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<rfc xmlns:xi="http://www.w3.org/2001/XInclude"
submissionType="independent"
category="exp"
ipr="trust200902"
number="8672"
docName="draft-sheffer-tls-pinning-ticket-12" category="exp">
obsoletes=""
updates=""
xml:lang="en"
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  <front>
    <title abbrev="Pinning with Tickets">TLS Server Identity Pinning with Tickets</title>
    <seriesInfo name="RFC" value="8672"/>
    <author initials="Y." surname="Sheffer" fullname="Yaron Sheffer">
      <organization>Intuit</organization>
      <address>
        <email>yaronf.ietf@gmail.com</email>
      </address>
    </author>
    <author initials="D." surname="Migault" fullname="Daniel Migault">
      <organization>Ericsson</organization>
      <address>
        <email>daniel.migault@ericsson.com</email>
      </address>
    </author>
    <date year="2019" month="June" day="26"/> month="October" year="2019"/>
    <area>General</area>

    <keyword>Internet-Draft</keyword>
    <keyword>public-key certificates, trust-on-first-use, TOFU</keyword>

    <abstract>
      <t>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.</t>
      <t>This document proposes experimental extensions to TLS with opaque
pinning tickets as a way to pin the server’s 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.</t>
    </abstract>
  </front>
  <middle>
    <section anchor="introduction" title="Introduction"> numbered="true" toc="default">
      <name>Introduction</name>
      <t>Misissued public-key certificates can prevent TLS <xref target="RFC8446"/> target="RFC8446" format="default"/> 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 large-scale
deployments of certificates within enterprises.</t>

      <t>This document proposes experimental extensions to TLS with opaque
pinning tickets as a way to pin the server’s server's identity. The approach
is intended to be easy to implement and deploy, and reuses some of
the ideas behind TLS session resumption <xref target="RFC5077"/>.</t> target="RFC5077" format="default"/>.</t>

      <t>Ticket pinning is a second factor 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 ticket pinning
does not pin certificate information and therefore is truly an
independent second factor second-factor authentication.</t>
      <t>Ticket pinning is a Trust On First Use 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
server's identity based on the server’s server's ability to decrypt the ticket,
in addition to normal PKI certificate authentication.</t>
      <t>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 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.</t>
      <t>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.</t>
      <t>Once the key exchange is completed completed, and the server is deemed
authenticated, the client generates locally the pinning secret and
caches the server’s server's identifiers to index the pinning secret as well as
the pinning ticket and its associated lifetime.</t>
      <t>When the client re-establishes 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 completed, and the server has been authenticated, the client
checks the pinning proof returned by the server using the client’s client's
stored pinning secret. If the proof matches, the client can conclude
that the server to which it is currently connecting to is is, in fact fact, the correct
server.</t>

      <t>This document only applies to TLS 1.3.
We believe that the
idea can also be back-fitted retrofitted into earlier versions of the protocol, but
this would require significant changes.
One example is that TLS 1.2 <xref target="RFC5246"/> target="RFC5246" format="default"/> and
earlier versions do not provide a generic facility of encrypted
handshake extensions, such as is used here to transport the ticket.</t>
      <t>The main advantages of this protocol over earlier pinning solutions are:</t>

<t><list style="symbols">
  <t>The
are the following:</t>
      <ul spacing="normal">
        <li>The protocol is at the TLS level, and as a result is not restricted to
HTTP at the application level.</t>
  <t>The level.</li>
        <li>The protocol is robust to changes in server IP, Certificate Authority IP address,
certification authority (CA), and public key changes. 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.</t>
  <t>Once protocol.</li>
        <li>Once a single parameter is configured (the ticket’s ticket's lifetime), operation
is fully automated. The server administrator need not bother with the
management of backup certificates or explicit pins.</t>
  <t>For pins.</li>
        <li>For server clusters, we reuse the existing <xref target="RFC5077"/> infrastructure <xref target="RFC5077" format="default"/>
where it exists.</t>
  <t>Pinning exists.</li>
        <li>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.</t>
</list></t> error.</li>
      </ul>
      <t>A note on terminology: unlike other solutions in this space, we do not
do “certificate pinning” "certificate pinning" (or “public "public key pinning”), pinning"), since the protocol
is oblivious to the server’s server's certificate. We prefer the term “server "server
identity pinning” 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.</t>
      <section anchor="conventions-used-in-this-document" title="Conventions used numbered="true" toc="default">
        <name>Conventions Used in this document">

<t>The This Document</name>
        <t>
    The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL
NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”,
“MAY”, "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL
    NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
    "<bcp14>MAY</bcp14>", and “OPTIONAL” "<bcp14>OPTIONAL</bcp14>" in this document are to be interpreted as
    described in BCP 14 BCP&nbsp;14 <xref target="RFC2119"/> <xref target="RFC8174"/>
    when, and only when, they appear in all capitals, as shown here.</t> here.
        </t>
      </section>
      <section anchor="scope-of-experimentation" title="Scope numbered="true" toc="default">
        <name>Scope of Experimentation"> Experimentation</name>
        <t>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.</t>
        <t>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 to only to strengthen the validation of the server’s server's identity (“second factor”). ("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.</t>
        <t>The goal of the experiment is to prove that:</t>

<t><list style="symbols">
  <t>Non-supporting
        <ul spacing="normal">
          <li>Non-supporting clients and servers are unaffected.</t>
  <t>Connectivity unaffected.</li>
          <li>Connectivity between supporting clients and servers is retained under normal circumstances,
whether the client connects to the server frequently (relative to the ticket’s ticket's lifetime) or very rarely.</t>
  <t>Enterprise rarely.</li>
          <li>Enterprise middleboxes do not interrupt such connectivity.</t>
  <t>Misissued connectivity.</li>
          <li>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.</t>
</list></t> restored.</li>
        </ul>
        <t>Following two years of successful deployment, the authors will publish a document that summarizes
the experiment’s experiment's findings and will resubmit the protocol for
consideration as a Proposed Standard.</t>
      </section>
    </section>
    <section anchor="protocol-overview" title="Protocol Overview"> numbered="true" toc="default">
      <name>Protocol Overview</name>
      <t>The protocol consists of two phases: the first time a particular client
connects to a server, and subsequent connections.</t>
      <t>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.</t>
      <t>The document presents some similarities with the ticket resumption
mechanism described in <xref target="RFC5077"/>. target="RFC5077" format="default"/>. However the scope of this document
differs from session resumption mechanisms implemented with <xref target="RFC5077"/> target="RFC5077" format="default"/>
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, resumption or to authenticate the client. 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.</t>
      <t><xref target="RFC5077"/> target="RFC5077" format="default"/> has been obsoleted
by <xref target="RFC8446"/> target="RFC8446" format="default"/>, and ticket resumption is
now defined by Sec. 2.2 of <xref target="RFC8446"/>. target="RFC8446" sectionFormat="of" section="2.2"  format="default"/>. This document references
<xref target="RFC5077"/> target="RFC5077" format="default"/> as an informational document since it contains a more
thorough discussion of stateless ticket resumption 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 document. writing.  This experience experience,
as well as deployment experience, can easily be re-used for identity pinning.</t>
      <t>With TLS 1.3, session resumption is based on a preshared key 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.</t>
      <t>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 <bcp14>MUST NOT</bcp14> include the extension defined here.</t>
      <section anchor="initial-connection" title="Initial Connection"> numbered="true" toc="default">
        <name>Initial Connection</name>
        <t>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 server's first response, in the returned PinningTicket extension.</t>

<figure><artwork><![CDATA[
        <artwork name="" type="ascii-art" align="left" alt=""><![CDATA[
 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.
]]></artwork></figure>
]]></artwork>
        <t>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.</t>
        <t>Upon receipt of the PinningTicket extension, the server computes a
pinning secret (<xref target="pinning-secret"/>), target="pinning-secret" format="default"/>) and sends the pinning ticket
(<xref target="pinning-ticket"/>) target="pinning-ticket" format="default"/>) encrypted with the pinning protection key
(<xref target="pinning-ticket-key"/>). target="pinning-ticket-key" format="default"/>).  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.</t>
        <t>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 <bcp14>MUST NOT</bcp14> be sent as part of a HelloRetryRequest.</t>
        <t>Upon receiving the pinning ticket, the client MUST NOT <bcp14>MUST NOT</bcp14> accept it until
the key exchange is completed and the server authenticated. If the key
exchange is not completed successfully, the client MUST <bcp14>MUST</bcp14> ignore the
received pinning ticket. Otherwise, the client computes the pinning
secret and SHOULD <bcp14>SHOULD</bcp14> cache the pinning secret and the pinning ticket for
the duration indicated by the pinning ticket lifetime. The client SHOULD <bcp14>SHOULD</bcp14>
clean up the cached values at the end of the indicated lifetime.</t>
      </section>
      <section anchor="subsequent-connections" title="Subsequent Connections"> numbered="true" toc="default">
        <name>Subsequent Connections</name>
        <t>When the client initiates a connection to a server it has previously
seen (see <xref target="indexing"/> target="indexing" format="default"/> on identifying servers), it SHOULD <bcp14>SHOULD</bcp14> send the
pinning ticket for that server.  The pinning ticket, pinning secret secret, and
pinning ticket lifetime computed during the establishment of the
previous TLS session are designated in this document as the “original” "original"
ones, to distinguish them from a new ticket that may be generated during
the current session.</t>
        <t>The server MUST <bcp14>MUST</bcp14> extract the original pinning_secret value from the
ticket and MUST <bcp14>MUST</bcp14> respond with a PinningTicket extension, which includes:</t>

<t><list style="symbols">
  <t>A
        <ul spacing="normal">
          <li>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 server's
(current) public key, as well as the ongoing TLS session. The proof is
mandatory and MUST <bcp14>MUST</bcp14> be included if a pinning ticket was sent by the client.</t>
  <t>A client.</li>
          <li>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 <bcp14>RECOMMENDED</bcp14> to include a fresh ticket with a non zero nonzero length with each
response.</t>
</list></t>
response.</li>
        </ul>
        <t>If the server cannot validate the received ticket, that might indicate
an earlier MITM attack on this client. The server MUST <bcp14>MUST</bcp14> then abort the
connection with a handshake_failure alert, alert and SHOULD <bcp14>SHOULD</bcp14> log this failure.</t>
        <t>The client MUST <bcp14>MUST</bcp14> verify the proof, and if it fails to do so, MUST
the client <bcp14>MUST</bcp14> issue a
handshake_failure alert and abort the connection (see also
<xref target="client_error"/>). target="client_error" format="default"/>).  It is important that the client does not attempt to
“fall back”
"fall back" by omitting the PinningTicket extension.</t>
        <t>When the connection is successfully set up, i.e. i.e., after the Finished
message is verified, the client SHOULD <bcp14>SHOULD</bcp14> store the new ticket along with
the corresponding pinning_secret, replacing the original ticket.</t>
        <t>Although this is an extension, if the client already has a ticket for a
server, the client MUST <bcp14>MUST</bcp14> interpret a missing PinningTicket extension in
the server’s server's response as an attack, because of the server’s server's prior
commitment to respect the ticket. The client MUST <bcp14>MUST</bcp14> abort the connection
in this case.  See also <xref target="ramp_down"/> target="ramp_down" format="default"/> on ramping down support for this
extension.</t>
      </section>
      <section anchor="indexing" title="Indexing numbered="true" toc="default">
        <name>Indexing the Pins"> Pins</name>

        <t>Each pin is associated with a set of identifiers which include that include, among
others host name,
others, hostname, protocol (TLS or DTLS) DTLS), and port
number.  In other words, the pin for port TCP/443 may be different from
that for DTLS 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 host name hostname is used, it MUST <bcp14>MUST</bcp14> be
the value sent inside the Server Name Indication (SNI) extension.  This
definition is similar to the concept of a Web Origin <xref target="RFC6454"/>, target="RFC6454" format="default"/>, but does not assume
the existence of a URL.</t>
        <t>The purpose of ticket pinning is to pin the server identity. As a
result, any information orthogonal to the server’s server's identity MUST NOT <bcp14>MUST NOT</bcp14> be
considered in indexing.  More particularly, IP addresses are ephemeral
and forbidden in SNI SNI, and therefore pins MUST NOT <bcp14>MUST NOT</bcp14> be associated with IP
addresses. Similarly, CA names or public keys associated with server
MUST NOT
<bcp14>MUST NOT</bcp14> be used for indexing as they may change over time.</t>
      </section>
    </section>
    <section anchor="message-definitions" title="Message Definitions"> numbered="true" toc="default">
      <name>Message Definitions</name>
      <t>This section defines the format of the PinningTicket extension.
We follow the message notation of <xref target="RFC8446"/>.</t>

<figure><artwork><![CDATA[ target="RFC8446" format="default"/>.</t>
      <sourcecode name="" type="c"><![CDATA[
 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;
]]></artwork></figure>

<t><list style="hanging">
  <t hangText='ticket'>
]]></sourcecode>
      <dl newline="false" spacing="normal" indent="10">
        <dt>ticket</dt>
        <dd>
  a pinning ticket sent by the client or returned by the server. The
ticket is opaque to the client. The extension MUST <bcp14>MUST</bcp14> contain exactly 0 or
1 tickets.</t>
  <t hangText='proof'> tickets.</dd>
        <dt>proof</dt>
        <dd>
  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 <bcp14>MUST</bcp14> contain exactly 0 or 1
proofs.</t>
  <t hangText='lifetime'>
proofs.</dd>
        <dt>lifetime</dt>
        <dd>
  the duration (in seconds) that the server commits to accept offered
tickets in the future.</t>
</list></t> future.</dd>
      </dl>
    </section>
    <section anchor="crypto" title="Cryptographic Operations"> numbered="true" toc="default">
      <name>Cryptographic Operations</name>
      <t>This section provides details on the cryptographic operations performed
by the protocol peers.</t>
      <section anchor="pinning-secret" title="Pinning Secret"> numbered="true" toc="default">
        <name>Pinning Secret</name>
        <t>The pinning secret is generated locally by the client and the server 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.</t>
        <t>The pinning secret is derived using the Derive-Secret function provided
by TLS 1.3, described in Section “Key Schedule” of <xref target="RFC8446"/>.</t>

<figure><artwork><![CDATA[ target="RFC8446" sectionFormat="of" section="7.1" format="default"/>.</t>
        <sourcecode name="" type="c"><![CDATA[
pinning secret = Derive-Secret(Handshake Secret, "pinning secret",
         ClientHello...ServerHello)
]]></artwork></figure>
]]></sourcecode>
      </section>
      <section anchor="pinning-ticket" title="Pinning Ticket"> numbered="true" toc="default">
        <name>Pinning Ticket</name>
        <t>The pinning ticket contains the pinning secret. The pinning ticket is
provided by the client to the server 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:</t>

<t><list style="symbols">
  <t>Pinning
        <ul spacing="normal">
          <li>Pinning tickets MUST <bcp14>MUST</bcp14> be encrypted and integrity-protected using strong
cryptographic algorithms.</t>
  <t>Pinning algorithms.</li>
          <li>Pinning tickets MUST <bcp14>MUST</bcp14> be protected with a long-term pinning protection
key.</t>
  <t>Pinning
key.</li>
          <li>Pinning tickets MUST <bcp14>MUST</bcp14> include a pinning protection key ID or serial
number as to enable the pinning protection key to be refreshed.</t>
  <t>The refreshed.</li>
          <li>The pinning ticket MAY <bcp14>MAY</bcp14> 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.</t>
</list></t> privacy.</li>
        </ul>

        <t>The pinning ticket’s ticket's format is not specified by this document, but we
RECOMMEND
a format similar to the one proposed by <xref target="RFC5077"/>.</t> target="RFC5077" format="default"/>
is <bcp14>RECOMMENDED</bcp14>.</t>
      </section>
      <section anchor="pinning-ticket-key" title="Pinning numbered="true" toc="default">
        <name>Pinning Protection Key"> Key</name>
        <t>The pinning protection key is only used only by the server and so remains
server implementation specific. <xref target="RFC5077"/> target="RFC5077" format="default"/> recommends the use of two
keys, but when using AEAD algorithms Authenticated Encryption with Associated Data (AEAD) algorithms,
 only a single key is required.</t>

        <t>When a single server terminates TLS for multiple virtual servers using
the Server Name Indication (SNI) SNI mechanism, we it is strongly RECOMMEND to <bcp14>RECOMMENDED</bcp14> 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.</t>
        <t>As noted in <xref target="cluster"/>, target="cluster" format="default"/>, if the server is actually a cluster of
machines, the protection key MUST <bcp14>MUST</bcp14> be synchronized between all the nodes
that accept TLS connections to the same server name.  When <xref target="RFC5077"/> target="RFC5077" format="default"/>
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:</t>

<figure><artwork><![CDATA[
        <sourcecode name="" type="c"><![CDATA[
pinning_protection_key = HKDF-Expand(resumption_protection_key,
                              "pinning protection", L)
]]></artwork></figure>
]]></sourcecode>
        <t>Where resumption_protection_key is the ticket protection key defined in
<xref target="RFC5077"/>. target="RFC5077" format="default"/>. Both resumption_protection_key and pinning_protection_key
are only used by the server.</t>

        <t>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, we RECOMMEND to have it is <bcp14>RECOMMENDED</bcp14> that the resumption_protection_key and pinning_protection_key as two,
be unrelated keys to the pinning_protection_key and that they are separately
shared among the relevant servers.</t>
      </section>
      <section anchor="pinning-proof" title="Pinning Proof"> numbered="true" toc="default">
        <name>Pinning Proof</name>
        <t>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.</t>
        <t>In order to address these requirements, the pinning proof is bound to
the TLS session as well as the public key of the server:</t>

<figure><artwork><![CDATA[
        <sourcecode name="" type="c"><![CDATA[
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))
]]></artwork></figure>
]]></sourcecode>
        <t>where HMAC <xref target="RFC2104"/> target="RFC2104" format="default"/> uses the Hash algorithm that was negotiated in
the handshake, and the same hash is also used over the server’s 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.</t>
      </section>
    </section>
    <section anchor="operational-considerations" title="Operational Considerations"> numbered="true" toc="default">
      <name>Operational Considerations</name>
      <t>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 error, and in the case of public key pinning, can
easily result in “server bricking”: "server bricking": the server becoming inaccessible to
some or all of its users. To achieve this goal 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.</t>
      <section anchor="cluster" title="Protection numbered="true" toc="default">
        <name>Protection Key Synchronization"> Synchronization</name>
        <t>The only operational requirement when deploying this protocol is that that, if
the server is part of a cluster, protection keys (the keys used to
encrypt tickets) MUST <bcp14>MUST</bcp14> be synchronized between all cluster members. The
protocol is designed so that if resumption ticket protection keys
<xref target="RFC5077"/> target="RFC5077" format="default"/> are already synchronized between cluster members, nothing
more needs to be done.</t>
        <t>Moreover, synchronization does not need to be instantaneous, e.g. e.g.,
protection keys can be distributed a few minutes or hours in advance of
their rollover. In such scenarios, each cluster member MUST <bcp14>MUST</bcp14> 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
that, when a client that receives a “new” ticket "new" ticket, it does not next hit a cluster
member that still rejects this ticket.</t>
        <t>Misconfiguration can lead to the server’s server's clock being off by a large
amount of time. Consider a case where a server’s 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.</t>
        <t>The decision to delete a key should at least consider
the largest value of the ticket lifetime as well as the expected time
desynchronisation
desynchronization between the servers of the cluster and the time
difference for distributing the new key among the different servers in
the cluster.</t>
      </section>
      <section anchor="ticket-lifetime" title="Ticket Lifetime"> numbered="true" toc="default">
        <name>Ticket Lifetime</name>
        <t>The lifetime of the ticket is a commitment by the server to retain the
ticket’s
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 <bcp14>SHOULD</bcp14> be between 7 and 31
days.</t>
      </section>
      <section anchor="certificate-renewal" title="Certificate Renewal"> numbered="true" toc="default">
        <name>Certificate Renewal</name>
        <t>The protocol ensures that the client will continue speaking to the
correct server even when the server’s server's certificate is renewed. In this
sense, pinning is not associated with certificates certificates, which is the reason we
designate the protocol described in this document as “server "server identity
pinning”.</t>
pinning".</t>
        <t>Note that this property is not impacted by the use of the server’s server's
public key in the pinning proof, proof because the scope of the public key
used is only the current TLS session.</t>
      </section>
      <section anchor="certificate-revocation" title="Certificate Revocation"> numbered="true" toc="default">
        <name>Certificate Revocation</name>
        <t>The protocol is orthogonal to certificate validation in the sense that,
if the server’s server's certificate has been revoked or is invalid for some
other reason, the client MUST <bcp14>MUST</bcp14> refuse to connect to it regardless of any
ticket-related behavior.</t>
      </section>
      <section anchor="ramp_down" title="Disabling Pinning"> numbered="true" toc="default">
        <name>Disabling Pinning</name>
        <t>A server implementing this protocol MUST <bcp14>MUST</bcp14> have a “ramp down” "ramp down" mode of
operation where:</t>

<t><list style="symbols">
  <t>The
        <ul spacing="normal">
          <li>The server continues to accept valid pinning tickets and responds
  correctly with a proof.</t>
  <t>The proof.</li>
          <li>The server does not send back a new pinning ticket.</t>
</list></t> ticket.</li>
        </ul>
        <t>After a while while, no clients will hold valid tickets any more 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 on using the ticket until its lifetime
expires. However, as detailed in section <xref target="privacy"/>, target="privacy" format="default"/>, re-use of a
ticket by the client may result in privacy concerns as the ticket value
may be used to correlate TLS sessions.</t>
        <t>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.</t>
      </section>
      <section anchor="server-compromise" title="Server Compromise"> numbered="true" toc="default">
        <name>Server Compromise</name>
        <t>If a server compromise is detected, the pinning protection key MUST <bcp14>MUST</bcp14> be
rotated immediately, but the server MUST <bcp14>MUST</bcp14> 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.</t>
        <t>The same procedure applies if the pinning protection key is compromised
directly, e.g. e.g., if a backup copy is inadvertently made public.</t>
      </section>
      <section anchor="disaster-recovery" title="Disaster Recovery"> numbered="true" toc="default">
        <name>Disaster Recovery</name>
        <t>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 server's
private key.</t>
        <t>Readers should note that <xref target="RFC5077"/> target="RFC5077" format="default"/> session resumption keys are more
security sensitive, sensitive and should normally not be backed up up, but rather
treated as ephemeral keys. Even when servers derive pinning secrets from
resumption keys (<xref target="pinning-secret"/>), target="pinning-secret" format="default"/>), they MUST NOT <bcp14>MUST NOT</bcp14> back up resumption
keys.</t>
      </section>
    </section>

    <section anchor="implementation-status" title="Implementation Status">

<t>Note to RFC Editor: please remove this section before publication,
including the reference to <xref target="RFC7942"/>.</t> anchor="security-considerations" numbered="true" toc="default">
      <name>Security Considerations</name>
      <t>This section records the status of known implementations of the protocol
defined by this specification at reviews several security aspects related to the time of posting of this
Internet-Draft, proposed
extension.</t>
      <section anchor="trust-on-first-use-tofu-and-mitm-attacks" numbered="true" toc="default">
        <name>Trust-on-First-Use (TOFU) and MITM Attacks</name>
        <t>This protocol is based on a proposal described in <xref target="RFC7942"></xref>.  The
description of implementations in this section is intended trust-on-first-use protocol. If a client initially
connects to assist the
IETF in its decision processes in progressing drafts to RFCs.  Please
note that "right" server, it will be protected against MITM
attackers for 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 exist.</t>

<t>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”.</t>

<section anchor="mint-fork" title="Mint Fork">

<section anchor="overview" title="Overview">

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

</section>
<section anchor="description" title="Description">

<t>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.</t>

</section>
<section anchor="level-of-maturity" title="Level of Maturity">
<t>This is a prototype.</t>

</section>
<section anchor="coverage" title="Coverage">
<t>The entire protocol is implemented.</t>

</section>
<section anchor="version-compatibility" title="Version Compatibility">
<t>The implementation is compatible with draft-sheffer-tls-pinning-ticket-02.</t>

</section>
<section anchor="licensing" title="Licensing">
<t>Mint itself and this fork are available under an MIT license.</t>

</section>
<section anchor="contact-information" title="Contact Information">
<t>See author details below.</t>

</section>
</section>
</section>
<section anchor="security-considerations" title="Security Considerations">

<t>This section reviews several security aspects related to the proposed
extension.</t>

<section anchor="trust-on-first-use-tofu-and-mitm-attacks" title="Trust on First Use (TOFU) and MITM Attacks">

<t>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 lifetime of each received ticket. If it connects
regularly (depending (depending, of course course, on the server-selected lifetime), it will
stay constantly protected against fake certificates.</t>
        <t>However if it initially connects to an attacker, subsequent connections
to the “right” "right" server will fail. Server operators might want to advise
clients on how to remove corrupted pins, once such large scale large-scale attacks
are detected and remediated.</t>
        <t>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 server's public key, key to ensure the client
that the proof was in fact generated by the server with which it is
initiating the connection.</t>
      </section>
      <section anchor="pervasive-monitoring" title="Pervasive Monitoring"> numbered="true" toc="default">
        <name>Pervasive Monitoring</name>
        <t>Some organizations, and even some countries countries, perform pervasive monitoring
on their constituents <xref target="RFC7258"/>. target="RFC7258" format="default"/>. 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.</t>
        <t>Pervasive monitoring may also result in privacy concerns detailed in
section
<xref target="privacy"/>.</t> target="privacy" format="default"/>.</t>
      </section>
      <section anchor="server_error" title="Server-Side numbered="true" toc="default">
        <name>Server-Side Error Detection"> Detection</name>
        <t>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.</t>
      </section>
      <section anchor="client_policy" title="Client numbered="true" toc="default">
        <name>Client Policy and SSL Proxies"> Proxies</name>
        <t>Like it or not, some clients are normally deployed behind an SSL proxy.
Similarly
Similar to <xref target="RFC7469"/>, target="RFC7469" format="default"/>, it is acceptable to allow pinning to be
disabled for some hosts according to local policy. For example,
a User Agent (UA) MAY <bcp14>MAY</bcp14>
disable pinning for hosts whose validated certificate chain terminates
at a user-defined trust anchor, rather than a trust anchor built-in to built into
the UA (or underlying platform). Moreover, a client MAY <bcp14>MAY</bcp14> accept an empty
PinningTicket extension from such hosts as a valid response.</t>
      </section>
      <section anchor="client_error" title="Client-Side numbered="true" toc="default">
        <name>Client-Side Error Behavior"> Behavior</name>

        <t>When a client receives a malformed or empty PinningTicket extension from
a pinned server, it MUST <bcp14>MUST</bcp14> abort the handshake and MUST NOT retry with no handshake. If the client
retries the request, it <bcp14>MUST NOT</bcp14> omit the
PinningTicket in the request. retry message. Doing otherwise would expose the client to
trivial fallback attacks, similar to those described in <xref target="RFC7507"/>.</t>

<t>This target="RFC7507" format="default"/>.</t>
        <t>However, this rule can however have negative affects on negatively impact clients that move from
behind SSL proxies into the open Internet Internet, and vice versa, if the advice
in <xref target="client_policy"/> target="client_policy" format="default"/> is not followed.  Therefore, we RECOMMEND
it is <bcp14>RECOMMENDED</bcp14>  that
browser and library vendors provide a documented way to remove stored
pins.</t>
      </section>
      <section anchor="stolen-and-forged-tickets" title="Stolen numbered="true" toc="default">
        <name>Stolen and Forged Tickets">

<t>Stealing Tickets</name>
        <t>An attacker gains no benefit from stealing pinning tickets tickets, even in conjunction with other pinning
parameters,
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 server.</t>
      </section>
      <section anchor="privacy" title="Client Privacy"> numbered="true" toc="default">
        <name>Client Privacy</name>

        <t>This protocol is designed so that an external attacker cannot correlate
between link
different requests of 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, ramp down, if the server does not provide
any
a new ticket ticket, and the client re-uses reuses the same ticket. To reduce or avoid such
privacy concerns, it is RECOMMENDED <bcp14>RECOMMENDED</bcp14> for the server to issue a fresh ticket with a
reduced life time. lifetime. This would at least reduce the time period under in which
the TLS session sessions of the client are correlated. can be linked. The server MAY <bcp14>MAY</bcp14> also
issue tickets with a zero second zero-second lifetime until it is confident all
tickets are expired.</t>
        <t>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 <bcp14>SHOULD</bcp14> allow the user to opt out of pinning, either in
general or for particular servers.</t>
        <t>This document does not define the exact content of tickets.
Including client-specific information in tickets would raise privacy concerns
and is NOT RECOMMENDED.</t> <bcp14>NOT RECOMMENDED</bcp14>.</t>
      </section>
      <section anchor="ticket-protection-key-management" title="Ticket numbered="true" toc="default">
        <name>Ticket Protection Key Management"> Management</name>

           <t>While the ticket format is not mandated by this document, we RECOMMEND protecting
the ticket using authenticated encryption to protect it. is <bcp14>RECOMMENDED</bcp14>. Some of the algorithms
commonly used for authenticated encryption, e.g. GCM, e.g., Galois/Counter Mode (GCM), are highly
vulnerable to nonce reuse, and this problem is magnified in a cluster
setting.  Therefore  Therefore, implementations that choose AES-GCM or any AEAD
equivalent MUST <bcp14>MUST</bcp14> adopt
one of these three alternatives:</t>

<t><list style="symbols">
  <t>Partition
        <ul spacing="normal">
          <li>Partition the nonce namespace between cluster members and use monotonic
counters on each member, e.g. e.g., by setting the nonce to the concatenation
of the cluster member ID and an incremental counter.</t>
  <t>Generate counter.</li>
          <li>Generate random nonces but avoid the so-called birthday bound, i.e. 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.</t>
  <t>An key.</li>
          <li>An alternative design which that has been attributed to Karthik Bhargavan is
as follows.  Start with a 128-bit master key “K_master” K_master and then for
each encryption, generate a 256-bit random nonce and compute: K =
HKDF(K_master, Nonce || “key”), "key"), then N = HKDF(K_master, Nonce ||
“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.</t>
</list></t>

</section>
</section>
<section anchor="iana-considerations" title="IANA Considerations">

<t>IANA is requested to allocate a TicketPinning extension value in the TLS
ExtensionType Registry.</t>

<t><xref target="RFC8447"/> defines the procedure and requirements and the necessary
information for the IANA to update the “TLS ExtensionType Values”
registry <xref target="TLS-EXT"/>.</t>

<t>According to <xref target="RFC8447"/> the update of the “TLS ExtensionType Values”
registry is “Specification Required” <xref target="RFC8126"/> which is fulfilled by
the current document, when it is published as an RFC.</t>

<t>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.</t>

<t>The TicketPinning Extension Name is expected to be ticket_pinning.</t>

<t>The TicketPinning Extension Recommended value should be set to “No” with
the publication of the current document as “Experimental”.</t>

<t>The TicketPinning Extension TLS.13 column should be set to CH, EE to
indicate that the TicketPinning Extension is present in ClientHello and
EncryptedExtensions messages.</t>

</section>
<section anchor="acknowledgements" title="Acknowledgements">

<t>The original idea behind this proposal was published in <xref target="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.</t>

<t>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.</t>

</section>

  </middle>

  <back>

    <references title='Normative References'>

<reference  anchor="RFC2119" target='https://www.rfc-editor.org/info/rfc2119'>
<front>
<title>Key words for use in RFCs to Indicate Requirement Levels</title>
<author initials='S.' surname='Bradner' fullname='S. Bradner'><organization /></author>
<date year='1997' month='March' />
<abstract><t>In many standards track documents several words are used to signify the requirements in the specification.  These words are often capitalized. This document defines these words as they should be interpreted in IETF documents.  This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.</t></abstract>
</front>
<seriesInfo name='BCP' value='14'/>
<seriesInfo name='RFC' value='2119'/>
<seriesInfo name='DOI' value='10.17487/RFC2119'/>
</reference>

<reference  anchor="RFC8126" target='https://www.rfc-editor.org/info/rfc8126'>
<front>
<title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
<author initials='M.' surname='Cotton' fullname='M. Cotton'><organization /></author>
<author initials='B.' surname='Leiba' fullname='B. Leiba'><organization /></author>
<author initials='T.' surname='Narten' fullname='T. Narten'><organization /></author>
<date year='2017' month='June' />
<abstract><t>Many protocols make use of points of extensibility that use constants to identify various protocol parameters.  To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper.  For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA).</t><t>To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed.  This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry.</t><t>This is the third edition of this document; it obsoletes RFC 5226.</t></abstract>
</front>
<seriesInfo name='BCP' value='26'/>
<seriesInfo name='RFC' value='8126'/>
<seriesInfo name='DOI' value='10.17487/RFC8126'/>
</reference>

<reference  anchor="RFC8446" target='https://www.rfc-editor.org/info/rfc8446'>
<front>
<title>The Transport Layer Security (TLS) Protocol Version 1.3</title>
<author initials='E.' surname='Rescorla' fullname='E. Rescorla'><organization /></author>
<date year='2018' month='August' />
<abstract><t>This document specifies version 1.3 of the Transport Layer Security (TLS) protocol.  TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery.</t><t>This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961.  This document also specifies new requirements for TLS 1.2 implementations.</t></abstract>
</front>
<seriesInfo name='RFC' value='8446'/>
<seriesInfo name='DOI' value='10.17487/RFC8446'/>
</reference>

<reference  anchor="RFC8447" target='https://www.rfc-editor.org/info/rfc8447'>
<front>
<title>IANA Registry Updates for TLS and DTLS</title>
<author initials='J.' surname='Salowey' fullname='J. Salowey'><organization /></author>
<author initials='S.' surname='Turner' fullname='S. Turner'><organization /></author>
<date year='2018' month='August' />
<abstract><t>This document describes a number of changes to TLS and DTLS IANA registries that range from adding notes to the registry all the way to changing the registration policy.  These changes were mostly motivated by WG review of the TLS- and DTLS-related registries undertaken as part of the TLS 1.3 development process.</t><t>This document updates the following RFCs: 3749, 5077, 4680, 5246, 5705, 5878, 6520, and 7301.</t></abstract>
</front>
<seriesInfo name='RFC' value='8447'/>
<seriesInfo name='DOI' value='10.17487/RFC8447'/>
</reference>

<reference  anchor="RFC8174" target='https://www.rfc-editor.org/info/rfc8174'>
<front>
<title>Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words</title>
<author initials='B.' surname='Leiba' fullname='B. Leiba'><organization /></author>
<date year='2017' month='May' />
<abstract><t>RFC 2119 specifies common key words that may be used in protocol  specifications.  This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the  defined special meanings.</t></abstract>
</front>
<seriesInfo name='BCP' value='14'/>
<seriesInfo name='RFC' value='8174'/>
<seriesInfo name='DOI' value='10.17487/RFC8174'/>
</reference>

    </references>

    <references title='Informative References'>

<reference  anchor="RFC2104" target='https://www.rfc-editor.org/info/rfc2104'>
<front>
<title>HMAC: Keyed-Hashing for Message Authentication</title>
<author initials='H.' surname='Krawczyk' fullname='H. Krawczyk'><organization /></author>
<author initials='M.' surname='Bellare' fullname='M. Bellare'><organization /></author>
<author initials='R.' surname='Canetti' fullname='R. Canetti'><organization /></author>
<date year='1997' month='February' />
<abstract><t>This document describes HMAC, a mechanism for message authentication using cryptographic hash functions. HMAC can be used with any iterative cryptographic hash function, e.g., MD5, SHA-1, in combination with a secret shared key.  The cryptographic strength of HMAC depends on the properties of the underlying hash function.  This memo provides information for the Internet community.  This memo does not specify an Internet standard of any kind</t></abstract>
</front>
<seriesInfo name='RFC' value='2104'/>
<seriesInfo name='DOI' value='10.17487/RFC2104'/>
</reference>

<reference  anchor="RFC5077" target='https://www.rfc-editor.org/info/rfc5077'>
<front>
<title>Transport Layer Security (TLS) Session Resumption without Server-Side State</title>
<author initials='J.' surname='Salowey' fullname='J. Salowey'><organization /></author>
<author initials='H.' surname='Zhou' fullname='H. Zhou'><organization /></author>
<author initials='P.' surname='Eronen' fullname='P. Eronen'><organization /></author>
<author initials='H.' surname='Tschofenig' fullname='H. Tschofenig'><organization /></author>
<date year='2008' month='January' />
<abstract><t>This document describes a mechanism that enables the Transport Layer Security (TLS) server to resume sessions and avoid keeping per-client session state.  The TLS server encapsulates the session state into a ticket and forwards it to the client.  The client can subsequently resume a session using the obtained ticket.  This document obsoletes RFC 4507.  [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='5077'/>
<seriesInfo name='DOI' value='10.17487/RFC5077'/>
</reference>

<reference  anchor="RFC5246" target='https://www.rfc-editor.org/info/rfc5246'>
<front>
<title>The Transport Layer Security (TLS) Protocol Version 1.2</title>
<author initials='T.' surname='Dierks' fullname='T. Dierks'><organization /></author>
<author initials='E.' surname='Rescorla' fullname='E. Rescorla'><organization /></author>
<date year='2008' month='August' />
<abstract><t>This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol.  The TLS protocol provides communications security over the Internet.  The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery.  [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='5246'/>
<seriesInfo name='DOI' value='10.17487/RFC5246'/>
</reference>

<reference  anchor="RFC6454" target='https://www.rfc-editor.org/info/rfc6454'>
<front>
<title>The Web Origin Concept</title>
<author initials='A.' surname='Barth' fullname='A. Barth'><organization /></author>
<date year='2011' month='December' />
<abstract><t>This document defines the concept of an &quot;origin&quot;, which is often used as the scope of authority or privilege by user agents.  Typically, user agents isolate content retrieved from different origins to prevent malicious web site operators from interfering with the operation of benign web sites.  In addition to outlining the principles that underlie the concept of origin, this document details how to determine the origin of a URI and how to serialize an origin into a string.  It also defines an HTTP header field, named &quot;Origin&quot;, that indicates which origins are associated with an HTTP request.   [STANDARDS-TRACK]</t></abstract>
</front>
<seriesInfo name='RFC' value='6454'/>
<seriesInfo name='DOI' value='10.17487/RFC6454'/>
</reference>

<reference  anchor="RFC6962" target='https://www.rfc-editor.org/info/rfc6962'>
<front>
<title>Certificate Transparency</title>
<author initials='B.' surname='Laurie' fullname='B. Laurie'><organization /></author>
<author initials='A.' surname='Langley' fullname='A. Langley'><organization /></author>
<author initials='E.' surname='Kasper' fullname='E. Kasper'><organization /></author>
<date year='2013' month='June' />
<abstract><t>This document describes an experimental protocol for publicly logging the existence of Transport Layer Security (TLS) certificates as they are issued or observed, in a manner that allows anyone to audit certificate authority (CA) activity and notice the issuance of suspect certificates as well as to audit the certificate logs themselves.  The intent is that eventually clients would refuse to honor certificates that do not appear in a log, effectively forcing CAs to add all issued certificates to the logs.</t><t>Logs are network services that implement the protocol operations for submissions and queries that are defined in this document.</t></abstract>
</front>
<seriesInfo name='RFC' value='6962'/>
<seriesInfo name='DOI' value='10.17487/RFC6962'/>
</reference>

<reference  anchor="RFC7258" target='https://www.rfc-editor.org/info/rfc7258'>
<front>
<title>Pervasive Monitoring Is an Attack</title>
<author initials='S.' surname='Farrell' fullname='S. Farrell'><organization /></author>
<author initials='H.' surname='Tschofenig' fullname='H. Tschofenig'><organization /></author>
<date year='2014' month='May' />
<abstract><t>Pervasive monitoring is a technical attack that should be mitigated in the design of IETF protocols, where possible.</t></abstract>
</front>
<seriesInfo name='BCP' value='188'/>
<seriesInfo name='RFC' value='7258'/>
<seriesInfo name='DOI' value='10.17487/RFC7258'/>
</reference>

<reference  anchor="RFC7469" target='https://www.rfc-editor.org/info/rfc7469'>
<front>
<title>Public Key Pinning Extension for HTTP</title>
<author initials='C.' surname='Evans' fullname='C. Evans'><organization /></author>
<author initials='C.' surname='Palmer' fullname='C. Palmer'><organization /></author>
<author initials='R.' surname='Sleevi' fullname='R. Sleevi'><organization /></author>
<date year='2015' month='April' />
<abstract><t>This document defines a new HTTP header that allows web host operators to instruct user agents to remember (&quot;pin&quot;) the hosts' cryptographic identities over a period of time.  During that time, user agents (UAs) will require that the host presents a certificate chain including at least one Subject Public Key Info structure whose fingerprint matches one of the pinned fingerprints for that host.  By effectively reducing the number of trusted authorities who can authenticate the domain during the lifetime of be stored and transmitted with the pin, pinning may reduce
ticket.</li>
        </ul>
      </section>
    </section>
    <section anchor="iana-considerations" numbered="true" toc="default">
      <name>IANA Considerations</name>
      <t>The IANA has allocated a TicketPinning extension value in the incidence of man-in-the-middle attacks due to compromised Certification Authorities.</t></abstract>
</front>
<seriesInfo name='RFC' value='7469'/>
<seriesInfo name='DOI' value='10.17487/RFC7469'/>
</reference>

<reference  anchor="RFC7507" target='https://www.rfc-editor.org/info/rfc7507'>
<front>
<title>TLS Fallback Signaling Cipher Suite Value (SCSV) for Preventing Protocol Downgrade Attacks</title>
<author initials='B.' surname='Moeller' fullname='B. Moeller'><organization /></author>
<author initials='A.' surname='Langley' fullname='A. Langley'><organization /></author>
<date year='2015' month='April' />
<abstract><t>This document "TLS
ExtensionType Values" registry.</t>
      <t><xref target="RFC8447" format="default"/> defines a Signaling Cipher Suite Value (SCSV) that prevents protocol downgrade attacks on the Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) protocols.  It updates RFCs 2246, 4346, 4347, 5246, procedure, requirements, and 6347.  Server update considerations are included.</t></abstract>
</front>
<seriesInfo name='RFC' value='7507'/>
<seriesInfo name='DOI' value='10.17487/RFC7507'/>
</reference>

<reference  anchor="RFC7942" target='https://www.rfc-editor.org/info/rfc7942'>
<front>
<title>Improving Awareness of Running Code: The Implementation Status Section</title>
<author initials='Y.' surname='Sheffer' fullname='Y. Sheffer'><organization /></author>
<author initials='A.' surname='Farrel' fullname='A. Farrel'><organization /></author>
<date year='2016' month='July' />
<abstract><t>This document describes a simple process that allows authors of Internet-Drafts to record the status of known implementations by including an Implementation Status section.  This will allow reviewers and working groups to assign due consideration necessary
information for the IANA to documents that have update the benefit of running code, which may serve "TLS ExtensionType Values"
registry <xref target="TLS-EXT" format="default"/>. The registration procedure
      is "Specification Required" <xref target="RFC8126"/>.</t>

      <t>The TicketPinning extension is registered as evidence of valuable experimentation and feedback that have made the implemented protocols more mature.</t><t>This process follows. (The extension is not mandatory.  Authors of Internet-Drafts are encouraged
      limited to consider using the process for their documents, Private Use, and working groups are invited to think about applying as such has its first byte in the process to all of their protocol specifications.  This document obsoletes RFC 6982, advancing it to a Best Current Practice.</t></abstract>
</front>
<seriesInfo name='BCP' value='205'/>
<seriesInfo name='RFC' value='7942'/>
<seriesInfo name='DOI' value='10.17487/RFC7942'/>
</reference>

<reference anchor="I-D.perrin-tls-tack">
<front>
<title>Trust Assertions for Certificate Keys</title>

<author initials='M' surname='Marlinspike' fullname='Moxie Marlinspike'>
    <organization />
</author>

<date month='January' day='7' year='2013' />

<abstract><t>This document defines a TLS Extension range
      0-254.)</t>
      <dl>
	<dt>Value:</dt><dd>32</dd>
	<dt>Name:</dt><dd>ticket_pinning</dd>
	<dt>Recommended:</dt><dd>No</dd>
	<dt>TLS 1.3:</dt><dd>CH, EE (to indicate that enables a TLS server to support "pinning" to a self-chosen signing key.  A client contacting a pinned host will require the server to extension is present a signature from the signing key over the TLS server's public key.</t></abstract>

</front>

<seriesInfo name='Internet-Draft' value='draft-perrin-tls-tack-02' />
<format type='TXT'
        target='http://www.ietf.org/internet-drafts/draft-perrin-tls-tack-02.txt' />
</reference>
	in ClientHello and EncryptedExtensions messages)</dd>
      </dl>
    </section>
  </middle>
  <back>
  <displayreference target="I-D.perrin-tls-tack" to="TLS-TACK"/>
    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8126.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8446.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8447.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
      </references>
      <references>

        <name>Informative References</name>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2104.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5077.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5246.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6454.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6962.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7258.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7469.xml"/>
        <xi:include
	    href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7507.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8555.xml"/>

        <!-- draft-ietf-draft-perrin-tls-tack-02 expired -->
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml3/reference.I-D.draft-perrin-tls-tack-02.xml"/>

        <reference anchor="Oreo" > anchor="Oreo">
          <front>
            <title>Firm Grip Handshakes: A Tool for Bidirectional Vouching</title>
            <author initials="O." surname="Berkman">
      <organization></organization>
              <organization/>
            </author>
            <author initials="B." surname="Pinkas">
      <organization></organization>
              <organization/>
            </author>
            <author initials="M." surname="Yung">
      <organization></organization>
              <organization/>
            </author>
            <date year="2012"/>
          </front>
            <seriesInfo name="Cryptology and Network Security, pp. 142-157" value=""/> Security" value="pp. 142-157"/>
        </reference>

        <reference anchor="Netcraft" target="http://news.netcraft.com/archives/2016/03/30/http-public-key-pinning-youre-doing-it-wrong.html">
                   target="https://news.netcraft.com/archives/2016/03/30/http-public-key-pinning-youre-doing-it-wrong.html">
          <front>
            <title>HTTP Public Key Pinning: You're doing it wrong!</title>
            <author initials="P." surname="Mutton" fullname="Paul Mutton">
      <organization></organization>
              <organization/>
            </author>
            <date year="2016" month="March"/>
          </front>
        </reference>

        <reference anchor="TLS-EXT" target="https://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtml#tls-extensiontype-values-1">
                   target="https://www.iana.org/assignments/tls-extensiontype-values/">
          <front>
            <title>TLS Extension Type Value</title>
    <author initials="." surname="IANA">
      <organization></organization>
            <author>
              <organization>IANA</organization>
            </author>
    <date year="2018"/>
          </front>
        </reference>
      </references>
    </references>
    <section anchor="previous-work" title="Previous Work"> numbered="true" toc="default">
      <name>Previous Work</name>
      <t>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 organization's approval (a misissued or “fake” "fake"
certificate), and use the certificate to impersonate the organization.
There are many attempts to resolve these weaknesses, including the
Certificate Transparency (CT) protocol <xref target="RFC6962"/>, target="RFC6962" format="default"/>,
HTTP Public Key Pinning (HPKP) <xref target="RFC7469"/>, target="RFC7469" format="default"/>,
and TACK Trust Assertions for Certificate Keys (TACK) <xref target="I-D.perrin-tls-tack"/>.</t> target="I-D.perrin-tls-tack" format="default"/>.</t>
      <t>CT requires
cooperation of a large portion of the hundreds of extant certificate
authorities (CAs) before it can be used “for real”, "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, use because many organizations
are not willing to expose their internal infrastructure publicly.</t>
      <t>TACK has some similarities
to the current proposal, but work on it seems to have stalled.  <xref target="tack"/> target="tack" format="default"/>
compares our proposal to TACK.</t>
      <t>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 in the ACME
WG
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 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 an HTTP header 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 server's certificate
management
management, and that can be entirely and easily automated. <xref target="hpkp"/> target="hpkp" format="default"/>
compares HPKP to the current document in more detail.</t>
      <t>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.</t>
      <t>This section compares ticket pinning to two earlier proposals, HPKP and TACK.</t>
      <section anchor="hpkp" title="Comparison: HPKP"> numbered="true" toc="default">
        <name>Comparison: HPKP</name>
        <t>The current IETF standard for pinning the identity of web servers is the
Public Key Pinning Extension for HTTP, or
HPKP <xref target="RFC7469"/>.</t> target="RFC7469" format="default"/>.</t>
        <t>The main differences between HPKP and the current document are the
following:</t>

<t><list style="symbols">
  <t>HPKP
        <ul spacing="normal">
          <li>HPKP limits its scope to HTTPS, while the current document considers all
application above TLS.</t>
  <t>HPKP TLS.</li>
          <li>HPKP pins the public key of the server (or another public key along the
certificate chain) chain), and as such 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.</t>
  <t>HPKP
management.</li>
          <li>HPKP pins public keys which that are public and used for the standard TLS
authentication.  Identity pinning relies on the ownership of the pinning
key
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 independent, second-factor authentication mechanism.</t>
  <t>HPKP mechanism.</li>
          <li>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.</t>
  <t>HPKP automated.</li>
          <li>HPKP relies on the client to report errors to the report-uri.  The
current document does not need any out-of band out-of-band mechanism, and the server is
informed automatically. This provides an easier and more reliable health
monitoring.</t>
</list></t>
monitoring.</li>
        </ul>
        <t>On the other hand, HPKP shares the following aspects with identity pinning:</t>

<t><list style="symbols">
  <t>Both
        <ul spacing="normal">
          <li>Both mechanisms provide hard failure.  With HPKP 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.</t>
  <t>Both failures.</li>
          <li>Both mechanisms are subject to a server compromise in which users are
provided with an invalid ticket (e.g. (e.g., a random one) or HTTP Header, header with
a very long lifetime. For identity pinning, this lifetime SHOULD NOT <bcp14>SHOULD NOT</bcp14> 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.</t>
</list></t> attacker.</li>
        </ul>
        <t>Unfortunately HPKP has not seen wide deployment yet.  As of March 2016,
the number of servers using HPKP was less than 3000 <xref target="Netcraft"/>. target="Netcraft" format="default"/>.  This
may simply be due to inertia, but we believe the main reason is the
interactions between HPKP and manual certificate management which that is
needed to implement HPKP for enterprise servers. The penalty for making
mistakes (e.g. (e.g., being too early or too late to deploy new pins) is having that
the server become becomes unusable for some of the clients.</t>
        <t>To demonstrate this point, we present a list of the steps involved in
deploying HPKP on a security-sensitive Web web server.</t>

<t><list style="numbers">
        <ol spacing="normal" type="1">
          <li>
            <t>Generate two public/private key-pairs key pairs on a computer that is not the
Live
live server. The second one is the “backup1” key-pair.  <vspace blankLines='1'/>
<spanx style="verb">openssl "backup1" key pair.  </t>
            <sourcecode type="bash">
openssl genrsa -out "example.com.key" 2048;</spanx>  <vspace blankLines='1'/>
<spanx style="verb">openssl 2048;

openssl genrsa -out "example.com.backup1.key" 2048;</spanx></t> 2048;
</sourcecode>
          </li>

          <li>
            <t>Generate hashes for both of the public keys. These will be used in
the HPKP header:  <vspace blankLines='1'/>
<spanx style="verb">openssl  </t>
            <sourcecode type="bash">
openssl rsa -in "example.com.key" -outform der -pubout | \
openssl dgst -sha256 -binary | openssl enc -base64</spanx>  <vspace blankLines='1'/>
<spanx style="verb">openssl -base64

openssl rsa -in "example.com.backup1.key" -outform der \
-pubout | openssl dgst -sha256 -binary | openssl enc -base64</spanx></t> -base64
</sourcecode>
          </li>
          <li>
            <t>Generate a single CSR (Certificate Signing Request) for the first
key-pair,
key pair, where you include the domain name in the CN (Common Name)
field:  <vspace blankLines='1'/>
<spanx style="verb">openssl  </t>
            <sourcecode type="bash">
openssl req -new -subj "/C=GB/ST=Area/L=Town/O=Company/CN=example.com" "/C=GB/ST=Area/L=Town/O=Org/ \
CN=example.com" -key "example.com.key" -out "example.com.csr";</spanx></t>
  <t>Send "example.com.csr";
</sourcecode>
          </li>
          <li>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.</t> two.</li>
          <li>
            <t>On the Live live server, upload and setup set up the first key-pair (and key pair and its
certificate).
certificate.  At this point point, you can add the “Public-Key-Pins” "Public-Key-Pins" header,
using the two hashes you created in step 2.  <vspace blankLines='1'/>  </t>
            <t>
Note that only the first key-pair key pair has been uploaded to the server so far.</t>
  <t>Store
          </li>
          <li>Store the second (backup1) key-pair key pair somewhere safe, probably
somewhere encrypted like a password manager.  It won’t won't expire, as it’s it's
just a key-pair, key pair; it just needs to be ready for when you need to get your
next certificate.</t>
  <t>Time passes… certificate.</li>
          <li>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 compromised, and you need to replace the key-pair key pair and certificate.</t>
  <t>Create certificate.</li>
          <li>Create a new CSR (Certificate Signing Request) using the “backup1”
key-pair, "backup1"
key pair, and get a new certificate from your CA.</t>
  <t>Generate CA.</li>
          <li>Generate a new backup key-pair key pair (backup2), get its hash, and store it
in a safe place (again, not on the Live server).</t>
  <t>Replace live server).</li>
          <li>Replace your old certificate and old key-pair, and key pair, update the
“Public-Key-Pins”
"Public-Key-Pins" header to remove the old hash, and add the new
“backup2” key-pair.</t>
</list></t>
"backup2" key pair.</li>
        </ol>

        <t>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, protocol <xref target="RFC8555"/>, key backup would be a
challenge to automate.</t>
      </section>
      <section anchor="tack" title="Comparison: TACK"> numbered="true" toc="default">
        <name>Comparison: TACK</name>
        <t>Compared with HPKP, TACK <xref target="I-D.perrin-tls-tack"/> target="I-D.perrin-tls-tack" format="default"/> is a lot 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:</t>

<t><list style="symbols">
  <t>Probably
        <ul spacing="normal">
          <li>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” "<bcp14>MAY</bcp14>" requirement (Sec. 5.3). (<xref target="I-D.perrin-tls-tack" sectionFormat="comma" section="5.3" format="default"/>).  With ticket pinning, certificate
validation by the client remains a MUST <bcp14>MUST</bcp14> requirement, and the ticket acts
only as a second factor. If the pinning secret is compromised, the
server’s
server's security is not immediately at risk.</t>
  <t>Both risk.</li>
          <li>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.</t>
  <t>TACK steps.</li>
          <li>TACK uses ECDSA Elliptic Curve Digital Signature Algorithm
(ECDSA) to sign the server’s 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.</t>
  <t>TACK
tickets.</li>
          <li>TACK allows multiple servers to share its public keys. Such sharing is
disallowed by the current document.</t>
  <t>TACK document.</li>
          <li>TACK does not allow the server to track a particular client, and so
has better privacy properties than the current document.</t>
  <t>TACK document.</li>
          <li>TACK has an interesting way to determine the pin’s 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 web sites which 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.</t>
</list></t>

</section>
</section>
<section anchor="document-history" title="Document History">

<section anchor="draft-sheffer-tls-pinning-ticket-12" title="draft-sheffer-tls-pinning-ticket-12">

<t><list style="symbols">
  <t>IETF-Conflict Review comments.</t>
  <t>IANA: removed request for a specific extension value.</t>
</list></t>

</section>
<section anchor="draft-sheffer-tls-pinning-ticket-11" title="draft-sheffer-tls-pinning-ticket-11">

<t><list style="symbols">
  <t>Comments by Ben Kaduk. Specifically, changed the derivation of the pinning proof
to make it more in line with the TLS 1.3 key schedule.</t>
</list></t>

</section>
<section anchor="draft-sheffer-tls-pinning-ticket-10" title="draft-sheffer-tls-pinning-ticket-10">

<t><list style="symbols">
  <t>ISE comments by Adrian Farrel, the ISE.</t>
</list></t> basis.</li>
        </ul>
      </section>
<section anchor="draft-sheffer-tls-pinning-ticket-09" title="draft-sheffer-tls-pinning-ticket-09">

<t><list style="symbols">
  <t>ISE comments by Yoav Nir.</t>
</list></t>

</section>
<section anchor="draft-sheffer-tls-pinning-ticket-08" title="draft-sheffer-tls-pinning-ticket-08">

<t><list style="symbols">
  <t>ISE comments by Rich Salz.</t>
</list></t>
    </section>

    <section anchor="draft-sheffer-tls-pinning-ticket-07" title="draft-sheffer-tls-pinning-ticket-07">

<t><list style="symbols">
  <t>Refer to anchor="acknowledgments" numbered="false" toc="default">
      <name>Acknowledgments</name>
      <t>The original idea behind this proposal was published RFCs.</t>
</list></t>

</section>
<section anchor="draft-sheffer-tls-pinning-ticket-06" title="draft-sheffer-tls-pinning-ticket-06">

<t><list style="symbols">
  <t>IANA Considerations in preparation for Experimental publication.</t>
</list></t>

</section>
<section anchor="draft-sheffer-tls-pinning-ticket-05" title="draft-sheffer-tls-pinning-ticket-05">

<t><list style="symbols">
  <t>Multiple comments from Eric Rescorla.</t>
</list></t>

</section>
<section anchor="draft-sheffer-tls-pinning-ticket-04" title="draft-sheffer-tls-pinning-ticket-04">

<t><list style="symbols">
  <t>Editorial changes.</t>
  <t>Two-phase rotation of protection key.</t>
</list></t>

</section>
<section anchor="draft-sheffer-tls-pinning-ticket-03" title="draft-sheffer-tls-pinning-ticket-03">

<t><list style="symbols">
  <t>Deleted redundant length fields in the extension’s formal definition.</t>
  <t>Modified cryptographic operations to align with the <xref target="Oreo" format="default"/> by
Moti Yung, Benny Pinkas, and Omer Berkman.  The current state protocol is
but a distant relative of TLS 1.3.</t>
  <t>Numerous textual improvements.</t>
</list></t>

</section>
<section anchor="draft-sheffer-tls-pinning-ticket-02" title="draft-sheffer-tls-pinning-ticket-02">

<t><list style="symbols">
  <t>Added an Implementation Status section.</t>
  <t>Added lengths into the extension structure.</t>
  <t>Changed original Oreo protocol, and any errors
are the computation responsibility of the pinning proof to be more robust.</t>
  <t>Clarified requirements on the length authors of the pinning_secret.</t>
  <t>Revamped the HPKP section this document alone.</t>
      <t>We would like to be more in line with current practices, thank Adrian Farrel, Dave Garrett,
Daniel Kahn Gillmor,
Alexey Melnikov,
Yoav Nir,
Eric Rescorla, Benjamin Kaduk, and added recent
statistics on HPKP deployment.</t>
</list></t>

</section>
<section anchor="draft-sheffer-tls-pinning-ticket-01" title="draft-sheffer-tls-pinning-ticket-01">

<t><list style="symbols">
  <t>Corrected the notation Rich Salz for variable-sized vectors.</t>
  <t>Added a section on disaster recovery and backup.</t>
  <t>Added a section their comments on privacy.</t>
  <t>Clarified the assumptions behind this document.
Special thanks to Craig Francis for contributing the HPKP procedure in the comparison section.</t>
  <t>Added a definition of pin indexing (origin).</t>
  <t>Adjusted deployment
script, and to the latest TLS 1.3 notation.</t>
</list></t>

</section>
<section anchor="draft-sheffer-tls-pinning-ticket-00" title="draft-sheffer-tls-pinning-ticket-00">

<t>Initial version.</t>

</section> Ralph Holz for several fruitful discussions.</t>
    </section>
  </back>

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