| Internet-Draft | TLS Cross-SNI Resumption | May 2021 |
| Vasiliev | Expires 14 November 2021 | [Page] |
This document specifies a way for the parties in the Transport Layer Security (TLS) protocol to indicate that an individual session ticket can be used to perform resumption even if the Server Name of the new connection does not match the Server Name of the original.¶
This note is to be removed before publishing as an RFC.¶
Discussion of this document takes place on the TLS Working Group mailing list (tls@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/tls/.¶
Source for this draft and an issue tracker can be found at https://github.com/vasilvv/tls-cross-sni-resumption.¶
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Transport Layer Security protocol [RFC8446] allows the clients to use an abbreviated handshake in cases where the client has previously established a secure session with the same server. This mechanism is known as "session resumption", and its positive impact on performance makes it desirable to be able to use it as frequently as possible.¶
Modern application-level protocols, HTTP in particular, often require accessing multiple servers within a single workflow. Since the identity of the server is established through its certificate, in the ideal case, the resumption would be possible to all of the domains for which the certificate is valid (see [PERF] for a survey of potential practical impact of such approach). TLS, starting with version 1.3, defines the SNI value to be a property of an individual connection that is not retained across sessions ([RFC8446], Section 4.2.11). However, in the absence of additional signals, it discourages using a session ticket when the SNI value does not match ([RFC8446], Section 4.6.1), as there is normally no reason to assume that all servers sharing the same certificate would also share the same session keys. The extension defined in this document allows the server to provide such a signal in-band.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
Resumption across server names is negotiated using the TLS flags extension [I-D.draft-ietf-tls-tlsflags]. The server MAY send a resumption_across_names(8) flag in a NewSessionTicket message. If the flag is sent, it indicates that the client MAY use the ticket for any SNI value for which the certificate presented by the server is valid. The server MUST handle the ticket correctly by either resuming and using a new SNI provided by the client, or by ignoring the ticket.¶
The server MAY send the flag if it reasonably believes that any server for any identity presented in its certificate would be capable of accepting that ticket. The server SHOULD NOT send the flag otherwise, since, if the client follows the single-use ticket policy recommended by [RFC8446], sending the ticket results in it being no longer usable regardless of whether resumption has succeeded.¶
This document does not alter any of the security requirements of [RFC8446], but merely lifts a performance-motivated "SHOULD NOT" recommendation from Section 4.6.1. Notably, it still relies on the server certificate being re-validated against the new SNI at the session resumption time.¶
If a client certificate has been associated with the session, the client MUST use the same policy on whether to present said certificate to the server as if it were a new TLS session. For instance, if the client would show a certificate choice prompt for every individual domain it connects to, it MUST show that prompt for the new host when performing cross-domain resumption.¶
Cross-domain resumption, like other similar mechanisms (e.g. cross-domain HTTP connection reuse), can incentivize the server deployments to create server certificates valid for a wider range of domains than they would otherwise. However, any increase in the scope of a certificate comes at a cost: the wider is the scope of the certificate, the wider is the impact of the key compromise for that certificate. In addition, creating a certificate that is valid for multiple hostnames can lead to complications if some of those hostnames change ownership, or otherwise require a different operational domain.¶
Session tickets can contain arbitrary information, and thus could be potentially used to re-identify a user from a previous connection. Cross-domain resumption expands the potential list of servers to which an individual ticket could be presented. Client applications should partition the session cache between connections that are meant to be uncorrelated. For example, the Web use case uses network partition keys to separate cache lookups [FETCH].¶
IANA (will add/has added) the following entry to the "TLS Flags" table of the "Transport Layer Security (TLS) Extensions" registry:¶
Cross-name resumption has been previously implemented in the QUIC Crypto protocol as a preloaded list of hostnames.¶
Erik Sy has previously proposed a similar mechanism for TLS, draft-sy-tls-resumption-group. This document incorporates ideas from that draft.¶
This document has benefited from contributions and suggestions from David Benjamin, Nick Harper, David Schinazi, Ryan Sleevi, Ian Swett and many others.¶