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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<body>
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<table summary="layout" width="66%" border="0" cellpadding="0" cellspacing="0"><tr><td><table summary="layout" width="100%" border="0" cellpadding="2" cellspacing="1">
<tr><td class="header">TLS Working Group</td><td class="header">M. Marlinspike</td></tr>
<tr><td class="header">Internet-Draft</td><td class="header">T. Perrin, Ed.</td></tr>
<tr><td class="header">Intended status: Standards Track</td><td class="header">May 12, 2012</td></tr>
<tr><td class="header">Expires: November 13, 2012</td><td class="header"> </td></tr>
</table></td></tr></table>
<h1><br />Trust Assertions for Certificate Keys<br />draft-tack.txt</h1>
<h3>Abstract</h3>
<p>
This document defines TACK, a TLS Extension that enables a TLS server to
assert the authenticity of its public key. A TACK contains a "TACK key"
which is used to sign the public key from the TLS server's certificate.
Hostnames can be "pinned" to a TACK key. TLS connections to a pinned
hostname require the server to present a TACK containing the pinned key
and a corresponding signature over the TLS server's public key.
</p>
<h3>Status of this Memo</h3>
<p>
This Internet-Draft is submitted in full
conformance with the provisions of BCP 78 and BCP 79.</p>
<p>
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/.</p>
<p>
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.”</p>
<p>
This Internet-Draft will expire on November 13, 2012.</p>
<h3>Copyright Notice</h3>
<p>
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.</p>
<p>
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.</p>
<a name="toc"></a><br /><hr />
<h3>Table of Contents</h3>
<p class="toc">
<a href="#intro">1.</a>
Introduction<br />
<a href="#anchor1">2.</a>
Requirements notation<br />
<a href="#anchor2">3.</a>
Overview<br />
<a href="#anchor3">3.1.</a>
TACK life cycle<br />
<a href="#anchor4">3.2.</a>
Pin life cycle<br />
<a href="#anchor5">4.</a>
TACK Extension<br />
<a href="#anchor6">4.1.</a>
Definition of TACK_Extension<br />
<a href="#anchor7">4.2.</a>
Explanation of TACK_Extension fields<br />
<a href="#anchor8">4.2.1.</a>
TACK fields<br />
<a href="#anchor9">4.2.2.</a>
TACK_Break_Sig fields<br />
<a href="#anchor10">4.2.3.</a>
TACK_Extension fields<br />
<a href="#clientproc">5.</a>
Client processing<br />
<a href="#anchor11">5.1.</a>
TACK pins, key records, and name records<br />
<a href="#anchor12">5.2.</a>
High-level client processing<br />
<a href="#details">5.3.</a>
Client processing details<br />
<a href="#wellformed">5.3.1.</a>
Check whether the TLS handshake is well-formed<br />
<a href="#anchor13">5.3.2.</a>
Check the TACK generation and update min_generation<br />
<a href="#anchor14">5.3.3.</a>
Check whether the TACK is expired<br />
<a href="#creating">5.3.4.</a>
Create and activate pins (optional)<br />
<a href="#anchor15">5.3.5.</a>
Discard pins based on break signatures<br />
<a href="#deleting">5.3.6.</a>
Deleting pins<br />
<a href="#anchor16">6.</a>
Variations on client processing<br />
<a href="#certver">6.1.</a>
TACK and certificate verification<br />
<a href="#anchor17">6.2.</a>
Application-specific pinning<br />
<a href="#anchor18">7.</a>
TACK IDs<br />
<a href="#anchor19">8.</a>
Advice<br />
<a href="#servadvice">8.1.</a>
For server operators<br />
<a href="#anchor20">8.2.</a>
For client implementers<br />
<a href="#security">9.</a>
Security considerations<br />
<a href="#anchor21">9.1.</a>
For server operators<br />
<a href="#security_client">9.2.</a>
For client implementers<br />
<a href="#future">9.3.</a>
Note on algorithm agility<br />
<a href="#anchor22">10.</a>
IANA considerations<br />
<a href="#anchor23">10.1.</a>
New entry for the TLS ExtensionType Registry<br />
<a href="#anchor24">11.</a>
Acknowledgements<br />
<a href="#rfc.references1">12.</a>
Normative references<br />
<a href="#rfc.authors">§</a>
Authors' Addresses<br />
</p>
<br clear="all" />
<a name="intro"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.1"></a><h3>1.
Introduction</h3>
<p>
Traditionally, a TLS client verifies a TLS server's public key using a
certificate chain issued by some public CA. "Pinning" is a way for clients to
obtain increased certainty in server public keys. Clients that employ pinning
check for some constant "pinned" element of the TLS connection when
contacting a particular TLS host.
</p>
<p>
Unfortunately, a number of problems arise when attempting to pin certificate
chains: the TLS servers at a given hostname may have different certificate
chains simultaneously deployed and may change their chains at any time, the
"more constant" elements of a chain (the CAs) may not be trustworthy, and the
client may be oblivious to key compromise events which render the pinned data
untrustworthy.
</p>
<p>
TACK addresses these problems by having the site sign its TLS server public
keys with a "TACK key". This enables clients to "pin" a hostname to the TACK
key without requiring sites to modify their existing certificate chains, and
without limiting a site's flexibility to deploy different certificate chains
on different servers or change certificate chains at any time. Since TACK pins
are based on TACK keys (instead of CA keys), trust in CAs is not required.
Additionally, the TACK key may be used to revoke previous TACK signatures (or
even itself) in order to handle the compromise of TLS or TACK private keys.
</p>
<p>
If requested, a compliant server will send a TLS Extension containing its
"TACK". Inside the TACK is a public key and signature. Once a client has seen
the same (hostname, TACK public key) pair multiple times, the client will
"activate" a pin between the hostname and TACK key for a period equal to the
length of time the pair has been observed for. This "pin activation" process
limits the impact of bad pins resulting from transient network attacks or
operator error.
</p>
<p>
TACK pins are easily shared between clients. For example, a TACK client
may scan the internet to discover TACK pins, then publish these pins for other
clients to rely upon.
</p>
<a name="anchor1"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.2"></a><h3>2.
Requirements notation</h3>
<p>The key words "MUST",
"MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as
described in <a class='info' href='#RFC2119'>[RFC2119]<span> (</span><span class='info'>Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.</span><span>)</span></a>.
</p>
<a name="anchor2"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.3"></a><h3>3.
Overview</h3>
<a name="anchor3"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.3.1"></a><h3>3.1.
TACK life cycle</h3>
<p>A server operator using TACK may perform several processes:
</p>
<blockquote class="text"><dl>
<dt>Selection of a TACK key:</dt>
<dd>
The server operator first chooses the ECDSA signing key to use for a set of
hostnames. It is safest to use a different signing key for each hostname,
though a signing key may be reused for closely-related hostnames (such as
aliases for the same host, or hosts sharing the same TLS key).
</dd>
<dt>Creating initial TACKs under a TACK key:</dt>
<dd>
The TACK private key is then used to sign the TLS public keys for all servers
associated with those hostnames. The TACK public key and signature are
combined with some metadata into each server's "TACK".
</dd>
<dt>Deploying initial TACKs:</dt>
<dd>
For each hostname, TACKs are deployed to TLS servers in a two-stage process.
First, each TLS server associated with the hostname is given a TACK. Once this
is completed, pin activation is enabled on the servers.
</dd>
<dt>Creating new TACKs under a TACK key:</dt>
<dd>
A TACK needs to be replaced whenever a server changes its TLS public key, or
when the TACK expires. TACKs may also need to be replaced with
later-generation TACKs if the TACK key's "min_generation" is updated (see
next).
</dd>
<dt>Revoking old TACKs:</dt>
<dd>
If a TLS private key is compromised, the TACKs signing this key can be revoked by
publishing a new TACK containing a higher "min_generation".
</dd>
<dt>Revoking TACK keys:</dt>
<dd>
If a TACK private key is compromised, or a server operator wishes to stop using
TACK or abruptly change its TACK key for any reason, a server can revoke an entire
TACK key (including all TACKs and pins referring to it) by publishing a "break
signature".
</dd>
</dl></blockquote><p>
</p>
<a name="anchor4"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.3.2"></a><h3>3.2.
Pin life cycle</h3>
<p>
A TACK client maintains a store of pins for verifying TLS connections. Pins
associate a hostname and a TACK key. When a client sees a new hostname and
TACK key combination, an inactive pin is created. Every subsequent time the
client sees the same pin, the pin is "activated" for a period equal to the
timespan between the first time the pin was seen and the most recent time, up
to a maximum period of 30 days.
</p>
<p>
Pin activation prevents an attacker with short-lived control of the hostname
from activating long-lived pins. It also makes it safer for sites to
experiment with TACKs, as a new TACK can be discarded without causing
long-lived problems. The 30 day limit guarantees that a worst-case pin can be
recovered from in reasonable time.
</p>
<p>In addition to creating and activating pins, a TLS connection can alter the
clients's pin store by publishing revocation data:</p>
<blockquote class="text"><dl>
<dt>Min_generation:</dt>
<dd>
Each pin stores the highest "min_generation" value it has seen from the pinned
TACK key, and rejects TACKs from earlier generations.
</dd>
<dt>Break signatures:</dt>
<dd>
A TLS handshake may send break signatures which cause all pins for the
broken key to be discarded.
</dd>
</dl></blockquote><p>
</p>
<a name="anchor5"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.4"></a><h3>4.
TACK Extension</h3>
<a name="anchor6"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.4.1"></a><h3>4.1.
Definition of TACK_Extension</h3>
<p>
A new TLS ExtensionType ("tack") is defined and MAY be included by a TLS
client in the ClientHello message defined in <a class='info' href='#RFC5246'>[RFC5246]<span> (</span><span class='info'>Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.</span><span>)</span></a>.
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
enum {tack(TBD), (65535)} ExtensionType;
</pre></div>
<p>
The "extension_data" field of this ClientHello SHALL be empty. A TLS server
which is not resuming a TLS session MAY respond with an extension of type
"tack" in the ServerHello. The "extension_data" field of this ServerHello
SHALL contain a "TACK_Extension", as defined below using the TLS presentation
language from <a class='info' href='#RFC5246'>[RFC5246]<span> (</span><span class='info'>Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.</span><span>)</span></a>.
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
enum (disabled(0), enabled(1)} TACK_Activation;
struct {
opaque public_key[64];
uint8 min_generation;
uint8 generation;
uint32 expiration;
opaque target_hash[32];
opaque signature[64];
} TACK; /* 166 bytes */
struct {
opaque public_key[64];
opaque signature[64];
} TACK_Break_Sig; /* 128 bytes */
struct {
TACK tack<0...166> /* 0 or 1 TACK */
TACK_Break_Sig break_sigs<0...1024> /* 0...8 Break Sigs */
TACK_Activation pin_activation;
} TACK_Extension;
</pre></div>
<p>
</p>
<a name="anchor7"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.4.2"></a><h3>4.2.
Explanation of TACK_Extension fields</h3>
<a name="anchor8"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.4.2.1"></a><h3>4.2.1.
TACK fields</h3>
<p>
</p>
<blockquote class="text"><dl>
<dt>public_key:</dt>
<dd>
This field specifies the TACK's public key. The field contains a pair
of integers (x, y) representing a point on the elliptic curve P-256 defined in
<a class='info' href='#FIPS186-3'>[FIPS186‑3]<span> (</span><span class='info'>National Institute of Standards and Technology, “Digital Signature Standard,” June 2009.</span><span>)</span></a>. Each integer is encoded as a 32-byte octet string
using the Integer-to-Octet-String algorithm from <a class='info' href='#RFC6090'>[RFC6090]<span> (</span><span class='info'>McGrew, D., Igoe, K., and M. Salter, “Fundamental Elliptic Curve Cryptography Algorithms,” February 2011.</span><span>)</span></a>, and
these strings are concatenated with the x value first. (NOTE: This is
equivalent to an uncompressed subjectPublicKey from <a class='info' href='#RFC5480'>[RFC5480]<span> (</span><span class='info'>Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk, “Elliptic Curve Cryptography Subject Public Key Information,” March 2009.</span><span>)</span></a>,
except that the initial 0x04 byte is omitted).
</dd>
<dt>min_generation:</dt>
<dd>
This field publishes a min_generation value.
</dd>
<dt>generation:</dt>
<dd>
This field assigns each TACK a generation. Generations less than a published
min_generation are considered revoked.
</dd>
<dt>expiration:</dt>
<dd>
This field specifies a time after which the TACK is considered expired. The
time is encoded as the number of minutes, excluding leap seconds, after
midnight UTC, January 1 1970.
</dd>
<dt>target_hash:</dt>
<dd>
This field is a hash of the TLS server's SubjectPublicKeyInfo <a class='info' href='#RFC5280'>[RFC5280]<span> (</span><span class='info'>Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” May 2008.</span><span>)</span></a> using the SHA256 algorithm from <a class='info' href='#FIPS180-2'>[FIPS180‑2]<span> (</span><span class='info'>National Institute of Standards and Technology, “Secure Hash Standard,” August 2002.</span><span>)</span></a>.
The SubjectPublicKeyInfo is typically conveyed as part of the server's X.509
certificate.
</dd>
<dt>signature:</dt>
<dd>
This field is an ECDSA signature by the TACK's public key over the 8 byte
ASCII string "tack_sig" followed by the contents of the TACK prior to the
"signature" field (i.e. the preceding 102 bytes). The field contains a pair of
integers (r, s) representing an ECDSA signature as defined in <a class='info' href='#FIPS186-3'>[FIPS186‑3]<span> (</span><span class='info'>National Institute of Standards and Technology, “Digital Signature Standard,” June 2009.</span><span>)</span></a>, using curve P-256 and SHA256. Each integer is encoded as
a 32-byte octet string using the Integer-to-Octet-String algorithm from <a class='info' href='#RFC6090'>[RFC6090]<span> (</span><span class='info'>McGrew, D., Igoe, K., and M. Salter, “Fundamental Elliptic Curve Cryptography Algorithms,” February 2011.</span><span>)</span></a>, and these strings are concatenated with the r value first.
</dd>
</dl></blockquote><p>
</p>
<a name="anchor9"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.4.2.2"></a><h3>4.2.2.
TACK_Break_Sig fields</h3>
<p>
</p>
<blockquote class="text"><dl>
<dt>public_key:</dt>
<dd>
This field specifies the TACK key being broken. The key is encoded as per
TACK.public_key.
</dd>
<dt>signature:</dt>
<dd>
This field is an ECDSA signature by the TACK_Break_Sig's public key over the
14 byte ASCII string "tack_break_sig". The field contains a pair of integers
(r, s) representing an ECDSA signature as defined in <a class='info' href='#FIPS186-3'>[FIPS186‑3]<span> (</span><span class='info'>National Institute of Standards and Technology, “Digital Signature Standard,” June 2009.</span><span>)</span></a>, using curve P-256 and SHA256. It is calculated and
encoded as per TACK.signature.
</dd>
</dl></blockquote><p>
</p>
<a name="anchor10"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.4.2.3"></a><h3>4.2.3.
TACK_Extension fields</h3>
<p>
</p>
<blockquote class="text"><dl>
<dt>tack:</dt>
<dd>
This field provides the server's TACK. It MAY be empty, or MAY contain a TACK.
</dd>
<dt>break_sigs:</dt>
<dd>
This field provides break signatures. It MAY be empty, or MAY contain up to 8
break signatures.
</dd>
<dt>pin_activation:</dt>
<dd>
If pin activation is enabled, then the TACK_Extension MAY be used by clients
to activate or extend the activation of TACK pins. This field is typically
toggled from a disabled to an enabled state once TACKs have been deployed to
all TLS servers for a hostname.
</dd>
</dl></blockquote><p>
</p>
<p> Note that both the "tack" and "break_sigs" fields MAY be empty.
</p>
<a name="clientproc"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.5"></a><h3>5.
Client processing</h3>
<a name="anchor11"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.5.1"></a><h3>5.1.
TACK pins, key records, and name records</h3>
<p>
A client supporting TACK SHALL have a local store of pins, consisting of "key
records" and "name records". Each name record is associated with a key record.
Multiple name records MAY be associated with one key record. A "pin" refers to
a (name record, key record) pair.
</p>
<p>
A "key record" contains:
</p>
<blockquote class="text">
<p>
TACK public key (or hash): A public key or a cryptographically-secure, second
preimage-resistant hash of a public key. A client SHALL NOT store multiple key
records referencing the same key.
</p>
<p>
Min_generation: A single byte used to detect revoked TACKs.
</p>
</blockquote><p>
</p>
<p>
A "name record" contains:
</p>
<blockquote class="text">
<p>
Name: A fully qualified DNS hostname. A client SHALL NOT store multiple name
records with the same name. The TLS server's hostname is considered the
"relevant name", and a pin whose name exactly matches the relevant name is
considered a "relevant pin".
</p>
<p>
Initial timestamp: A timestamp noting when this pin was created.
</p>
<p>
Active period end: Empty or a timestamp. If empty or set to a time in the
past, the pin is "inactive". If set to a future time, the pin is "active"
until that time.
</p>
</blockquote><p>
</p>
<a name="anchor12"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.5.2"></a><h3>5.2.
High-level client processing</h3>
<p>
A TACK client SHALL send the "tack" extension defined previously, as well as
the "server_name" extension from <a class='info' href='#RFC6066'>[RFC6066]<span> (</span><span class='info'>Eastlake, D., “Transport Layer Security (TLS) Extensions: Extension Definitions,” January 2011.</span><span>)</span></a> indicating the
hostname the client is contacting. If not resuming a session, the server MAY
respond with a TACK_Extension. A TACK client SHALL perform the following steps
prior to using a non-resumed connection:
</p>
<ol class="text">
<li>Check whether the TLS handshake is "well-formed".
</li>
<li>Check the TACK generation and update min_generation.
</li>
<li>Check whether the TACK is expired.
</li>
<li>Create and activate pins (optional).
</li>
<li>Discard pins based on break signatures.
</li>
</ol><p>
These steps SHALL be performed in order. If there is any error, the client
SHALL send a fatal error alert and close the connection, skipping the
remaining steps (see <a class='info' href='#details'>Section 5.3<span> (</span><span class='info'>Client processing details</span><span>)</span></a> for details).
</p>
<p>
After the above steps, if there is a relevant active pin and a TACK whose key
is referenced by the pin, then the connection is "accepted" by the pin. If
there is a relevant active pin but no such TACK, the connection is "rejected"
by the pin. If there is no relevant active pin, the connection is "unpinned".
</p>
<p> A rejected connection might indicate a network attack. If the connection
is rejected the client SHOULD send a fatal "access_denied" error alert and
close the connection.
</p>
<p> A client MAY perform additional verification steps before using an
accepted or unpinned connection. See <a class='info' href='#certver'>Section 6.1<span> (</span><span class='info'>TACK and certificate verification</span><span>)</span></a> for an
example.
</p>
<a name="details"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.5.3"></a><h3>5.3.
Client processing details</h3>
<a name="wellformed"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.5.3.1"></a><h3>5.3.1.
Check whether the TLS handshake is well-formed</h3>
<p>
A TLS handshake is "well-formed" if the following are true (the error alert to
be sent on a failure is indicated in parentheses):
</p>
<ol class="text">
<li>The handshake protocol negotiates a cryptographically secure ciphersuite
and finishes succesfully (else see <a class='info' href='#RFC5246'>[RFC5246]<span> (</span><span class='info'>Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.</span><span>)</span></a>).
</li>
<li>The handshake contains either no TACK_Extension or a syntactically-valid
TACK_Extension (else "decode_error").
</li>
<li>If break signatures are present, the signatures are correct (else
"decrypt_error"). This step is optional, as break signature verification MAY
be deferred till later.
</li>
<li>If a TACK is present, it is "well-formed" by the rules below.
</li>
</ol><p>
</p>
<p>
A TACK is "well-formed" if:
</p>
<ol class="text">
<li>"public_key" is a valid elliptic curve public key on the curve P-256 (else
"decrypt_error").
</li>
<li>"generation" is >= "min_generation" (else "decode_error").
</li>
<li>"target_hash" is equal to the SHA256 hash of the server's
SubjectPublicKeyInfo (else "illegal_parameter").
</li>
<li>"signature" is a correct ECDSA signature (else "decrypt_error").
</li>
</ol><p>
</p>
<a name="anchor13"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.5.3.2"></a><h3>5.3.2.
Check the TACK generation and update min_generation</h3>
<p>
If there is a TACK and a key record referencing the TACK key, and the TACK's
generation is less than the key record's min_generation, then the TACK is
revoked and the client SHALL send the "certificate_revoked" alert and close
the connection.
</p>
<p> Otherwise, if there is a TACK and a key record referencing the TACK key,
and the TACK's min_generation is greater than the key record's min_generation,
then the key record's min_generation SHALL be set to the TACK's value.
</p>
<a name="anchor14"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.5.3.3"></a><h3>5.3.3.
Check whether the TACK is expired</h3>
<p>
If there is a TACK and the TACK's "expiration" field specifies a time in the
past, the client SHALL send the "certificate_expired" alert and close the
connection.
</p>
<a name="creating"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.5.3.4"></a><h3>5.3.4.
Create and activate pins (optional)</h3>
<p>
The TLS connection MAY be used to create, delete, and activate pins as
described in this section. Note that this section is optional; a client MAY
rely on an external source of pins, provided the external pins are produced by
a client following the below algorithms.
</p>
<p>
If there is a TACK and a relevant pin referencing the TACK key, and pin
activation is enabled, the name record's "active period end" SHALL be set
using the below formula (where "current" is the current time, and "initial" is
the "initial timestamp" from the name record):
</p><div style='display: table; width: 0; margin-left: 3em; margin-right: auto'><pre>
active_period_end = current + MIN(30 days, current - initial)
</pre></div>
<p>
If there is a TACK and either no relevant pin or an inactive relevant pin that
does not reference the TACK key, a new pin SHALL be created:
</p>
<ol class="text">
<li>If the TACK key is referenced by an existing
key record, the key record is reused, otherwise a new key record is created
with the TACK's key and min_generation.
</li>
<li>A new name record is created
containing the relevant name, an "initial timestamp" equal to the current
time, and an empty "active period end".
</li>
<li>If there is an existing relevant pin, the pin SHALL be deleted (see <a class='info' href='#deleting'>Section 5.3.6<span> (</span><span class='info'>Deleting pins</span><span>)</span></a>).
</li>
</ol><p>
</p>
<p> If there is no TACK and the relevant pin is inactive, the pin SHALL be
deleted (see <a class='info' href='#deleting'>Section 5.3.6<span> (</span><span class='info'>Deleting pins</span><span>)</span></a>).
</p>
<p>
The following table summarizes this behavior based on whether the relevant pin
is active and references the TACK key. The "(*)" means "if pin activation is
enabled".
</p><table class="full" align="center" border="0" cellpadding="2" cellspacing="2">
<col align="left"><col align="left"><col align="left">
<tr><th align="left">Pin status</th><th align="left">Pin references TACK</th><th align="left">Result</th></tr>
<tr>
<td align="left">Active</td>
<td align="left">Yes</td>
<td align="left">Extend activation period (*)</td>
</tr>
<tr>
<td align="left">Active</td>
<td align="left">No (or no TACK)</td>
<td align="left">Rejected</td>
</tr>
<tr>
<td align="left">Inactive</td>
<td align="left">Yes</td>
<td align="left">Activate pin (*)</td>
</tr>
<tr>
<td align="left">Inactive</td>
<td align="left">No</td>
<td align="left">Replace with new inactive pin</td>
</tr>
<tr>
<td align="left">Inactive</td>
<td align="left">No TACK</td>
<td align="left">Delete pin</td>
</tr>
<tr>
<td align="left">No pin</td>
<td align="left">-</td>
<td align="left">Create new inactive pin</td>
</tr>
<tr>
<td align="left">No pin</td>
<td align="left">No TACK</td>
<td align="left">-</td>
</tr>
</table>
<br clear="all" />
<a name="anchor15"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.5.3.5"></a><h3>5.3.5.
Discard pins based on break signatures</h3>
<p>
All key records broken by break signatures SHALL be discarded, along with
their associated name records. A key record is broken by a break signature if
the break signature passes the following checks:
</p>
<ol class="text">
<li>"public_key" is referenced by the key record.
</li>
<li>"signature" is a correct ECDSA signature (else "decrypt_error").
</li>
</ol><p>
</p>
<a name="deleting"></a><br /><hr />
<table summary="layout" cellpadding="0" cellspacing="2" class="TOCbug" align="right"><tr><td class="TOCbug"><a href="#toc"> TOC </a></td></tr></table>
<a name="rfc.section.5.3.6"></a><h3>5.3.6.
Deleting pins</h3>
<p>
A client might need to delete a pin from its store as a result of the
algorithms in <a class='info' href='#creating'>Section 5.3.4<span> (</span><span class='info'>Create and activate pins (optional)</span><span>)</span></a>. A client MAY also delete pins from
its store at any time, whether to save space, protect privacy, or for any
other reason. To delete a pin, its name record SHALL be removed. If this
leaves a key record with no associated name records, the key record MAY be
removed as well. Pins MAY be deleted regardless of whether they are active or
inactive, however for security concerns regarding pin deletion, see <a class='info' href='#security_client'>Section 9.2<span> (</span><span class='info'>For client implementers</span><span>)</span></a>.
</p>
<p>
</p>
<p>
Deleting pins unnecessarily will reduce the benefits of TACK, so SHOULD be