Network Working Group Y. Sheffer
Internet-Draft Intuit
Intended status: Standards Track February 06, 2016
Expires: August 9, 2016

TLS Server Identity Pinning with Tickets


Fake public-key certificates are an ongoing problem for users of TLS. Several solutions have been proposed, but none is currently in wide use. This document proposes to extend TLS with opaque tickets, similar to those being used for TLS session resumption, as a way to pin the server’s identity. That is, to ensure the client that it is connecting to the right server even in the presence of corrupt certificate authorities and fake certificates. The main advantage of this solution is that no manual management actions are required.

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Table of Contents

1. Introduction

The weaknesses of the global PKI system are by now widely known. Essentially, any valid CA may issue a certificate for any organization without the organization’s approval (a misissued or “fake” certificate), and use the certificate to impersonate the organization. There are many attempts to resolve these weaknesses, including Certificate Transparency (CT) [RFC6962], HTTP Public Key Pinning (HPKP) [RFC7469], and TACK [I-D.perrin-tls-tack]. CT requires cooperation of a large portion of the hundreds of extant certificate authorities (CAs) before it can be used “for real”, in enforcing mode. It is noted that the relevant industry forum (CA/Browser Forum) is indeed pushing for such extensive adoption. TACK has some similarities to the current proposal, but work on it seems to have stalled. Section 6.2 compares our proposal to TACK. HPKP is a standard, but so far has proven hard to deploy (see Section 6.1). This proposal augments these mechanisms with a much easier to implement and deploy solution for server identity pinning, by reusing some of the mechanisms behind TLS session resumption.

When a client first connects to a server, the server responds with a ticket and a committed lifetime. The ticket is modeled on the session resumption ticket, but is distinct from it. Specifically, the ticket acts as a “second factor” for proving the server’s identity; the ticket does not authenticate the client. The committed lifetime indicates for how long the server promises to retain the server-side ticket-encryption key, which allows it to complete the protocol exchange correctly and prove its identity. The committed lifetime is typically on the order of weeks or months. We follow the Trust On First Use (TOFU) model, in that the first server authentication is only based on PKI certificate validation, but for any follow-on sessions, the client is further ensuring the server’s identity based on the server’s ability to decrypt the ticket and complete the handshake correctly.

This version of the draft only discusses TLS 1.3. We believe that the idea can also be back-fitted into earlier versions of the protocol.

The main advantages of this protocol over earlier pinning solutions are:

A note on terminology: unlike other solutions in this space, we do not do “certificate pinning” (or “public key pinning”), since the protocol is oblivious to the server’s certificate. We prefer the term “server identity pinning” for this new solution.

1.1. Conventions used in this document

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC2119].

2. Protocol Overview

The protocol consists of two phases: the first time a particular client connects to a server, and subsequent connections.

This protocol supports full TLS handshakes, as well as 0-RTT handshakes. Below we present it in the context of a full handshake, but behavior in 0-RTT handshakes should be identical.

The preshared key (PSK) variant of TLS 1.3 is orthogonal to this protocol. A TLS session can be established using PKI and a pinning ticket, and later resumed with PSK. The PSK handshake MUST NOT include the extension defined here.

2.1. Initial Connection

When a client first connects to a server, it requests a pinning ticket by sending an empty PinningTicket extension, and receives it as part of the server’s first response, in the returned PinningTicket extension.

 Client                                               Server

   + key_share
   + PinningTicket         -------->
                                                 + key_share
                                            + PinningTicket}
                           <--------              {Finished}
 {Finished}                -------->
 [Application Data]        <------->      [Application Data]

        *  Indicates optional or situation-dependent
           messages that are not always sent.

        {} Indicates messages protected using keys
           derived from the ephemeral secret.

        [] Indicates messages protected using keys
           derived from the master secret.

The server computes a pinning_secret value (Section 4.1) in order to generate the ticket. When the connection setup is complete, the client computes the same pinning_secret value and saves it locally, together with the received ticket.

The client SHOULD cache the ticket and the pinning_secret for the lifetime received from the server. The client MUST forget these values at the end of this duration.

The returned ticket is sent as a ServerHello protected extension, and MUST NOT be sent as part of a HelloRetryRequest.

2.2. Subsequent Connections

When the client initiates a connection to a server it has previously seen (see Section 2.3 on identifying servers and origins), it SHOULD send the pinning ticket for that server.

The server MUST extract the original pinning_secret from the ticket and MUST respond with a PinningTicket extension, which includes:

If the server cannot validate the ticket, that might indicate an earlier MITM attack on this client. The server MUST then abort the connection with a handshake_failure alert, and SHOULD log this failure.

The client MUST verify the proof, and if it fails to do so, MUST issue a handshake_failure alert and abort the connection (see also Section 7.5). When the connection is successfully set up, the client SHOULD store the new ticket along with the corresponding pinning_secret.

Although this is an extension, if the client already has a ticket for a server, the client MUST interpret a missing PinningTicket extension in the server’s response as an attack, because of the server’s prior commitment to respect the ticket. The client MUST abort the connection in this case. See also Section 5.4 on ramping down support for this extension.

2.3. Indexing the Pins

Each pin is associated with a host name, protocol (TLS or DTLS) and port number. In other words, the pin for port TCP/443 may be different from that for DTLS or from the pin for port TCP/8443. The host name MUST be the value sent inside the Server Name Indication (SNI) extension. This definition is similar to a Web Origin [RFC6454], but does not assume the existence of a URL.

IP addresses are ephemeral and forbidden in SNI and therefore Pins MUST NOT be associated with IP addresses.

3. Message Definitions

This section defines the format of the PinningTicket extension. We follow the message notation of [I-D.ietf-tls-tls13].

 opaque pinning_ticket<0..2^16-1>;

 opaque pinning_proof<0..2^8-1>;

 struct {
   select (Role) {
     case client:
       pinning_ticket ticket<0..2^16-1>; //omitted on 1st connection

     case server:
       pinning_proof proof<0..2^8-1>; //no proof on 1st connection
       pinning_ticket ticket<0..2^16-1>; //omitted on ramp down
       uint32 lifetime;
} PinningTicketExtension;

a pinning ticket sent by the client or returned by the server. The ticket is opaque to the client. The extension MUST contain exactly 0 or 1 tickets.
a demonstration by the server that it understands the ticket and therefore that it is in possession of the secret that was used to generate it originally. The proof is further bound to the server’s public key to prevent some MITM attacks. The extension MUST contain exactly 0 or 1 proofs.
the duration (in seconds) that the server commits to accept the newly offered ticket in the future. This period MUST be at least 604800 (one week).

4. Cryptographic Operations

This section provides details on the cryptographic operations performed by the protocol peers.

4.1. Pinning Secret

On each connection that includes the PinningTicket extension, both peers derive the the value pinning_secret from the shared Diffie Hellman secret. They compute:

pinning_secret = HKDF(xSS, xES, "pinning secret", L)

using the notation of [I-D.ietf-tls-tls13], sec. Key Schedule. This secret is used by the server to generate the new ticket that it returns to the client.

4.2. Pinning Ticket

The pinning ticket’s format is not specified by this document, but it MUST be encrypted and integrity-protected using a long-term pinning-ticket protection key. The server MUST rotate the protection key periodically, and therefore the ticket MUST contain a protection key ID or serial number. The ticket MUST allow the server to recover the pinning_secret value, and MAY include additional information.

As noted in Section 5.1, if the server is actually a cluster of machines, the protection key MUST be synchronized between them. An easy way to do it is to derive it from the session-ticket protection key, which is already synchronized. For example:

pinning_protection_key = HKDF(0, resumption_protection_key,
                              "pinning protection", L)

4.3. Pinning Proof

The proof sent by the server consists of this value:

proof = HMAC(original_pinning_secret, "pinning proof" + '\0' +
             client.random + server.random + Hash(server_public_key))

where HMAC [RFC2104] uses the Hash algorithm for the handshake, and the same hash is also used over the server’s public key.

5. Operational Considerations

The main motivation behind the current protocol is to enable identity pinning without the need for manual operations. Manual operations are susceptible to human error and in the case of public key pinning, can easily result in “server bricking”: the server becoming inaccessible to some or all of its users.

5.1. Protection Key Synchronization

The only operational requirement when deploying this protocol is that if the server is part of a cluster, protection keys (the keys used to encrypt tickets) MUST be synchronized between all cluster members. The protocol is designed so that if resumption ticket protection keys [RFC5077] are already synchronized between cluster members, nothing more needs to be done.

Moreover, synchronization does not need to be instantaneous, e.g. protection keys can be distributed a few minutes or hours in advance of their rollover.

5.2. Certificate Renewal

The protocol ensures that the client will continue speaking to the correct server even when the server’s certificate is renewed. In this sense, we are not “pinning certificates” and the protocol should more precisely be called “server identity pinning”.

5.3. Certificate Revocation

The protocol is orthogonal to certificate validation, in the sense that, if the server’s certificate has been revoked or is invalid for some other reason, the client MUST refuse to connect to it.

5.4. Disabling Pinning

A server implementing this protocol MUST have a “ramp down” mode of operation where:

After a while no clients will hold valid tickets any more and the feature may be disabled.

5.5. Server Compromise

If a server compromise is detected, the pinning secret MUST be rotated immediately, but the server MUST still accept valid tickets that use the old, compromised key. Clients who still hold old pinning tickets will remain vulnerable to MITM attacks, but those that connect to the correct server will immediately receive new tickets.

5.6. Disaster Recovery

All web servers in production need to be backed up, so that they can be recovered if a disaster (including a malicious activity) ever wipes them out. Backup typically includes the certificate and its private key, which must be backed up securely. The pinning secret, including earlier versions that are still being accepted, must be backed up regularly. However since it is only used as an authentication second factor, it does not require the same level of confidentiality as the server’s private key.

Readers should note that [RFC5077] session resumption keys are more security sensitive, and should normally not be backed up but rather treated as ephemeral keys. Even when servers derive pinning secrets from resumption keys (Section 4.1), they MUST NOT back up resumption keys.

6. Previous Work

This section compares ticket pinning to two earlier proposals, HPKP and TACK.

6.1. Comparison: HPKP Deployment

The current IETF standard for pinning the identity of web servers is the Public Key Pinning Extension for HTTP, or HPKP [RFC7469]. Unfortunately HPKP has not seen wide deployment yet. This may simply be due to inertia, but we believe the main reason is the onerous manual certificate management which is needed to implement HPKP for enterprise servers. The penalty for making mistakes (e.g. being too early or too late to deploy new pins) is often bricking the server for some clients.

To demonstrate this point, we present an analysis of what it would take to deploy HPKP for a security-sensitive Web server.

  1. Pin only end-entity certificates. Pinning an intermediate certificate means that the enterprise is at risk if the CA makes sudden operational changes. Pinning the root certificate is useless: it still allows every “brand” (sub-CA) to issue a fake certificate for the servers.
  2. Make sure the default reminder period from the certificate management system is long, e.g. 3 months. This is assuming a pin period (“max age”) of 1 month.
  3. Issue two certificates with the same validity period, the main and a backup one.
  4. Once we get the expiration reminder, issue two new certificates and install the new “main” certificate on servers. Change the HPKP header to send the old main certificate as the main pin (actually, what is sent is the certificate’s SPKI), the new main certificate as the backup, and the new backup certificate as a secondary backup (in case the new main certificate gets compromised). This transition period must be at least one month, so as not to break clients who still pin to the old main certificate.
  5. Shortly before expiration, change the HPKP header again to send the new main certificate’s SPKI as the main pin and the new backup certificate as the backup pin.

To summarize:

Period Main server certificate Backup pin Secondary backup pin
Regular operation: before rotation Old main certificate Old backup certificate
>1 month before expiration of old certificates Old main certificate New main certificate New backup certificate
Shortly before expiration but not earlier than the previous change + 1 month New main certificate New backup certificate
Regular operation: after rotation New main certificate New backup certificate

The above assumes that public keys are normally associated with certificates, that is, the certificate is issued shortly after the public key is generated. This is true for many enterprise deployment, where certificates are managed by certificate management applications or directly with the CA, and there is no facility for secure and resilient long-term storage of public (and private) keys. HPKP is easier to deploy securely where such facilities do exist.

6.2. Comparison: TACK

Compared with HPKP, TACK [I-D.perrin-tls-tack] is a lot more similar to the current draft. It can even be argued that this document is a symmetric-cryptography variant of TACK. That said, there are still a few significant differences:

7. Security Considerations

This section reviews several security aspects related to the proposed extension.

7.1. Trust on First Use (TOFU) and MITM Attacks

This protocol is a “trust on first use” protocol. If a client initially connects to the “right” server, it will be protected against MITM attackers for the lifetime of each received ticket. If it connects regularly (depending of course on the server-selected lifetime), it will stay constantly protected against fake certificates.

However if it initially connects to an attacker, subsequent connections to the “right” server will fail. Server operators might want to advise clients on how to remove corrupted pins, once such large scale attacks are detected and remediated.

The protocol is designed so that it is not vulnerable to an active MITM attacker who has real-time access to the original server. The pinning proof includes a hash of the server’s public key, to ensure the client that the proof was in fact generated by the server with which it is initiating the connection.

7.2. Pervasive Monitoring

Some organizations, and even some countries perform pervasive monitoring on their constituents [RFC7258]. This often takes the form of SSL proxies. Because of the TOFU property, this protocol does not provide any security in such cases.

7.3. Server-Side Error Detection

Uniquely, this protocol allows the server to detect clients that present incorrect tickets and therefore can be assumed to be victims of a MITM attack. Server operators can use such cases as indications of ongoing attacks, similarly to fake certificate attacks that took place in a few countries in the past.

7.4. Client Policy

Like it or not, some clients are normally deployed behind an SSL proxy. Similarly to [RFC7469], it is acceptable to allow pinning to be disabled for some hosts according to local policy. For example, a UA MAY disable pinning for hosts whose validated certificate chain terminates at a user-defined trust anchor, rather than a trust anchor built-in to the UA (or underlying platform). Moreover, a client MAY accept an empty PinningTicket extension from such hosts as a valid response.

7.5. Client-Side Error Behavior

When a client receives an incorrect or empty PinningTicket from a pinned server, it MUST abort the handshake and MUST NOT retry with no PinningTicket in the request. Doing otherwise would expose the client to trivial fallback attacks, similar to those described in [RFC7507].

This rule can however have negative affects on clients that move from behind SSL proxies into the open Internet. Therefore, browser and library vendors MUST provide a documented way to remove stored pins.

7.6. Client Privacy

This protocol is designed so that an external attacker cannot correlate between different requests of a single client, provided the client requests and receives a fresh ticket upon each connection.

On the other hand, the server to which the client is connecting can easily track the client. This may be an issue when the client expects to connect to the server (e.g., a mail server) with multiple identities. Implementations SHOULD allow the user to opt out of pinning, either in general or for particular servers.

8. IANA Considerations

IANA is requested to allocate a TicketPinning extension value in the TLS ExtensionType Registry.

No registries are defined by this document.

9. Acknowledgements

The original idea behind this proposal was published in [Oreo] by Moty Yung, Benny Pinkas and Omer Berkman. The current protocol is but a distant relative of the original Oreo protocol, and any errors are the draft author’s alone.

I would like to thank Dave Garrett, Daniel Kahn Gillmor and Yoav Nir for their comments on this draft.

10. References

10.1. Normative References

[I-D.ietf-tls-tls13] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", Internet-Draft draft-ietf-tls-tls13-11, December 2015.
[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC5077] Salowey, J., Zhou, H., Eronen, P. and H. Tschofenig, "Transport Layer Security (TLS) Session Resumption without Server-Side State", RFC 5077, DOI 10.17487/RFC5077, January 2008.

10.2. Informative References

[I-D.perrin-tls-tack] Marlinspike, M., "Trust Assertions for Certificate Keys", Internet-Draft draft-perrin-tls-tack-02, January 2013.
[Oreo] Berkman, O., Pinkas, B. and M. Yung, "Firm Grip Handshakes: A Tool for Bidirectional Vouching", Cryptology and Network Security, pp. 142-157 , 2012.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, DOI 10.17487/RFC6454, December 2011.
[RFC6962] Laurie, B., Langley, A. and E. Kasper, "Certificate Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 2014.
[RFC7469] Evans, C., Palmer, C. and R. Sleevi, "Public Key Pinning Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April 2015.
[RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol Downgrade Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015.

Appendix A. Document History

A.1. draft-sheffer-tls-pinning-ticket-01

A.2. draft-sheffer-tls-pinning-ticket-00

Initial version.

Author's Address

Yaron Sheffer Intuit EMail: