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Security TLS Working Group Internet-Draft 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. Discussion Venues 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.

Introduction Transport Layer Security protocol 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 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 (, Section 4.2.11). However, in the absence of additional signals, it discourages using a session ticket when the SNI value does not match (, 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.

Conventions and Definitions 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 when, and only when, they appear in all capitals, as shown here.

The Flag Resumption across server names is negotiated using the TLS flags extension . The server MAY send a resumption_across_names(8) flag in a NewSessionTicket message; the flag is an assertion by the server that any server for any identity presented in its certificate would be capable of accepting that ticket. A client receiving a ticket with this flag MAY attempt resumption for any server name corresponding to an identity in the server certificate even if the new server name value does not match the one used in the original session; note that this requires the client to retain the list of the names specified in the original server certificate. The flag cannot be used in TLS versions before 1.3, as the NewSessionTicket message does not exist in those versions.

Security Considerations This document does not alter any of the security requirements of , but merely lifts a performance-motivated "SHOULD NOT" recommendation from Section 4.6.1. Notably, it still relies on the client ensuring that the server certificate is valid for the new SNI at the time of session resumption. If the original server's assertion regarding supporting cross-name resumption turns out to be incorrect, a different server that receives a misdirected ticket will not be able to decrypt it and will therefore be unable to resume. The protocol will gracefully recover from such situations, as session resumption may be safely rejected for any reason. However, such misconfiguration will waste tickets stored in the client's cache, as TLS tickets may be single-use, leading to a potential performance regression. When providing the SNI value to the application, TLS 1.3 requires the value from the most recent ClientHello to be used (). If the server TLS implementation violates that requirement and instead reports the SNI value of the original session, this can lead to a confusion attack where the client and the server disagree on the server name being used (similar to the attacks described in ). The implementers MUST ensure that this aspect of SNI processing is handled correctly before enabling cross-name resumption. Cross-domain resumption implies that any certificate the client provides for one host would become available to the other hosts using the same server certificate. Because of that, when performing cross-domain resumption, 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 .

IANA Considerations IANA (will add/has added) the following entry to the "TLS Flags" table of the "Transport Layer Security (TLS) Extensions" registry:

Value

0x8

Flag Name

resumption_across_names

Message

NST

Recommended

N

Reference

This document

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Dell Technologies A number of extensions are proposed in the TLS working group that carry no interesting information except the 1-bit indication that a certain optional feature is supported. Such extensions take 4 octets each. This document defines a flags extension that can provide such indications at an average marginal cost of 1 bit each. More precisely, it provides as many flag extensions as needed at 4 + the order of the last set bit divided by 8. Informative References WHATWG

Acknowledgments 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 Carrick Bartle, David Benjamin, Nick Harper, Eric Rescorla, David Schinazi, Ryan Sleevi, Ian Swett, Martin Thomson, Christopher Wood, and many others.