RFC 8672 | Pinning with Tickets | October 2019
Sheffer & Migault | Experimental | [Page]

Stream:
  Independent Submission
RFC:
  8672
Category:
  Experimental
Published:
  October 2019
ISSN:
  2070-1721
Authors:
  Y. Sheffer
  Intuit
  D. Migault
  Ericsson

# RFC 8672

# TLS Server Identity Pinning with Tickets

## Abstract

Misissued public-key certificates can prevent TLS clients from appropriately authenticating the TLS server. Several alternatives have been proposed to detect this situation and prevent a client from establishing a TLS session with a TLS end point authenticated with an illegitimate public-key certificate. These mechanisms are either not widely deployed or limited to public web browsing.¶

This document proposes experimental extensions to TLS with opaque pinning tickets as a way to pin the server's identity. During an initial TLS session, the server provides an original encrypted pinning ticket. In subsequent TLS session establishment, upon receipt of the pinning ticket, the server proves its ability to decrypt the pinning ticket and thus the ownership of the pinning protection key. The client can now safely conclude that the TLS session is established with the same TLS server as the original TLS session. One of the important properties of this proposal is that no manual management actions are required.¶

## Status of This Memo

This document is not an Internet Standards Track specification; it is published for examination, experimental implementation, and evaluation.¶

This document defines an Experimental Protocol for the Internet community. This is a contribution to the RFC Series, independently of any other RFC stream. The RFC Editor has chosen to publish this document at its discretion and makes no statement about its value for implementation or deployment. Documents approved for publication by the RFC Editor are not candidates for any level of Internet Standard; see Section 2 of RFC 7841.¶

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8672.¶

## Copyright Notice

Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.¶

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document.¶

## 1. Introduction

Misissued public-key certificates can prevent TLS [RFC8446] clients from appropriately authenticating the TLS server. This is a significant risk in the context of the global public key infrastructure (PKI), and similarly for large-scale deployments of certificates within enterprises.¶

This document proposes experimental extensions to TLS with opaque pinning tickets as a way to pin the server's identity. The approach is intended to be easy to implement and deploy, and reuses some of the ideas behind TLS session resumption [RFC5077].¶

Ticket pinning is a second-factor server authentication method and is not proposed as a substitute for the authentication method provided in the TLS key exchange. More specifically, the client only uses the pinning identity method after the TLS key exchange is successfully completed. In other words, the pinning identity method is only performed over an authenticated TLS session. Note that ticket pinning does not pin certificate information and therefore is truly an independent second-factor authentication.¶

Ticket pinning is a trust-on-first-use (TOFU) mechanism, 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, in addition to normal PKI certificate authentication.¶

During initial TLS session establishment, the client requests a pinning ticket from the server. Upon receiving the request the server generates a pinning secret that is expected to be unpredictable for peers other than the client or the server. In our case, the pinning secret is generated from parameters exchanged during the TLS key exchange, so client and server can generate it locally and independently. The server constructs the pinning ticket with the necessary information to retrieve the pinning secret. The server then encrypts the ticket and returns the pinning ticket to the client with an associated pinning lifetime.¶

The pinning lifetime value 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 server commitment (and ticket lifetime) is typically on the order of weeks.¶

Once the key exchange is completed, and the server is deemed authenticated, the client generates locally the pinning secret and caches the server's identifiers to index the pinning secret as well as the pinning ticket and its associated lifetime.¶

When the client reestablishes a new TLS session with the server, it sends the pinning ticket to the server. Upon receiving it, the server returns a proof of knowledge of the pinning secret. Once the key exchange is completed, and the server has been authenticated, the client checks the pinning proof returned by the server using the client's stored pinning secret. If the proof matches, the client can conclude that the server to which it is currently connecting is, in fact, the correct server.¶

This document only applies to TLS 1.3. We believe that the idea can also be retrofitted into earlier versions of the protocol, but this would require significant changes. One example is that TLS 1.2 [RFC5246] and earlier versions do not provide a generic facility of encrypted handshake extensions, such as is used here to transport the ticket.¶

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

- The protocol is at the TLS level, and as a result is not restricted to HTTP at the application level.¶
- The protocol is robust to changes in server IP address, certification authority (CA), and public key. The server is characterized by the ownership of the pinning protection key, which is never provided to the client. Server configuration parameters such as the CA and the public key may change without affecting the pinning ticket protocol.¶
- Once a single parameter is configured (the ticket's lifetime), operation is fully automated. The server administrator need not bother with the management of backup certificates or explicit pins.¶
- For server clusters, we reuse the existing infrastructure [RFC5077] where it exists.¶
- Pinning errors, presumably resulting from man-in-the-middle (MITM) attacks, can be detected both by the client and the server. This allows for server-side detection of MITM attacks using large-scale analytics, and with no need to rely on clients to explicitly report the error.¶

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. In our solution, the server proves its identity by generating a proof that it can read and decrypt an encrypted ticket. As a result, the identity proof relies on proof of ownership of the pinning protection key. However, this key is never exchanged with the client or known by it, and so cannot itself be pinned.¶

### 1.1. Conventions Used in This Document

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.¶

### 1.2. Scope of Experimentation

This document describes an experimental extension to the TLS protocol. This section defines constraints on this experiment and how it can yield useful information, potentially resulting in a standard.¶

The protocol is designed so that if the server does not support it, the client and server fall back to a normal TLS exchange, with the exception of a single PinningTicket extension being initially sent by the client. In addition, the protocol is designed only to strengthen the validation of the server's identity ("second factor"). As a result, implementation or even protocol errors should not result in weakened security compared to the normal TLS exchange. Given these two points, experimentation can be run on the open Internet between consenting client and server implementations.¶

The goal of the experiment is to prove that:¶

- Non-supporting clients and servers are unaffected.¶
- Connectivity between supporting clients and servers is retained under normal circumstances, whether the client connects to the server frequently (relative to the ticket's lifetime) or very rarely.¶
- Enterprise middleboxes do not interrupt such connectivity.¶
- Misissued certificates and rogue TLS-aware middleboxes do result in broken connectivity, and these cases are detected on the client and/or server side. Clients and servers can be recovered even after such events and the normal connectivity restored.¶

Following two years of successful deployment, the authors will publish a document that summarizes the experiment's findings and will resubmit the protocol for consideration as a Proposed Standard.¶

## 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 document presents some similarities with the ticket resumption mechanism described in [RFC5077]. However the scope of this document differs from session resumption mechanisms implemented with [RFC5077] or with other mechanisms. Specifically, the pinning ticket does not carry any state associated with a TLS session and thus cannot be used for session resumption or client authentication. Instead, the pinning ticket only contains the encrypted pinning secret. The pinning ticket is used by the server to prove its ability to decrypt it, which implies ownership of the pinning protection key.¶

[RFC5077] has been obsoleted by [RFC8446], and ticket resumption is now defined by Section 2.2 of [RFC8446]. This document references [RFC5077] as an informational document since it contains a more thorough discussion of stateless ticket resumption, and because ticket resumption benefits from significant operational experience with TLS 1.2 that is still widely deployed at the time of writing. This experience, as well as deployment experience, can easily be re-used for identity pinning.¶

With TLS 1.3, session resumption is based on a Pre-Shared Key (PSK). This is orthogonal to this protocol. With TLS 1.3, a TLS session can be established using PKI and a pinning ticket, and later resumed with PSK.¶

However, the protocol described in this document addresses the problem of misissued certificates. Thus, it is not expected to be used outside a certificate-based TLS key exchange, such as in PSK. As a result, PSK handshakes 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

     ClientHello
       + key_share
       + signature_algorithms
       + PinningTicket         -------->
                                                     ServerHello
                                                     + key_share
                                           {EncryptedExtensions
                                                + PinningTicket}
                                           {CertificateRequest*}
                                                  {Certificate*}
                                            {CertificateVerify*}
                               <--------              {Finished}
     {Certificate*}
     {CertificateVerify*}
     {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.

¶

If a client supports the PinningTicket extension and does not have any pinning ticket associated with the server, the exchange is considered as an initial connection. Other reasons the client may not have a pinning ticket include the client having flushed its pinning ticket store, or the committed lifetime of the pinning ticket having expired.¶

Upon receipt of the PinningTicket extension, the server computes a pinning secret (Section 4.1) and sends the pinning ticket (Section 4.2) encrypted with the pinning protection key (Section 4.3). The pinning ticket is associated with a lifetime value by which the server assumes the responsibility of retaining the pinning protection key and being able to decrypt incoming pinning tickets during the period indicated by the committed lifetime.¶

Once the pinning ticket has been generated, the server returns the pinning ticket and the committed lifetime in a PinningTicket extension embedded in the EncryptedExtensions message. We note that a PinningTicket extension MUST NOT be sent as part of a HelloRetryRequest.¶

Upon receiving the pinning ticket, the client MUST NOT accept it until the key exchange is completed and the server authenticated. If the key exchange is not completed successfully, the client MUST ignore the received pinning ticket. Otherwise, the client computes the pinning secret and SHOULD cache the pinning secret and the pinning ticket for the duration indicated by the pinning ticket lifetime. The client SHOULD clean up the cached values at the end of the indicated lifetime.¶

### 2.2. Subsequent Connections

When the client initiates a connection to a server it has previously seen (see Section 2.3 on identifying servers), it SHOULD send the pinning ticket for that server. The pinning ticket, pinning secret, and pinning ticket lifetime computed during the establishment of the previous TLS session are designated in this document as the "original" ones, to distinguish them from a new ticket that may be generated during the current session.¶

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

- A proof that the server can understand the ticket that was sent by the client; this proof also binds the pinning ticket to the server's (current) public key, as well as the ongoing TLS session. The proof is mandatory and MUST be included if a pinning ticket was sent by the client.¶
- A fresh pinning ticket. The main reason for refreshing the ticket on each connection is privacy: to avoid the ticket serving as a fixed client identifier. While a fresh pinning ticket might be of zero length, it is RECOMMENDED to include a fresh ticket with a nonzero length with each response.¶

If the server cannot validate the received 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, the client MUST issue a handshake_failure alert and abort the connection (see also Section 6.5). It is important that the client does not attempt to "fall back" by omitting the PinningTicket extension.¶

When the connection is successfully set up, i.e., after the Finished message is verified, the client SHOULD store the new ticket along with the corresponding pinning_secret, replacing the original ticket.¶

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 5.5 on ramping down support for this extension.¶

### 2.3. Indexing the Pins

Each pin is associated with a set of identifiers that include, among others, hostname, 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. These identifiers are expected to be relevant to characterize the identity of the server as well as the establishing TLS session. When a hostname is used, it MUST be the value sent inside the Server Name Indication (SNI) extension. This definition is similar to the concept of a Web Origin [RFC6454], but does not assume the existence of a URL.¶

The purpose of ticket pinning is to pin the server identity. As a result, any information orthogonal to the server's identity MUST NOT be considered in indexing. More particularly, IP addresses are ephemeral and forbidden in SNI, and therefore pins MUST NOT be associated with IP addresses. Similarly, CA names or public keys associated with server MUST NOT be used for indexing as they may change over time.¶

## 3. Message Definitions

This section defines the format of the PinningTicket extension. We follow the message notation of [RFC8446].¶

     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;

¶

ticket
  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.¶
proof
  a demonstration by the server that it understands the received ticket and therefore that it is in possession of the secret that was used to generate it originally. The extension MUST contain exactly 0 or 1 proofs.¶
lifetime
  the duration (in seconds) that the server commits to accept offered tickets in the future.¶

## 4. Cryptographic Operations

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

### 4.1. Pinning Secret

The pinning secret is generated locally by the client and the server, which means they must use the same inputs to generate it. This value must be generated before the ServerHello message is sent, as the server includes the corresponding pinning ticket in the same flight as the ServerHello message. In addition, the pinning secret must be unpredictable to any party other than the client and the server.¶

The pinning secret is derived using the Derive-Secret function provided by TLS 1.3, described in Section 7.1 of [RFC8446].¶

    pinning secret = Derive-Secret(Handshake Secret, "pinning secret",
             ClientHello...ServerHello)

¶

### 4.2. Pinning Ticket

The pinning ticket contains the pinning secret. The pinning ticket is provided by the client to the server, which decrypts it in order to extract the pinning secret and responds with a pinning proof. As a result, the characteristics of the pinning ticket are:¶

- Pinning tickets MUST be encrypted and integrity-protected using strong cryptographic algorithms.¶
- Pinning tickets MUST be protected with a long-term pinning protection key.¶
- Pinning tickets MUST include a pinning protection key ID or serial number as to enable the pinning protection key to be refreshed.¶
- The pinning ticket MAY include other information, in addition to the pinning secret. When additional information is included, a careful review needs to be performed to evaluate its impact on privacy.¶

The pinning ticket's format is not specified by this document, but a format similar to the one proposed by [RFC5077] is RECOMMENDED.¶

### 4.3. Pinning Protection Key

The pinning protection key is used only by the server and so remains server implementation specific. [RFC5077] recommends the use of two keys, but when using Authenticated Encryption with Associated Data (AEAD) algorithms, only a single key is required.¶

When a single server terminates TLS for multiple virtual servers using the SNI mechanism, it is strongly RECOMMENDED that the server use a separate protection key for each one of them, in order to allow migrating virtual servers between different servers while keeping pinning active.¶

As noted in Section 5.1, if the server is actually a cluster of machines, the protection key MUST be synchronized between all the nodes that accept TLS connections to the same server name. When [RFC5077] is deployed, an easy way to do it is to derive the protection key from the session-ticket protection key, which is already synchronized. For example:¶

    pinning_protection_key = HKDF-Expand(resumption_protection_key,
                                  "pinning protection", L)

¶

Where resumption_protection_key is the ticket protection key defined in [RFC5077]. Both resumption_protection_key and pinning_protection_key are only used by the server.¶

The above solution attempts to minimize code changes related to management of the resumption_protection_key. The drawback is that this key would be used both to directly encrypt session tickets and to derive the pinning_protection_key, and such mixed usage of a single key is not in line with cryptographic best practices. Where possible, it is RECOMMENDED that the resumption_protection_key be unrelated to the pinning_protection_key and that they are separately shared among the relevant servers.¶

### 4.4. Pinning Proof

The pinning proof is sent by the server to demonstrate that it has been able to decrypt the pinning ticket and to retrieve the pinning secret. The proof must be unpredictable and must not be replayed. Similarly to the pinning ticket, the pinning proof is sent by the server in the ServerHello message. In addition, it must not be possible for a MITM server with a fake certificate to obtain a pinning proof from the original server.¶

In order to address these requirements, the pinning proof is bound to the TLS session as well as the public key of the server:¶

    pinning_proof_secret=Derive-Secret(Handshake Secret,
                 "pinning proof 1", ClientHello...ServerHello)

    proof = HMAC(original_pinning_secret, "pinning proof 2" +
                 pinning_proof_secret + Hash(server_public_key))

¶

where HMAC [RFC2104] uses the Hash algorithm that was negotiated in the handshake, and the same hash is also used over the server's public key. The original_pinning_secret value refers to the secret value extracted from the ticket sent by the client, to distinguish it from a new pinning secret value that is possibly computed in the current exchange. The server_public_key value is the DER representation of the public key, specifically the SubjectPublicKeyInfo structure as-is.¶

## 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. To achieve this goal, operations described in identity pinning are only performed within the current TLS session, and there is no dependence on any TLS configuration parameters such as CA identity or public keys. As a result, configuration changes are unlikely to lead to desynchronized state between the client and the server.¶

### 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. In such scenarios, each cluster member MUST be able to accept tickets protected with a new version of the protection key, even while it is still using an old version to generate keys. This ensures that, when a client receives a "new" ticket, it does not next hit a cluster member that still rejects this ticket.¶

Misconfiguration can lead to the server's clock being off by a large amount of time. Consider a case where a server's clock is misconfigured, for example, to be 1 year in the future, and the system is allowed to delete expired keys automatically. The server will then delete many outstanding keys because they are now long expired and will end up rejecting valid tickets that are stored by clients. Such a scenario could make the server inaccessible to a large number of clients.¶

The decision to delete a key should at least consider the largest value of the ticket lifetime as well as the expected time desynchronization between the servers of the cluster and the time difference for distributing the new key among the different servers in the cluster.¶

### 5.2. Ticket Lifetime

The lifetime of the ticket is a commitment by the server to retain the ticket's corresponding protection key for this duration, so that the server can prove to the client that it knows the secret embedded in the ticket. For production systems, the lifetime SHOULD be between 7 and 31 days.¶

### 5.3. 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, pinning is not associated with certificates, which is the reason we designate the protocol described in this document as "server identity pinning".¶

Note that this property is not impacted by the use of the server's public key in the pinning proof because the scope of the public key used is only the current TLS session.¶

### 5.4. 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 regardless of any ticket-related behavior.¶

### 5.5. Disabling Pinning

A server implementing this protocol MUST have a "ramp down" mode of operation where:¶

- The server continues to accept valid pinning tickets and responds correctly with a proof.¶
- The server does not send back a new pinning ticket.¶

After a while, no clients will hold valid tickets, and the feature may be disabled. Note that clients that do not receive a new pinning ticket do not necessarily need to remove the original ticket. Instead, the client may keep using the ticket until its lifetime expires. However, as detailed in Section 6.7, re-use of a ticket by the client may result in privacy concerns as the ticket value may be used to correlate TLS sessions.¶

Issuing a new pinning ticket with a shorter lifetime would only delay the ramp down process, as the shorter lifetime can only affect clients that actually initiated a new connection. Other clients would still see the original lifetime for their pinning tickets.¶

### 5.6. Server Compromise

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

The same procedure applies if the pinning protection key is compromised directly, e.g., if a backup copy is inadvertently made public.¶

### 5.7. 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 often 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. Security Considerations

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

### 6.1. 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.¶

### 6.2. Pervasive Monitoring

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

Pervasive monitoring may also result in privacy concerns detailed in Section 6.7.¶

### 6.3. 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.¶

### 6.4. Client Policy and SSL Proxies

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

### 6.5. Client-Side Error Behavior

When a client receives a malformed or empty PinningTicket extension from a pinned server, it MUST abort the handshake. If the client retries the request, it MUST NOT omit the PinningTicket in the retry message. Doing otherwise would expose the client to trivial fallback attacks, similar to those described in [RFC7507].¶

However, this rule can negatively impact clients that move from behind SSL proxies into the open Internet, and vice versa, if the advice in Section 6.4 is not followed. Therefore, it is RECOMMENDED that browser and library vendors provide a documented way to remove stored pins.¶

### 6.6. Stolen and Forged Tickets

An attacker gains no benefit from stealing pinning tickets, even in conjunction with other pinning parameters such as the associated pinning secret, since pinning tickets are used to secure the client rather than the server. Similarly, it is useless to forge a ticket for a particular server.¶

### 6.7. Client Privacy

This protocol is designed so that an external attacker cannot link different requests to a single client, provided the client requests and receives a fresh ticket upon each connection. This may be of concern particularly during ramp down, if the server does not provide a new ticket, and the client reuses the same ticket. To reduce or avoid such privacy concerns, it is RECOMMENDED for the server to issue a fresh ticket with a reduced lifetime. This would at least reduce the time period in which the TLS sessions of the client can be linked. The server MAY also issue tickets with a zero-second lifetime until it is confident all tickets are expired.¶

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.¶

This document does not define the exact content of tickets. Including client-specific information in tickets would raise privacy concerns and is NOT RECOMMENDED.¶

### 6.8. Ticket Protection Key Management

While the ticket format is not mandated by this document, protecting the ticket using authenticated encryption is RECOMMENDED. Some of the algorithms commonly used for authenticated encryption, e.g., Galois/Counter Mode (GCM), are highly vulnerable to nonce reuse, and this problem is magnified in a cluster setting. Therefore, implementations that choose AES-GCM or any AEAD equivalent MUST adopt one of these three alternatives:¶

- Partition the nonce namespace between cluster members and use monotonic counters on each member, e.g., by setting the nonce to the concatenation of the cluster member ID and an incremental counter.¶
- Generate random nonces but avoid the so-called birthday bound, i.e., never generate more than the maximum allowed number of encrypted tickets (2**64 for AES-128-GCM) for the same ticket pinning protection key.¶
- An alternative design that has been attributed to Karthik Bhargavan is as follows. Start with a 128-bit master key K_master and then for each encryption, generate a 256-bit random nonce and compute: K = HKDF(K_master, Nonce || "key"), then N = HKDF(K_master, Nonce || "nonce"). Use these values to encrypt the ticket, AES-GCM(K, N, data). This nonce should then be stored and transmitted with the ticket.¶

## 7. IANA Considerations

The IANA has allocated a TicketPinning extension value in the "TLS ExtensionType Values" registry.¶

[RFC8447] defines the procedure, requirements, and the necessary information for the IANA to update the "TLS ExtensionType Values" registry [TLS-EXT]. The registration procedure is "Specification Required" [RFC8126].¶

The TicketPinning extension is registered as follows. (The extension is not limited to Private Use, and as such has its first byte in the range 0-254.)¶

Value:
  32¶
Name:
  ticket_pinning¶
Recommended:
  No¶
TLS 1.3:
  CH, EE (to indicate that the extension is present in ClientHello and EncryptedExtensions messages)¶

## 8. References

### 8.1. Normative References

[RFC2119]
  Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.
[RFC8126]
  Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, <https://www.rfc-editor.org/info/rfc8126>.
[RFC8174]
  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8446]
  Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, <https://www.rfc-editor.org/info/rfc8446>.
[RFC8447]
  Salowey, J. and S. Turner, "IANA Registry Updates for TLS and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018, <https://www.rfc-editor.org/info/rfc8447>.

### 8.2. Informative References

[Netcraft]
  Mutton, P., "HTTP Public Key Pinning: You're doing it wrong!", March 2016, <https://news.netcraft.com/archives/2016/03/30/http-public-key-pinning-youre-doing-it-wrong.html>.
[Oreo]
  Berkman, O., Pinkas, B., and M. Yung, "Firm Grip Handshakes: A Tool for Bidirectional Vouching", Cryptology and Network Security pp. 142-157, 2012.
[RFC2104]
  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, DOI 10.17487/RFC2104, February 1997, <https://www.rfc-editor.org/info/rfc2104>.
[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, <https://www.rfc-editor.org/info/rfc5077>.
[RFC5246]
  Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008, <https://www.rfc-editor.org/info/rfc5246>.
[RFC6454]
  Barth, A., "The Web Origin Concept", RFC 6454, DOI 10.17487/RFC6454, December 2011, <https://www.rfc-editor.org/info/rfc6454>.
[RFC6962]
  Laurie, B., Langley, A., and E. Kasper, "Certificate Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013, <https://www.rfc-editor.org/info/rfc6962>.
[RFC7258]
  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 2014, <https://www.rfc-editor.org/info/rfc7258>.
[RFC7469]
  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April 2015, <https://www.rfc-editor.org/info/rfc7469>.
[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, <https://www.rfc-editor.org/info/rfc7507>.
[RFC8555]
  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. Kasten, "Automatic Certificate Management Environment (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019, <https://www.rfc-editor.org/info/rfc8555>.
[TLS-EXT]
  IANA, "TLS Extension Type Value", <https://www.iana.org/assignments/tls-extensiontype-values/>.
[TLS-TACK]
  Marlinspike, M., "Trust Assertions for Certificate Keys", Work in Progress, Internet-Draft, draft-perrin-tls-tack-02, 7 January 2013, <https://tools.ietf.org/html/draft-perrin-tls-tack-02>.

## Appendix A. Previous Work

The global PKI system relies on the trust of a CA issuing certificates. As a result, a corrupted trusted 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 the Certificate Transparency (CT) protocol [RFC6962], HTTP Public Key Pinning (HPKP) [RFC7469], and Trust Assertions for Certificate Keys (TACK) [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. However the public nature of CT often makes it inappropriate for enterprise use because many organizations are not willing to expose their internal infrastructure publicly.¶

TACK has some similarities to the current proposal, but work on it seems to have stalled. Appendix A.2 compares our proposal to TACK.¶

HPKP is an IETF standard, but so far has proven hard to deploy. HPKP pins (fixes) a public key, one of the public keys listed in the certificate chain. As a result, HPKP needs to be coordinated with the certificate management process. Certificate management impacts HPKP and thus increases the probability of HPKP failures. This risk is made even higher given the fact that, even though work has been done in the Automated Certificate Management Environment (ACME) working group to automate certificate management, in many or even most cases, certificates are still managed manually. As a result, HPKP cannot be completely automated, resulting in error-prone manual configuration. Such errors could prevent the web server from being accessed by some clients. In addition, HPKP uses an HTTP header, which makes this solution HTTPS specific and not generic to TLS. On the other hand, the current document provides a solution that is independent of the server's certificate management, and that can be entirely and easily automated. Appendix A.1 compares HPKP to the current document in more detail.¶

The ticket pinning proposal augments these mechanisms with a much easier to implement and deploy solution for server identity pinning, by reusing some of the ideas behind TLS session resumption.¶

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

## A.1. Comparison: HPKP

The current IETF standard for pinning the identity of web servers is HPKP [RFC7469].¶

The main differences between HPKP and the current document are the following:¶

- HPKP limits its scope to HTTPS, while the current document considers all application above TLS.¶
- HPKP pins the public key of the server (or another public key along the certificate chain), and as such, is highly dependent on the management of certificates. Such dependency increases the potential error surface, especially as certificate management is not yet largely automated. The current proposal, on the other hand, is independent of certificate management.¶
- HPKP pins public keys that are public and used for the standard TLS authentication. Identity pinning relies on the ownership of the pinning key, which is not disclosed to the public and not involved in the standard TLS authentication. As a result, identity pinning is a completely independent, second-factor authentication mechanism.¶
- HPKP relies on a backup key to recover the misissuance of a key. We believe such backup mechanisms add excessive complexity and cost. Reliability of the current mechanism is primarily based on its being highly automated.¶
- HPKP relies on the client to report errors to the report-uri. The current document does not need any out-of-band mechanism, and the server is informed automatically. This provides an easier and more reliable health monitoring.¶

On the other hand, HPKP shares the following aspects with identity pinning:¶

- Both mechanisms provide hard failure. With HPKP, only the client is aware of the failure, while with the current proposal both client and server are informed of the failure. This provides room for further mechanisms to automatically recover from such failures.¶
- Both mechanisms are subject to a server compromise in which users are provided with an invalid ticket (e.g., a random one) or HTTP header with a very long lifetime. For identity pinning, this lifetime SHOULD NOT be longer than 31 days. In both cases, clients will not be able to reconnect the server during this lifetime. With the current proposal, an attacker needs to compromise the TLS layer, while with HPKP, the attacker needs to compromise the HTTP server. Arguably, the TLS-level compromise is typically more difficult for the attacker.¶

Unfortunately HPKP has not seen wide deployment yet. As of March 2016, the number of servers using HPKP was less than 3000 [Netcraft]. This may simply be due to inertia, but we believe the main reason is the interactions between HPKP and manual certificate management that 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 that the server becomes unusable for some of the clients.¶

To demonstrate this point, we present a list of the steps involved in deploying HPKP on a security-sensitive web server.¶

1. Generate two public/private key pairs on a computer that is not the live server. The second one is the "backup1" key pair.¶

       openssl genrsa -out "example.com.key" 2048;

       openssl genrsa -out "example.com.backup1.key" 2048;

   ¶

2. Generate hashes for both of the public keys. These will be used in the HPKP header:¶

       openssl rsa -in "example.com.key" -outform der -pubout | \
       openssl dgst -sha256 -binary | openssl enc -base64

       openssl rsa -in "example.com.backup1.key" -outform der \
       -pubout | openssl dgst -sha256 -binary | openssl enc -base64

   ¶

3. Generate a single CSR (Certificate Signing Request) for the first key pair, where you include the domain name in the CN (Common Name) field:¶

       openssl req -new -subj "/C=GB/ST=Area/L=Town/O=Org/ \
       CN=example.com" -key "example.com.key" -out "example.com.csr";

   ¶
4. Send this CSR to the CA and go though the dance to prove you own the domain. The CA will give you a single certificate that will typically expire within a year or two.¶

5. On the live server, upload and set up the first key pair and its certificate. At this point, you can add the "Public-Key-Pins" header, using the two hashes you created in step 2.¶

   Note that only the first key pair has been uploaded to the server so far.¶

6. Store the second (backup1) key pair somewhere safe, probably somewhere encrypted like a password manager. It won't expire, as it's just a key pair; it just needs to be ready for when you need to get your next certificate.¶
7. Time passes -- probably just under a year (if waiting for a certificate to expire), or maybe sooner if you find that your server has been compromised, and you need to replace the key pair and certificate.¶
8. Create a new CSR using the "backup1" key pair, and get a new certificate from your CA.¶
9. Generate a new backup key pair (backup2), get its hash, and store it in a safe place (again, not on the live server).¶
10. Replace your old certificate and old key pair, update the "Public-Key-Pins" header to remove the old hash, and add the new "backup2" key pair.¶

Note that in the above steps, both the certificate issuance as well as the storage of the backup key pair involve manual steps. Even with an automated CA that runs the ACME protocol [RFC8555], key backup would be a challenge to automate.¶

## A.2. Comparison: TACK

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

- Probably the most important difference is that with TACK, validation of the server certificate is no longer required, and in fact TACK specifies it as a "MAY" requirement ([TLS-TACK], Section 5.3). With ticket pinning, certificate validation by the client remains a MUST requirement, and the ticket acts only as a second factor. If the pinning secret is compromised, the server's security is not immediately at risk.¶
- Both TACK and the current document are mostly orthogonal to the server certificate as far as their life cycle, and so both can be deployed with no manual steps.¶
- TACK uses Elliptic Curve Digital Signature Algorithm (ECDSA) to sign the server's public key. This allows cooperating clients to share server assertions between themselves. This is an optional TACK feature, and one that cannot be done with pinning tickets.¶
- TACK allows multiple servers to share its public keys. Such sharing is disallowed by the current document.¶
- TACK does not allow the server to track a particular client, and so has better privacy properties than the current document.¶
- TACK has an interesting way to determine the pin's lifetime, setting it to the time period since the pin was first observed, with a hard upper bound of 30 days. The current document makes the lifetime explicit, which may be more flexible to deploy. For example, web sites that are only visited rarely by users may opt for a longer period than other sites that expect users to visit on a daily basis.¶

## Acknowledgments

The original idea behind this proposal was published in [Oreo] by Moti Yung, Benny Pinkas, and Omer Berkman. The current protocol is but a distant relative of the original Oreo protocol, and any errors are the responsibility of the authors of this document alone.¶

We would like to thank Adrian Farrel, Dave Garrett, Daniel Kahn Gillmor, Alexey Melnikov, Yoav Nir, Eric Rescorla, Benjamin Kaduk, and Rich Salz for their comments on this document. Special thanks to Craig Francis for contributing the HPKP deployment script, and to Ralph Holz for several fruitful discussions.¶

## Authors' Addresses