dnsop                                                        W. Hardaker
Internet-Draft                                             Parsons, Inc.
Intended status: Standards Track                               W. Kumari
Expires: February 4, 2017                                         Google
                                                          August 3, 2016

             Security Considerations for RFC5011 Publishers


   This document describes the minimum requirements which a publisher of
   a zone must wait before using a new DNSKEY advertised using the
   RFC5011 DNSKEY rollover process.

Status of This Memo

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

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   This Internet-Draft will expire on February 4, 2017.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements notation . . . . . . . . . . . . . . . . . .   3
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Timing associated with RFC5011 processing . . . . . . . . . .   3
   5.  Denial of Service Attack                   Considerations . .   3
     5.1.  Numerical Concrete Attack                    Example  . .   3
       5.1.1.  Attack Timing Breakdown . . . . . . . . . . . . . . .   4
   6.  Proper Timing Requirements  . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Operational
       Considerations  . . . . . . . . . . . . . . . . . . . . . . .   6
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   10. Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Appendix A.  Changes / Author Notes.  . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   RFC5011 [RFC5011] defines a mechanism by which DNSSEC validators can
   extend their list of trust anchors when they've seen a new key.
   However, RFC5011 [intentionally] provides no guidance to publishers
   of DNSKEYs about how long they must wait before such a new key is
   actually usable.  Because of this lack of guidance, DNSSEC publishers
   may derive incorrect assumptions about safe usage of the RFC5011
   process.  This document describes the minimum security requirements
   from a publishers point of view and is intended to compliment the
   guidance offered in RFC5011 (which is designed to solely represent
   the Validating Resolvers point of view).

   The authors reached out to 5 DNSSEC experts and asked them how long
   they must wait before using a new KSK that was being rolled according
   to the 5011 process.  All 5 experts answered with an insecure value,
   and thus the authors have determined that this lack of operational
   guidance is causing security concerns.  This document will hopefully
   help rectify this problem.

   One important (temporary?) note about ICANN's upcoming KSK rolling
   plan for the root zone: the timing values, at the time of this
   writing, chosen for rolling the KSK in the root zone appear
   completely safe, and are not in any way affected by the timing
   concerns introduced by this draft

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1.1.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Background

   The RFC5011 process describes a process by which a Validating
   Resolver may accept a newly published KSK as a trust anchor for
   validating future DNSSEC signed records.  This document augments that
   information with additional constraints, as required from the DNSKEY
   publication point of view.  Note that it does not define any other
   operational guidance or recommendations about the RFC5011 process
   from a publication point of view and restricts itself to solely the
   security and operational ramifications of switching to a new key too
   soon.  Failure of a DNSKEY publisher to follow the minimum
   recommendations associated with this draft will result in potential
   denial-of-service attack opportunities against validating resolvers.

3.  Terminology

   foo  bar

4.  Timing associated with RFC5011 processing


5.  Denial of Service Attack Considerations

   If an attacker is able to provide a RFC5011 validating engine with
   past responses, such as when it is in-path or able to otherwise
   perform any number of cache poising attacks, she may be able to leave
   the RFC5011-compliant validator without an appropriate DNSKEY trust

   The following timeline illustrates this situation.

5.1.  Numerical Concrete Attack Example

   These assumptions are used in the example scenario within this

   TTL (all records)  1 day

   DNSKEY RRSIG Signature Validity  10 days

   Zone resigned every  1 day

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   Given these assumptions, the following sequence of events depicts how
   a Trust Anchor Publisher (XXX: TERM!) which waits for only the
   RFC5011 hold time timer length of 30 days subjects its users to a
   potential Denial of Service attack.  The timing schedule listed below
   is based on a new Key Signing Key (KSK) being published at T+0, and
   where all numbers in this sequence refer to days before and after
   such an event.  Thus, T-1 is the day before the introduction of the
   new key, and T+15 is the 15th day after the key was introduced into
   the zone being discussed..

   In this dialog, we consider two keys being published:

   K_old  The older KSK being replaced.

   K_new  The new KSK being transitioned into active use, using the
      RFC5011 process.

   In this dialog, the following actors are discussed:

   Zone Signer  The owner of a zone intending to publish a new Key-
      Signing-Keys (KSKs) that will become a trust anchor by validators
      following the RFC5011 process.

   RFC5011 Validator  A DNSSEC validator that is using the RFC5011
      processes to track and update trust anchors.

   Attacker  An attacker intent on foiling the RFC5011 Validator's
      ability to successfully adopt the Zone Signer's K_new key as a
      trust anchor.

5.1.1.  Attack Timing Breakdown

   The following series of steps depicts the timeline in which an attack
   occurs that foils the publisher of a new key who revokes the old key
   too quickly.

   T-1  The last signatures are published by the Zone Signer that signs
      only K_old using K_old.

   T-0  The Zone Signer adds K_new to his zone and signs the zone's key
      set with K_old.  The RFC5011 Validator retrieves the new key set
      and corresponding signature set and notices the publication of
      K_new.  The RFC5011 Validator starts the hold-down timer for

   T+5  The RFC5011 Validator queries for the zone's keyset per the
      Active Refresh schedule, discussed in Section 2.3 of RFC5011.
      Instead of receiving the intended published keyset, the Attacker

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      successfully replays the keyset and associated signatures that
      they recorded at T-1.  Because the signature lifetime is 10 days
      (in this example), the replayed signature and keyset is accepted
      as valid (being only 6 days old) and the RFC5011 Validator cancels
      the hold-down timer for K_new.

   T+10  The RFC5011 Validator queries for the zone's keyset and
      discovers K_new again, signed by K_old (the attacker is unable to
      replay the records at T-1, because they have now expired).  It
      starts the hold-timer for K_new again.

   ...  The RFC5011 Validator continues checking the zone's key set and
      lets the hold-down timer keep running without resetting it.

   T+30  The Zone Signer believes that this is the first time at which
      some validators might accept K_new as a new trust anchor.  The
      hold-down timer of our RFC5011 Validator is at 20 days.

   T+35  The Zone Signer mistakenly believes that all validators
      following the Active Refresh schedule should have accepted K_new
      as a the new trust anchor (since 30 days + 1/2 the signature
      validity period would have passed).  The hold-time timer of our
      RFC5011 Validator is at 25 days and has not actually reached its
      30 day requirement though.

   T+36  The Zone Signer, believing K_new is safe to use, switches their
      active KSK to K_new and publishes a new key set signature using
      K_new as the signing key.  Because our RFC5011 Validator still has
      a hold-down timer for K_new at 26 days, it will fail to validate
      this new key set and the zone contents will be treated as invalid.

6.  Proper Timing Requirements

   Given the attack description in Section 5, the correct length of time
   required for the Zone Signer to wait before using K_new is:

     waitTime = addHoldDownTime
                + 3 * (DNSKEY RRSIG Signature Validity) / 2
                + 2 * MAX(TTL of all records)

   For the parameters listed in Section 5.1, this becomes:

     waitTime = 30 + 3 * (10) / 2 + 2 * (1)  (days)
     waitTime = 47                           (days)

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7.  IANA Considerations

   This document contains no IANA considerations.

8.  Operational Considerations

   A companion document to RFC5011 was expected to be published that
   describes the best operational considerations from the perspective of
   a zone publisher.  However, the companion document was never written
   but the authors of this document hope that it will at some point in
   the future.  This document is intended only to fill a single
   operational void that results in security ramifications (specifically
   a denial of service attack against an RFC5011 Validator).  This
   document does not attempt to document any other missing operational
   guidance for zone publishers.

9.  Security Considerations

   This document, is solely about the security considerations with
   respect to the publisher of RFC5011 trust anchors / keys.

10.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,

   [RFC5011]  StJohns, M., "Automated Updates of DNS Security (DNSSEC)
              Trust Anchors", STD 74, RFC 5011, DOI 10.17487/RFC5011,
              September 2007, <http://www.rfc-editor.org/info/rfc5011>.

Appendix A.  Changes / Author Notes.

   -00 to -01:

   o  Renamed Kold, Knew, we Knew it would be confusing to read.

   o  Added "Proper Timing Requirements" with equasion.  Somehow we
      forgot that in original version.

Authors' Addresses

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   Wes Hardaker
   Parsons, Inc.
   P.O. Box 382
   Davis, CA  95617

   Email: ietf@hardakers.net

   Warren Kumari
   1600 Amphitheatre Parkway
   Mountain View, CA  94043

   Email: warren@kumari.net

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