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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 8672

Network Working Group                                         Y. Sheffer
Internet-Draft                                                    Intuit
Intended status: Experimental                                 D. Migault
Expires: September 19, 2018                                     Ericsson
                                                          March 18, 2018

                TLS Server Identity Pinning with Tickets


   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, but none is currently in wide

   This document proposes to extend 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 if 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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 19, 2018.

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Copyright Notice

   Copyright (c) 2018 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
   (http://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.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions used in this document . . . . . . . . . . . .   6
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Initial Connection  . . . . . . . . . . . . . . . . . . .   6
     2.2.  Subsequent Connections  . . . . . . . . . . . . . . . . .   8
     2.3.  Indexing the Pins . . . . . . . . . . . . . . . . . . . .   9
   3.  Message Definitions . . . . . . . . . . . . . . . . . . . . .   9
   4.  Cryptographic Operations  . . . . . . . . . . . . . . . . . .  10
     4.1.  Pinning Secret  . . . . . . . . . . . . . . . . . . . . .  10
     4.2.  Pinning Ticket  . . . . . . . . . . . . . . . . . . . . .  10
     4.3.  Pinning Protection Key  . . . . . . . . . . . . . . . . .  11
     4.4.  Pinning Proof . . . . . . . . . . . . . . . . . . . . . .  11
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .  12
     5.1.  Protection Key Synchronization  . . . . . . . . . . . . .  12
     5.2.  Ticket Lifetime . . . . . . . . . . . . . . . . . . . . .  12
     5.3.  Certificate Renewal . . . . . . . . . . . . . . . . . . .  13
     5.4.  Certificate Revocation  . . . . . . . . . . . . . . . . .  13
     5.5.  Disabling Pinning . . . . . . . . . . . . . . . . . . . .  13
     5.6.  Server Compromise . . . . . . . . . . . . . . . . . . . .  13
     5.7.  Disaster Recovery . . . . . . . . . . . . . . . . . . . .  14
   6.  Previous Work . . . . . . . . . . . . . . . . . . . . . . . .  14
     6.1.  Comparison: HPKP  . . . . . . . . . . . . . . . . . . . .  14
     6.2.  Comparison: TACK  . . . . . . . . . . . . . . . . . . . .  17
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .  17
     7.1.  Mint Fork . . . . . . . . . . . . . . . . . . . . . . . .  18
       7.1.1.  Overview  . . . . . . . . . . . . . . . . . . . . . .  18
       7.1.2.  Description . . . . . . . . . . . . . . . . . . . . .  18
       7.1.3.  Level of Maturity . . . . . . . . . . . . . . . . . .  18
       7.1.4.  Coverage  . . . . . . . . . . . . . . . . . . . . . .  18
       7.1.5.  Version Compatibility . . . . . . . . . . . . . . . .  18
       7.1.6.  Licensing . . . . . . . . . . . . . . . . . . . . . .  19

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       7.1.7.  Contact Information . . . . . . . . . . . . . . . . .  19
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
     8.1.  Trust on First Use (TOFU) and MITM Attacks  . . . . . . .  19
     8.2.  Pervasive Monitoring  . . . . . . . . . . . . . . . . . .  19
     8.3.  Server-Side Error Detection . . . . . . . . . . . . . . .  19
     8.4.  Client Policy and SSL Proxies . . . . . . . . . . . . . .  20
     8.5.  Client-Side Error Behavior  . . . . . . . . . . . . . . .  20
     8.6.  Stolen and Forged Tickets . . . . . . . . . . . . . . . .  20
     8.7.  Client Privacy  . . . . . . . . . . . . . . . . . . . . .  20
     8.8.  Ticket Protection Key Management  . . . . . . . . . . . .  21
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     11.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  25
     A.1.  draft-sheffer-tls-pinning-ticket-06 . . . . . . . . . . .  25
     A.2.  draft-sheffer-tls-pinning-ticket-05 . . . . . . . . . . .  25
     A.3.  draft-sheffer-tls-pinning-ticket-04 . . . . . . . . . . .  25
     A.4.  draft-sheffer-tls-pinning-ticket-03 . . . . . . . . . . .  25
     A.5.  draft-sheffer-tls-pinning-ticket-02 . . . . . . . . . . .  25
     A.6.  draft-sheffer-tls-pinning-ticket-01 . . . . . . . . . . .  25
     A.7.  draft-sheffer-tls-pinning-ticket-00 . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

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 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 at the ACME WG to automate certificate management, in many or

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   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 a 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.  Section 6.1 compares HPKP to the
   current draft 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.

   Ticket pinning is a second factor server authentication method and is
   not proposed as a substitute of 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 as such should be
   considered a "real" 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

   During initial TLS session establishment, the client requests a
   pinning ticket from the server.  Upon receiving the request the
   server generates a pinning secret which 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
   committed lifetime is typically on the order of weeks or months.

   Once the key exchange is completed and the server is deemed
   authenticated, the client generates locally the pinning secret and

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   caches the server's identifiers to index the pinning secret as well
   as the pinning ticket and its associated lifetime.

   When the client re-establishes 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 it is currently connecting to is in fact the
   correct server.

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

   The main advantages of this protocol over earlier pinning solutions

   -  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 server IP, CA, and public key changes.
      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

   -  For server clusters, we reuse the existing [RFC5077]
      infrastructure where it exists.

   -  Pinning errors, presumably resulting from 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

   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 out
   solution, the server proves its identity by generating a proof that
   it can read and decrypt an encrypted ticket.  As a result, the

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   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",
   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 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 to authenticate the client.  Instead, the
   pinning ticket only contains the Pinning Secret used to generate the

   With TLS 1.3, session resumption is based on a preshared 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

   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

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

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    Client                                               Server

      + key_share
      + signature_algorithms*
      + 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.

   If a client supports the pinning ticket 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

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   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

   -  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 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.  It is RECOMMENDED to include a fresh
      ticket 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, MUST
   issue a handshake_failure alert and abort the connection (see also
   Section 8.5).  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.

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   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 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.

   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 [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;

   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.

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   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 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 "Key Schedule" of

   pinning secret = Derive-Secret(Handshake Secret, "pinning secret",

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.

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

4.3.  Pinning Protection Key

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

   When a single server terminates TLS for multiple virtual servers
   using the Server Name Indication (SNI) mechanism, we strongly
   RECOMMEND to 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)

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 retrieve the pinning
   secret.  The proof must be unpredictable and must not be replayed.
   Similarly to the pinning secret, 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:

   proof = HMAC(original_pinning_secret, "pinning proof" +
                Handshake-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

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   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.  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.  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.  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 a client that receives a "new" ticket 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.  Therefore we RECOMMEND never to automatically delete
   protection keys, even when they are long expired.

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.

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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, we are not "pinning certificates" and the protocol should
   more precisely be called "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 any more and the
   feature may be disabled.  Note that clients that do not receive a new
   pinning ticket do not remove the original ticket.  Instead, the
   client keeps on using the ticket until its lifetime expires.

   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.

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

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 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

6.  Previous Work

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

6.1.  Comparison: HPKP

   The current IETF standard for pinning the identity of web servers is
   the Public Key Pinning Extension for HTTP, or HPKP [RFC7469].

   The main differences between HPKP and the current document are the

   -  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 which 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,

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      identity pinning is a completely independent second factor
      authentication mechanism.

   -  HPKP relies on a backup key to recover the mis-issuance 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 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

   -  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 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 cannot 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 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 having the server become 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;"

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   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=Company/
        CN=example.com" -key "example.com.key" -out "example.com.csr";"

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

   5.   On the Live server, upload and setup 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 (Certificate Signing Request) 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, and update the
        "Public-Key-Pins" header to remove the old hash, and add the new
        "backup2" key-pair.

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   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, key backup would be
   a challenge to automate.

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:

   -  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 (Sec. 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 draft 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 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 draft.

   -  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 draft makes the
      lifetime explicit, which may be more flexible to deploy.  For
      example, Web sites which are only visited rarely by users may opt
      for a longer period than other sites that expect users to visit on
      a daily basis.

7.  Implementation Status

   Note to RFC Editor: please remove this section before publication,
   including the reference to [RFC7942].

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   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may

   According to RFC 7942, "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

7.1.  Mint Fork

7.1.1.  Overview

   A fork of the Mint TLS 1.3 implementation, developed by Yaron Sheffer
   and available at https://github.com/yaronf/mint.

7.1.2.  Description

   This is a fork of the TLS 1.3 implementation, and includes client and
   server code.  In addition to the actual protocol, several utilities
   are provided allowing to manage pinning protection keys on the server
   side, and pinning tickets on the client side.

7.1.3.  Level of Maturity

   This is a prototype.

7.1.4.  Coverage

   The entire protocol is implemented.

7.1.5.  Version Compatibility

   The implementation is compatible with draft-sheffer-tls-pinning-

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7.1.6.  Licensing

   Mint itself and this fork are available under an MIT license.

7.1.7.  Contact Information

   See author details below.

8.  Security Considerations

   This section reviews several security aspects related to the proposed

8.1.  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

   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.

8.2.  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.

8.3.  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.

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8.4.  Client Policy and SSL Proxies

   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

8.5.  Client-Side Error Behavior

   When a client receives a malformed or empty PinningTicket extension
   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

   This rule can however have negative affects on clients that move from
   behind SSL proxies into the open Internet and vice versa, if the
   advice in Section 8.4 is not followed.  Therefore, we RECOMMEND that
   browser and library vendors provide a documented way to remove stored

8.6.  Stolen and Forged Tickets

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

8.7.  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.

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8.8.  Ticket Protection Key Management

   While the ticket format is not mandated by this document, we
   RECOMMEND using authenticated encryption to protect it.  Some of the
   algorithms commonly used for authenticated encryption, e.g.  GCM, are
   highly vulnerable to nonce reuse, and this problem is magnified in a
   cluster setting.  Therefore implementations that choose AES-128-GCM
   MUST adopt one of these two 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

   -  Generate random nonces but avoid the so-called birthday bound,
      i.e. never generate more than 2**64 encrypted tickets for the same
      ticket pinning protection Key.

   An alternative design which 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")
   N = HKDF(K_master, Nonce || "nonce")

   And use these values to encrypt the ticket, AES-GCM(K, N, <data>).

9.  IANA Considerations

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

   [I-D.ietf-tls-iana-registry-updates] defines the procedure and
   requirements and the necessary information for the IANA to update the
   "TLS ExtensionType Values" registry [TLS-EXT].

   According to [I-D.ietf-tls-iana-registry-updates] the update of the
   "TLS ExtensionType Values" registry is "Specification Required"
   [RFC8126] which is fulfilled by the current document, when it is
   published as an RFC.

   The TicketPinning Extension is not limited to Private use and as such
   the TicketPinning Extension Value is expected to have its first byte
   in the range 0-254.  A value of 26 would address this requirement.

   The TicketPinning Extension Name is expected to be ticket_pinning.

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   The TicketPinning Extension Recommended value should be set to "No"
   with the publication of the current document as "Experimental".

   The TicketPinning Extension TLS.13 column should be set to CH, SH to
   indicate that the TicketPinning Extension is present in ClientHello
   and ServerHello messages.

10.  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 authors' alone.

   We would like to thank Dave Garrett, Daniel Kahn Gillmor, Eric
   Rescorla and Yoav Nir for their comments on this draft.  Special
   thanks to Craig Francis for contributing the HPKP deployment script,
   and to Ralph Holz for several fruitful discussions.

11.  References

11.1.  Normative References

              Salowey, J. and S. Turner, "IANA Registry Updates for TLS
              and DTLS", draft-ietf-tls-iana-registry-updates-04 (work
              in progress), February 2018.

              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-27 (work in progress),
              March 2018.

   [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, <http://www.rfc-editor.org/info/rfc5077>.

   [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,

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11.2.  Informative References

              Marlinspike, M., "Trust Assertions for Certificate Keys",
              draft-perrin-tls-tack-02 (work in progress), January 2013.

              Mutton, P., "HTTP Public Key Pinning: You're doing it
              wrong!", March 2016,

   [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-

   [RFC6454]  Barth, A., "The Web Origin Concept", RFC 6454,
              DOI 10.17487/RFC6454, December 2011, <https://www.rfc-

   [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, <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,

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,

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   [TLS-EXT]  IANA, ., "TLS Extension Type Value", 2018,

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Appendix A.  Document History

A.1.  draft-sheffer-tls-pinning-ticket-06

   -  IANA Considerations in preparation for Experimental publication.

A.2.  draft-sheffer-tls-pinning-ticket-05

   -  Multiple comments from Eric Rescorla.

A.3.  draft-sheffer-tls-pinning-ticket-04

   -  Editorial changes.

   -  Two-phase rotation of protection key.

A.4.  draft-sheffer-tls-pinning-ticket-03

   -  Deleted redundant length fields in the extension's formal

   -  Modified cryptographic operations to align with the current state
      of TLS 1.3.

   -  Numerous textual improvements.

A.5.  draft-sheffer-tls-pinning-ticket-02

   -  Added an Implementation Status section.

   -  Added lengths into the extension structure.

   -  Changed the computation of the pinning proof to be more robust.

   -  Clarified requirements on the length of the pinning_secret.

   -  Revamped the HPKP section to be more in line with current
      practices, and added recent statistics on HPKP deployment.

A.6.  draft-sheffer-tls-pinning-ticket-01

   -  Corrected the notation for variable-sized vectors.

   -  Added a section on disaster recovery and backup.

   -  Added a section on privacy.

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   -  Clarified the assumptions behind the HPKP procedure in the
      comparison section.

   -  Added a definition of pin indexing (origin).

   -  Adjusted to the latest TLS 1.3 notation.

A.7.  draft-sheffer-tls-pinning-ticket-00

   Initial version.

Authors' Addresses

   Yaron Sheffer

   EMail: yaronf.ietf@gmail.com

   Daniel Migault

   EMail: daniel.migault@ericsson.com

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