draft-tack.txt

TLS Working Group                                         M. Marlinspike
Internet-Draft                                            T. Perrin, Ed.
Intended status: Standards Track                             May 5, 2012
Expires: November 6, 2012


                 Trust Assertions for Certificate Keys
                             draft-tack.txt

Abstract

   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.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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

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

   This Internet-Draft will expire on November 6, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of



Marlinspike & Perrin    Expires November 6, 2012                [Page 1]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  4
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  TACK life cycle  . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Pin life cycle . . . . . . . . . . . . . . . . . . . . . .  6
   4.  TACK Extension . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Definition of TACK_Extension . . . . . . . . . . . . . . .  7
     4.2.  Explanation of TACK_Extension fields . . . . . . . . . . .  8
       4.2.1.  TACK fields  . . . . . . . . . . . . . . . . . . . . .  8
       4.2.2.  TACK_Break_Sig fields  . . . . . . . . . . . . . . . .  8
       4.2.3.  TACK_Extension fields  . . . . . . . . . . . . . . . .  9
   5.  Client processing  . . . . . . . . . . . . . . . . . . . . . . 10
     5.1.  TACK pins, key records, and name records . . . . . . . . . 10
     5.2.  High-level client processing . . . . . . . . . . . . . . . 10
     5.3.  Client processing details  . . . . . . . . . . . . . . . . 11
       5.3.1.  Check whether the TLS handshake is well-formed . . . . 11
       5.3.2.  Check the TACK generation and update min_generation  . 12
       5.3.3.  Check whether the TACK is expired  . . . . . . . . . . 12
       5.3.4.  Create and activate pins (optional)  . . . . . . . . . 12
       5.3.5.  Discard pins based on break signatures . . . . . . . . 13
       5.3.6.  Deleting pins  . . . . . . . . . . . . . . . . . . . . 13
   6.  Variations on client processing  . . . . . . . . . . . . . . . 14
     6.1.  TACK and certificate verification  . . . . . . . . . . . . 14
     6.2.  Application-specific pinning . . . . . . . . . . . . . . . 14
   7.  TACK IDs . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   8.  Advice . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     8.1.  For server operators . . . . . . . . . . . . . . . . . . . 16
     8.2.  For client implementers  . . . . . . . . . . . . . . . . . 17
   9.  Security considerations  . . . . . . . . . . . . . . . . . . . 18
     9.1.  For server operators . . . . . . . . . . . . . . . . . . . 18
     9.2.  For client implementers  . . . . . . . . . . . . . . . . . 18
     9.3.  Note on algorithm agility  . . . . . . . . . . . . . . . . 19
   10. IANA considerations  . . . . . . . . . . . . . . . . . . . . . 20
     10.1. New entry for the TLS ExtensionType Registry . . . . . . . 20
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
   12. Normative references . . . . . . . . . . . . . . . . . . . . . 22
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23








Marlinspike & Perrin    Expires November 6, 2012                [Page 2]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


1.  Introduction

   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.

   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.

   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.

   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.

   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.














Marlinspike & Perrin    Expires November 6, 2012                [Page 3]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


2.  Requirements notation

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














































Marlinspike & Perrin    Expires November 6, 2012                [Page 4]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


3.  Overview

3.1.  TACK life cycle

   A server operator using TACK may perform several processes:

   Selection of a TACK key:  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).

   Creating initial TACKs under a TACK key:  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".

   Deploying initial TACKs:  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.

   Creating new TACKs under a TACK key:  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).

   Revoking old TACKs:  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".

   Revoking TACK keys:  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".















Marlinspike & Perrin    Expires November 6, 2012                [Page 5]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


3.2.  Pin life cycle

   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.

   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.

   In addition to creating and activating pins, a TLS connection can
   alter the clients's pin store by publishing revocation data:

   Min_generation:  Each pin stores the highest "min_generation" value
      it has seen from the pinned TACK key, and rejects TACKs from
      earlier generations.

   Break signatures:  A TLS handshake may send break signatures which
      cause all pins for the broken key to be discarded.


























Marlinspike & Perrin    Expires November 6, 2012                [Page 6]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


4.  TACK Extension

4.1.  Definition of TACK_Extension

   A new TLS ExtensionType ("tack") is defined and MAY be included by a
   TLS client in the ClientHello message defined in [RFC5246].

   enum {tack(TBD), (65535)} ExtensionType;

   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 [RFC5246].

   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;















Marlinspike & Perrin    Expires November 6, 2012                [Page 7]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


4.2.  Explanation of TACK_Extension fields

4.2.1.  TACK fields

   public_key:  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 [FIPS186-3].  Each integer is
      encoded as a 32-byte octet string using the Integer-to-Octet-
      String algorithm from [RFC6090], and these strings are
      concatenated with the x value first.  (NOTE: This is equivalent to
      an uncompressed subjectPublicKey from [RFC5480], except that the
      initial 0x04 byte is omitted).

   min_generation:  This field publishes a min_generation value.

   generation:  This field assigns each TACK a generation.  Generations
      less than a published min_generation are considered revoked.

   expiration:  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.

   target_hash:  This field is a hash of the TLS server's
      SubjectPublicKeyInfo [RFC5280] using the SHA256 algorithm from
      [FIPS180-2].  The SubjectPublicKeyInfo is typically conveyed as
      part of the server's X.509 certificate.

   signature:  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 [FIPS186-3], using curve P-256
      and SHA256.  Each integer is encoded as a 32-byte octet string
      using the Integer-to-Octet-String algorithm from [RFC6090], and
      these strings are concatenated with the r value first.

4.2.2.  TACK_Break_Sig fields

   public_key:  This field specifies the TACK key being broken.  The key
      is encoded as per TACK.public_key.

   signature:  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 [FIPS186-3], using curve P-256 and SHA256.
      It is calculated and encoded as per TACK.signature.





Marlinspike & Perrin    Expires November 6, 2012                [Page 8]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


4.2.3.  TACK_Extension fields

   tack:  This field provides the server's TACK.  It MAY be empty, or
      MAY contain a TACK.

   break_sigs:  This field provides break signatures.  It MAY be empty,
      or MAY contain up to 8 break signatures.

   pin_activation:  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.

   Note that both the "tack" and "break_sigs" fields MAY be empty.




































Marlinspike & Perrin    Expires November 6, 2012                [Page 9]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


5.  Client processing

5.1.  TACK pins, key records, and name records

   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.

   A "key record" contains:

      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.

      Min_generation: A single byte used to detect revoked TACKs.

   A "name record" contains:

      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".

      Initial timestamp: A timestamp noting when this pin was created.

      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.

5.2.  High-level client processing

   A TACK client SHALL send the "tack" extension defined previously, as
   well as the "server_name" extension from [RFC6066] 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:

   1.  Check whether the TLS handshake is "well-formed".

   2.  Check the TACK generation and update min_generation.

   3.  Check whether the TACK is expired.

   4.  Create and activate pins (optional).

   5.  Discard pins based on break signatures.




Marlinspike & Perrin    Expires November 6, 2012               [Page 10]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


   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 Section 5.3 for details).

   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".

   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.

   A client MAY perform additional verification steps before using an
   accepted or unpinned connection.  See Section 6.1 for an example.

5.3.  Client processing details

5.3.1.  Check whether the TLS handshake is well-formed

   A TLS handshake is "well-formed" if the following are true (the error
   alert to be sent on a failure is indicated in parentheses):

   1.  The handshake protocol negotiates a cryptographically secure
       ciphersuite and finishes succesfully (else see [RFC5246]).

   2.  The handshake contains either no TACK_Extension or a
       syntactically-valid TACK_Extension (else "decode_error").

   3.  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.

   4.  If a TACK is present, it is "well-formed" by the rules below.

   A TACK is "well-formed" if:

   1.  "public_key" is a valid elliptic curve public key on the curve
       P-256 (else "decrypt_error").

   2.  "generation" is >= "min_generation" (else "decode_error").

   3.  "target_hash" is equal to the SHA256 hash of the server's
       SubjectPublicKeyInfo (else "illegal_parameter").

   4.  "signature" is a correct ECDSA signature (else "decrypt_error").




Marlinspike & Perrin    Expires November 6, 2012               [Page 11]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


5.3.2.  Check the TACK generation and update min_generation

   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.

   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.

5.3.3.  Check whether the TACK is expired

   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.

5.3.4.  Create and activate pins (optional)

   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.

   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):

   active_period_end = current + MIN(30 days, current - initial)

   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:

   1.  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.

   2.  A new name record is created containing the relevant name, an
       "initial timestamp" equal to the current time, and an empty
       "active period end".

   3.  If there is an existing relevant pin, the pin SHALL be deleted
       (see Section 5.3.6).

   If there is no TACK and the relevant pin is inactive, the pin SHALL
   be deleted (see Section 5.3.6).



Marlinspike & Perrin    Expires November 6, 2012               [Page 12]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


   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".

   +------------+---------------------+-------------------------------+
   | Pin status | Pin references TACK | Result                        |
   +------------+---------------------+-------------------------------+
   | Active     | Yes                 | Extend activation period (*)  |
   |            |                     |                               |
   | Active     | No (or no TACK)     | Rejected                      |
   |            |                     |                               |
   | Inactive   | Yes                 | Activate pin (*)              |
   |            |                     |                               |
   | Inactive   | No                  | Replace with new inactive pin |
   |            |                     |                               |
   | Inactive   | No TACK             | Delete pin                    |
   |            |                     |                               |
   | No pin     | -                   | Create new inactive pin       |
   |            |                     |                               |
   | No pin     | No TACK             | -                             |
   +------------+---------------------+-------------------------------+

5.3.5.  Discard pins based on break signatures

   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:

   1.  "public_key" is referenced by the key record.

   2.  "signature" is a correct ECDSA signature (else "decrypt_error").

5.3.6.  Deleting pins

   A client might need to delete a pin from its store as a result of the
   algorithms in Section 5.3.4.  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 Section 9.2.

   Deleting pins unnecessarily will reduce the benefits of TACK, so
   SHOULD be avoided.  Note that a pin SHOULD NOT be deleted simply
   because it has become inactive.  Instead, such a pin SHOULD be
   retained, so that it can be re-activated in the future by the
   algorithms in Section 5.3.4.



Marlinspike & Perrin    Expires November 6, 2012               [Page 13]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


6.  Variations on client processing

6.1.  TACK and certificate verification

   A TACK client MAY choose to perform some form of certificate
   verification in addition to TACK processing.  When combining
   certificate verification and TACK processing, the TACK processing
   described in Section 5 SHALL be followed, with the exception that
   TACK processing MAY be terminated early (or skipped) if some fatal
   certificate error is discovered.

   If TACK processing and certificate verification both complete without
   a fatal error, the client SHALL apply some policy to decide whether
   to accept the connection.  The policy is up to the client.  An
   example policy would be to accept the connection only if it passes
   certificate verification and is not rejected by a pin, or if the user
   elects to "connect anyway" despite certificate and/or pin failures.

6.2.  Application-specific pinning

   In addition to the hostname-based pinning described in Section 5,
   some applications may require "application-specific pins", where an
   application-layer name is pinned to a TACK key.  For example, an SMTP
   MTA may wish to authenticate receiving MTAs by pinning email domains
   to the receiving MTAs' TACK keys.

   Application-specific pins may require redefinition of the name
   record's "name" field, the "relevant name" for the TLS connection,
   and the "pin activation" signal.  With these items redefined, the
   client processing rules in Section 5 may be reused.

   Note that a server using application-specific pins is still subject
   to hostname pins, and a client MAY apply either or both forms of
   pinning.

   The specification of application-specific pinning for particular
   applications is outside the scope of this document.














Marlinspike & Perrin    Expires November 6, 2012               [Page 14]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


7.  TACK IDs

   A "TACK ID" MAY be used to represent a TACK public key to users in a
   form that is relatively easy to compare and transcribe.  A TACK ID
   consists of the first 25 characters from the base32 encoding of
   SHA256(public_key), split into 5 groups of 5 characters separated by
   periods.  Base32 encoding is as specified in [RFC4648], except
   lowercase is used.

   Example TACK IDs:

      quxiz.kpldu.uuedc.j5znm.7mqst

      a334f.bt7ts.ljb3b.y24ij.6zhwm

      ebsx7.z22qt.okobu.ibhut.xzdny



































Marlinspike & Perrin    Expires November 6, 2012               [Page 15]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


8.  Advice

8.1.  For server operators

   Key reuse:  All servers that are pinned to a single TACK key are able
      to impersonate each other, since clients will perceive their TACKs
      as equivalent.  Thus, TACK keys SHOULD NOT be reused with
      different hostnames unless these hostnames are closely related.
      Examples where it would be safe to reuse a TACK key are hostnames
      aliased to the same host, hosts sharing the same TLS key, or
      hostnames for a group of near-identical servers.

   Aliases:  A TLS server may be referenced by multiple hostnames.
      Clients may pin any of these hostnames.  Server operators should
      be careful when using such DNS aliases that hostnames are not
      pinned inadvertently.

   Generations:  To revoke older generations of TACKs, the server
      operator SHOULD first provide all servers with a new generation of
      TACKs, and only then provide servers with new TACKs containing the
      new min_generation.  Otherwise, a client may receive a
      min_generation update from one server but then try to contact an
      older-generation server which has not yet been updated.

   Signature expiration:  It is convenient to set the TACK expiration
      equal to the end-entity certificate expiration, so that the TACK
      and certificate may both be replaced at the same time.
      Alternatively, short-lived TACKs may be used so that a compromised
      TLS private key has limited value to an attacker.

   Break signatures:  A break signature only needs to be published for a
      time interval equal to the maximum active period of any affected
      pins.  For example, if a TACK has been only been published on a
      website for 24 hours, to remove the TACK only requires publishing
      the break signature for 24 hours.

   Pin activation:  Pin activation SHOULD only be enabled once all TLS
      servers sharing the same hostname have a TACK.  Otherwise, a
      client may activate a pin by contacting one server, then contact a
      different server at the same hostname that does not yet have a
      TACK.

   Pin deactivation:  The pin_activation field can be used to phase out
      TACKs for a hostname.  If all servers at a hostname disable pin
      activation, all existing pins for the hostname will eventually
      become inactive, at which point the servers' TACKs can be removed.





Marlinspike & Perrin    Expires November 6, 2012               [Page 16]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


8.2.  For client implementers

   Sharing pin information:  It is possible for a client to maintain a
      pin store based entirely on its own TLS connections.  However,
      such a client runs the risk of creating incorrect pins, failing to
      keep its pins active, or failing to receive revocation information
      (min_generation updates and break signatures).  Clients are
      advised to collaborate so that pin data can be aggregated and
      shared.  This will require additional protocols outside the scope
      of this document.

   Clock synchronization:  A client SHOULD take measures to prevent
      TACKs from being erroneously rejected due to an inaccurate client
      clock.  Such methods MAY include using time synchronization
      protocols such as NTP [RFC5905], or accepting seemingly-expired
      TACKs if they expired less than T minutes ago, where T is a
      "tolerance bound" set to the client's maximum expected clock
      error.

































Marlinspike & Perrin    Expires November 6, 2012               [Page 17]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


9.  Security considerations

9.1.  For server operators

   All servers pinned to the same TACK key can impersonate each other
   (see Section 8.1).  Think carefully about this risk if using the same
   TACK key for multiple hostnames.

   Make backup copies of the TACK private key and keep all copies in
   secure locations where they can't be compromised.

   A TACK private key MUST NOT be used to perform any non-TACK
   cryptographic operations.  For example, using a TACK key for email
   encryption, code-signing, or any other purpose MUST NOT be done.

   HTTP cookies [RFC6265] set by a pinned host can be stolen by a
   network attacker who can forge web and DNS responses so as to cause a
   client to send the cookies to a phony subdomain of the pinned host.
   To prevent this, TACK HTTPS Servers SHOULD set the "secure" attribute
   and omit the "domain" attribute on all security-sensitive cookies,
   such as session cookies.  These settings tell the browser that the
   cookie should only be presented back to the originating host (not its
   subdomains), and should only be sent over HTTPS (not HTTP) [RFC6265].

9.2.  For client implementers

   A TACK pin store may contain private details of the client's
   connection history.  An attacker may be able to access this
   information by hacking or stealing the client.  Some information
   about the client's connection history could also be gleaned by
   observing whether the client accepts or rejects connections to phony
   TLS servers without correct TACKs.  To mitigate these risks, a TACK
   client SHOULD allow the user to edit or clear the pin store.

   Aside from rejecting TLS connections, clients SHOULD NOT take any
   actions which would reveal to a network observer the state of the
   client's pin store, as this would allow an attacker to know in
   advance whether a "man-in-the-middle" attack on a particular TLS
   connection will succeed or be detected.

   An attacker may attempt to flood a client with spurious TACKs for
   different hostnames, causing the client to delete old pins to make
   space for new ones.  To defend against this, clients SHOULD NOT
   delete active pins to make space for new pins.  Clients instead
   SHOULD delete inactive pins.  If there are no inactive pins to
   delete, then the pin store is full and there is no space for new
   pins.  To select an inactive pin for deletion, the client SHOULD
   delete the pin with the oldest "active_period_end".



Marlinspike & Perrin    Expires November 6, 2012               [Page 18]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


9.3.  Note on algorithm agility

   If the need arises for TACKs using different cryptographic algorithms
   (e.g., if SHA256 or ECDSA are shown to be weak), a "v2" version of
   TACKs could be defined, requiring assignment of a new TLS Extension
   number.  TACKs as defined in this document would then be known as
   "v1" TACKs.












































Marlinspike & Perrin    Expires November 6, 2012               [Page 19]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


10.  IANA considerations

10.1.  New entry for the TLS ExtensionType Registry

   IANA is requested to add an entry to the existing TLS ExtensionType
   registry, defined in [RFC5246], for tack(TBD) as defined in this
   document.












































Marlinspike & Perrin    Expires November 6, 2012               [Page 20]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


11.  Acknowledgements

   Valuable feedback has been provided by Adam Langley, Chris Palmer,
   Nate Lawson, and Joseph Bonneau.















































Marlinspike & Perrin    Expires November 6, 2012               [Page 21]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


12.  Normative references

   [FIPS180-2]
              National Institute of Standards and Technology, "Secure
              Hash Standard", FIPS PUB 180-2, August 2002, <http://
              csrc.nist.gov/publications/fips/fips180-2/fips180-2.pdf>.

   [FIPS186-3]
              National Institute of Standards and Technology, "Digital
              Signature Standard", FIPS PUB 186-3, June 2009, <http://
              csrc.nist.gov/publications/fips/fips186-3/fips_186-3.pdf>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5280]  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", RFC 5280, May 2008.

   [RFC5480]  Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
              "Elliptic Curve Cryptography Subject Public Key
              Information", RFC 5480, March 2009.

   [RFC5905]  Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network
              Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, June 2010.

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

   [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", RFC 6090, February 2011.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              April 2011.









Marlinspike & Perrin    Expires November 6, 2012               [Page 22]

Internet-Draft    Trust Assertions for Certificate Keys         May 2012


Authors' Addresses

   Moxie Marlinspike


   Trevor Perrin (editor)













































Marlinspike & Perrin    Expires November 6, 2012               [Page 23]