. It
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makes no sense to send HTTP/2 requests to a server that only knows HTTP/1.1.)
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#### Updates
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When a new TLS session ticket is received by a filter, it adds it to the
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cache using its peer_key and SSL configuration. The cache looks for
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a matching entry and, should it find one, adds the ticket for this
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peer.
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### Put, Take and Return
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when a filter accesses the session cache, it *takes*
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a ticket from the cache, meaning a returned ticket is removed. The filter
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then configures its TLS backend and *returns* the ticket to the cache.
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The cache needs to treat tickets from TLSv1.2 and 1.3 differently. 1.2 tickets
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should be reused, but 1.3 tickets SHOULD NOT (RFC 8446). The session cache
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drops 1.3 tickets when they are returned after use, but keeps a 1.2
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ticket.
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When a ticket is *put* into the cache, there is also a difference. There
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can be several 1.3 tickets at the same time, but only a single 1.2 ticket.
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TLSv1.2 tickets replace any other. 1.3 tickets accumulate up to a max
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amount.
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By having a "put/take/return" we reflect the 1.3 use case nicely. Two
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concurrent connections do not reuse the same ticket.
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## Session Ticket Persistence
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#### Privacy and Security
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As mentioned above, ssl peer keys are not intended for storage in a file
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system. They clearly show which hosts the user talked to. This is not only
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privacy relevant, but also has security implications as an attacker might find
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worthy targets among your peer keys.
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Also, we do not recommend to persist TLSv1.2 tickets.
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### Salted Hashes
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The TLS session cache offers an alternative to storing peer keys:
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it provides a salted SHA256 hash of the peer key for import and export.
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#### Export
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The salt is generated randomly for each peer key on export. The SHA256 makes
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sure that the peer key cannot be reversed and that a slightly different key
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still produces a different result.
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This means an attacker cannot "grep" a session file for a particular entry,
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e.g. if they want to know if you accessed a specific host. They *can* however
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compute the SHA256 hashes for all salts in the file and find a specific entry.
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They *cannot* find a hostname they do not know. They would have to brute force
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by guessing.
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#### Import
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When session tickets are imported from a file, curl only gets the salted
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hashes. The imported tickets belong to an *unknown* peer key.
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When a connection filter tries to *take* a session ticket, it passes its peer
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key. This peer key initially does not match any tickets in the cache. The
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cache then checks all entries with unknown peer keys if the passed key matches
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their salted hash. If it does, the peer key is recovered and remembered at the
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cache entry.
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This is a performance penalty in the order of "unknown" peer keys which
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diminishes over time when keys are rediscovered. Note that this also works for
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putting a new ticket into the cache: when no present entry matches, a new one
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with peer key is created. This peer key then no longer bears the cost of hash
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computes.
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