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dnsop                                                        W. Hardaker
Internet-Draft                                                   USC/ISI
Updates: 7583 (if approved)                                    W. Kumari
Intended status: Standards Track                                  Google
Expires: June 22, 2018                                 December 19, 2017


             Security Considerations for RFC5011 Publishers
          draft-ietf-dnsop-rfc5011-security-considerations-10

Abstract

   This document extends the RFC5011 rollover strategy with timing
   advice that must be followed by the publisher in order to maintain
   security.  Specifically, this document describes the math behind the
   minimum time-length that a DNS zone publisher must wait before
   signing exclusively with recently added DNSKEYs.  This document also
   describes the minimum time-length that a DNS zone publisher must wait
   after publishing a revoked DNSKEY before assuming that all active
   RFC5011 resolvers should have seen the revocation-marked key and
   removed it from their list of trust anchors.

   This document contains much math and complicated equations, but the
   summary is that the key rollover / revocation time is much longer
   than intuition would suggest.  If you are not both publishing a
   DNSSEC DNSKEY, and using RFC5011 to advertise this DNSKEY as a new
   Secure Entry Point key for use as a trust anchor, you probably don't
   need to read this document.

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 June 22, 2018.






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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Document History and Motivation . . . . . . . . . . . . .   3
     1.2.  Safely Rolling the Root Zone's KSK in 2017/2018 . . . . .   3
     1.3.  Requirements notation . . . . . . . . . . . . . . . . . .   4
   2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Timing Associated with RFC5011 Processing . . . . . . . . . .   5
     4.1.  Timing Associated with Publication  . . . . . . . . . . .   5
     4.2.  Timing Associated with Revocation . . . . . . . . . . . .   5
   5.  Denial of Service Attack Walkthrough  . . . . . . . . . . . .   6
     5.1.  Enumerated Attack Example . . . . . . . . . . . . . . . .   6
       5.1.1.  Attack Timing Breakdown . . . . . . . . . . . . . . .   7
   6.  Minimum RFC5011 Timing Requirements . . . . . . . . . . . . .   8
     6.1.  Equation Components . . . . . . . . . . . . . . . . . . .   9
       6.1.1.  addHoldDownTime . . . . . . . . . . . . . . . . . . .   9
       6.1.2.  lastSigExpirationTime . . . . . . . . . . . . . . . .   9
       6.1.3.  sigExpirationTime . . . . . . . . . . . . . . . . . .   9
       6.1.4.  sigExpirationTimeRemaining  . . . . . . . . . . . . .   9
       6.1.5.  activeRefresh . . . . . . . . . . . . . . . . . . . .   9
       6.1.6.  activeRefreshOffset . . . . . . . . . . . . . . . . .  10
       6.1.7.  driftSafetyMargin . . . . . . . . . . . . . . . . . .  10
       6.1.8.  timingSafetyMargin  . . . . . . . . . . . . . . . . .  10
       6.1.9.  retrySafetyMargin . . . . . . . . . . . . . . . . . .  11
     6.2.  Timing Requirements For Adding a New KSK  . . . . . . . .  12
       6.2.1.  Wait Timer Based Calculation  . . . . . . . . . . . .  12
       6.2.2.  Wall-Clock Based Calculation  . . . . . . . . . . . .  13
       6.2.3.  Timing Constraint Summary . . . . . . . . . . . . . .  13
       6.2.4.  Additional Considerations for RFC7583 . . . . . . . .  14
       6.2.5.  Example Scenario Calculations . . . . . . . . . . . .  14
     6.3.  Timing Requirements For Revoking an Old KSK . . . . . . .  14
       6.3.1.  Wait Timer Based Calculation  . . . . . . . . . . . .  15



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       6.3.2.  Wall-Clock Based Calculation  . . . . . . . . . . . .  15
       6.3.3.  Additional Considerations for RFC7583 . . . . . . . .  16
       6.3.4.  Example Scenario Calculations . . . . . . . . . . . .  16
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   8.  Operational Considerations  . . . . . . . . . . . . . . . . .  16
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  17
   Appendix A.  Real World Example: The 2017 Root KSK Key Roll . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   [RFC5011] defines a mechanism by which DNSSEC validators can update
   their list of trust anchors when they've seen a new key published in
   a zone or revoke a properly marked key from a trust anchor list.
   However, RFC5011 [intentionally] provides no guidance to the
   publishers of DNSKEYs about how long they must wait before switching
   to exclusively using recently published keys for signing records, or
   how long they must wait before ceasing publication of a revoked key.
   Because of this lack of guidance, zone publishers may derive
   incorrect assumptions about safe usage of the RFC5011 DNSKEY
   advertising, rolling and revocation process.  This document describes
   the minimum security requirements from a publisher's point of view
   and is intended to complement the guidance offered in RFC5011 (which
   is written to provide timing guidance solely to a Validating
   Resolver's point of view).

1.1.  Document History and Motivation

   To verify this lack of understanding is wide-spread, the authors
   reached out to 5 DNSSEC experts to ask them how long they thought
   they must wait before signing a zone exclusively with a new KSK
   [RFC4033] that was being introduced according to the 5011 process.
   All 5 experts answered with an insecure value, and we determined that
   this lack of mathematical understanding might cause security concerns
   in deployment.  We hope that this companion document to RFC5011 will
   rectify this understanding and provide better guidance to zone
   publishers that wish to make use of the RFC5011 rollover process.

1.2.  Safely Rolling the Root Zone's KSK in 2017/2018

   One important note about ICANN's (currently in process) 2017/2018 KSK
   rollover plan for the root zone: the timing values chosen for rolling
   the KSK in the root zone appear completely safe, and are not affected
   by the timing concerns introduced by this draft





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1.3.  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].

2.  Background

   The RFC5011 process describes a process by which a RFC5011 Resolver
   may accept a newly published KSK as a trust anchor for validating
   future DNSSEC signed records.  It also describes the process for
   publicly revoking a published KSK.  This document augments that
   information with additional constraints, from the SEP publisher's
   points of view.  Note that this document does not define any other
   operational guidance or recommendations about the RFC5011 process and
   restricts itself to solely the security and operational ramifications
   of switching to exclusively using recently added keys or removing
   revoked keys too soon.

   Failure of a DNSKEY publisher to follow the minimum recommendations
   associated with this draft can result in potential denial-of-service
   attack opportunities against validating resolvers.  Failure of a
   DNSKEY publisher to publish a revoked key for a long enough period of
   time may result in RFC5011 Resolvers leaving that key in their trust
   anchor storage beyond the key's expected lifetime.

3.  Terminology

   SEP Publisher  The entity responsible for publishing a DNSKEY (with
      the Secure Entry Point (SEP) bit set) that can be used as a trust
      anchor.

   Zone Signer  The owner of a zone intending to publish a new Key-
      Signing-Key (KSK) that may become a trust anchor for validators
      following the RFC5011 process.

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

   Attacker  An entity intent on foiling the RFC5011 Resolver's ability
      to successfully adopt the Zone Signer's new DNSKEY as a new trust
      anchor or to prevent the RFC5011 Resolver from removing an old
      DNSKEY from its list of trust anchors.

   sigExpirationTime  The amount of time between the DNSKEY RRSIG's
      Signature Inception field and the Signature Expiration field.





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   Also see Section 2 of [RFC4033] and [RFC7719] for additional
   terminology.

4.  Timing Associated with RFC5011 Processing

   These sections define a high-level overview of [RFC5011] processing.
   These steps are not sufficient for proper RFC5011 implementation, but
   provide enough background for the reader to follow the discussion in
   this document.  Readers need to fully understand [RFC5011] as well to
   fully comprehend the content and importance of this document.

4.1.  Timing Associated with Publication

   RFC5011's process of safely publishing a new DNSKEY and then assuming
   RFC5011 Resolvers have adopted it for trust falls into a number of
   high-level steps to be performed by the SEP Publisher.  This document
   discusses the following scenario, which the principle way RFC5011 is
   currently being used (even though Section 6 of RFC5011 suggests
   having a stand-by key available):

   1.  Publish a new DNSKEY in a zone, but continue to sign the zone
       with the old one.

   2.  Wait a period of time.

   3.  Begin to exclusively use recently published DNSKEYs to sign the
       appropriate resource records.

   This document discusses the time required to wait during step 2 of
   the above process.  Some interpretations of RFC5011 have erroneously
   determined that the wait time is equal to RFC5011's "hold down time".
   Section 5 describes an attack based on this (common) erroneous
   belief, which can result in a denial of service attack against the
   zone.

4.2.  Timing Associated with Revocation

   RFC5011's process of advertising that an old key is to be revoked
   from RFC5011 Resolvers falls into a number of high-level steps:

   1.  Set the revoke bit on the DNSKEY to be revoked.

   2.  Sign the revoked DNSKEY with itself.

   3.  Wait a period of time.

   4.  Remove the revoked key from the zone.




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   This document discusses the time required to wait in step 3 of the
   above process.  Some interpretations of RFC5011 have erroneously
   determined that the wait time is equal to RFC5011's "hold down time".
   This document describes an attack based on this (common) erroneous
   belief, which results in a revoked DNSKEY potentially remaining as a
   trust anchor in a RFC5011 Resolver long past its expected usage.

5.  Denial of Service Attack Walkthrough

   This section serves as an illustrative example of the problem being
   discussed in this document.  Note that in order to keep the example
   simple enough to understand, some simplifications were made (such as
   by not creating a set of pre-signed RRSIGs and by not using values
   that result in the addHoldDownTime not being evenly divisible by the
   activeRefresh value); the mathematical formulas in Section 6 are,
   however, complete.

   If an attacker is able to provide a RFC5011 Resolver with past
   responses, such as when it is in-path or able to perform any number
   of cache poisoning attacks, the attacker may be able to leave
   compliant RFC5011 Resolvers without an appropriate DNSKEY trust
   anchor.  This scenario will remain until an administrator manually
   fixes the situation.

   The time-line below illustrates an example of this situation.

5.1.  Enumerated Attack Example

   The following example settings are used in the example scenario
   within this section:

   TTL (all records)  1 day

   sigExpirationTime  10 days

   Zone resigned every  1 day

   Given these settings, the sequence of events in Section 5.1.1 depicts
   how a SEP Publisher that 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 SEP Publisher
   publishing a new Key Signing Key (KSK), with the intent that it will
   later be used as a trust anchor.  We label this publication time as
   "T+0".  All numbers in this sequence refer to days before and after
   this initial publication 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 fictitious zone being discussed.




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   In this dialog, we consider two keys within the example zone:

   K_old:  An older KSK and Trust Anchor being replaced.

   K_new:  A new KSK being transitioned into active use and expected to
      become a Trust Anchor via the RFC5011 automated trust anchor
      update process.

5.1.1.  Attack Timing Breakdown

   The steps shows an attack that foils the adoption of a new DNSKEY by
   a 5011 Resolver when the SEP Publisher that starts signing and
   publishing with the new DNSKEY too quickly.

   T-1  The K_old based RRSIGs are being published by the Zone Signer.
      [It may also be signing ZSKs as well, but they are not relevant to
      this event so we will not talk further about them; we are only
      considering the RRSIGs that cover the DNSKEYs in this document.]
      The Attacker queries for, retrieves and caches this DNSKEY set and
      corresponding RRSIG signatures.

   T+0  The Zone Signer adds K_new to their zone and signs the zone's
      key set with K_old.  The RFC5011 Resolver (later to be under
      attack) retrieves this new key set and corresponding RRSIGs and
      notices the publication of K_new.  The RFC5011 Resolver starts the
      (30-day) hold-down timer for K_new.  [Note that in a more real-
      world scenario there will likely be a further delay between the
      point where the Zone Signer publishes a new RRSIG and the RFC5011
      Resolver notices its publication; though not shown in this
      example, this delay is accounted for in the equation in Section 6
      below]

   T+5  The RFC5011 Resolver queries for the zone's keyset per the
      RFC5011 Active Refresh schedule, discussed in Section 2.3 of
      RFC5011.  Instead of receiving the intended published keyset, the
      Attacker successfully replays the keyset and associated signatures
      recorded at T-1 to the victim RFC5011 Resolver.  Because the
      signature lifetime is 10 days (in this example), the replayed
      signature and keyset is accepted as valid (being only 6 days old,
      which is less than sigExpirationTime) and the RFC5011 Resolver
      cancels the (30-day) hold-down timer for K_new, per the RFC5011
      algorithm.

   T+10  The RFC5011 Resolver queries for the zone's keyset and
      discovers a signed keyset that includes K_new (again), and is
      signed by K_old.  Note: the attacker is unable to replay the
      records cached at T-1, because the signatures have now expired.




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      Thus at T+10, the RFC5011 Resolver starts (anew) the hold-timer
      for K_new.

   T+11 through T+29  The RFC5011 Resolver continues checking the zone's
      key set at the prescribed regular intervals.  During this period,
      the attacker can no longer replay traffic to their benefit.

   T+30  The Zone Signer knows that this is the first time at which some
      validators might accept K_new as a new trust anchor, since the
      hold-down timer of a RFC5011 Resolver not under attack that had
      queried and retrieved K_new at T+0 would now have reached 30 days.
      However, the hold-down timer of our attacked RFC5011 Resolver is
      only at 20 days.

   T+35  The Zone Signer (mistakenly) believes that all validators
      following the Active Refresh schedule (Section 2.3 of RFC5011)
      should have accepted K_new as a the new trust anchor (since the
      hold down time (30 days) + the query interval [which is just 1/2
      the signature validity period in this example] would have passed).
      However, the hold-down timer of our attacked RFC5011 Resolver is
      only at 25 days (T+35 minus T+10); thus the RFC5011 Resolver won't
      consider it a valid trust anchor addition yet, as the required 30
      days have not yet elapsed.

   T+36  The Zone Signer, believing K_new is safe to use, switches their
      active signing KSK to K_new and publishes a new RRSIG, signed with
      (only) K_new, covering the DNSKEY set.  Non-attacked RFC5011
      validators, with a hold-down timer of at least 30 days, would have
      accepted K_new into their set of trusted keys.  But, because our
      attacked RFC5011 Resolver now has a hold-down timer for K_new of
      only 26 days, it failed to ever accept K_new as a trust anchor.
      Since K_old is no longer being used to sign the zone's DNSKEYs,
      all the DNSKEY records from the zone will be treated as invalid.
      Subsequently, all of the records in the DNS tree below the zone's
      apex will be deemed invalid by DNSSEC.

6.  Minimum RFC5011 Timing Requirements

   This section defines the minimum timing requirements for making
   exclusive use of newly added DNSKEYs and timing requirements for
   ceasing the publication of DNSKEYs to be revoked.  We break our
   timing solution requirements into two primary components: the
   mathematically-based security analysis of the RFC5011 publication
   process itself, and an extension of this that takes operational
   realities into account that further affect the recommended timings.

   First, we define the term components used in all equations in
   Section 6.1.



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6.1.  Equation Components

6.1.1.  addHoldDownTime

   The addHoldDownTime is defined in Section 2.4.1 of [RFC5011] as:

       The add hold-down time is 30 days or the expiration time of the
       original TTL of the first trust point DNSKEY RRSet that contained
       the new key, whichever is greater.  This ensures that at least
       two validated DNSKEY RRSets that contain the new key MUST be seen
       by the resolver prior to the key's acceptance.

6.1.2.  lastSigExpirationTime

   The latest value (i.e. the future most date and time) of any RRSig
   Signature Expiration field covering any DNSKEY RRSet containing only
   the old trust anchor(s) that are being superseded.  Note that for
   organizations pre-creating signatures this time may be fairly far in
   the future unless they can be significantly assured that none of
   their pre-generated signatures can be replayed at a later date.

6.1.3.  sigExpirationTime

   The amount of time between the DNSKEY RRSIG's Signature Inception
   field and the Signature Expiration field.

6.1.4.  sigExpirationTimeRemaining

   sigExpirationTimeRemaining is defined in Section 3.

6.1.5.  activeRefresh

   activeRefresh time is defined by RFC5011 by

     A resolver that has been configured for an automatic update
     of keys from a particular trust point MUST query that trust
     point (e.g., do a lookup for the DNSKEY RRSet and related
     RRSIG records) no less often than the lesser of 15 days, half
     the original TTL for the DNSKEY RRSet, or half the RRSIG
     expiration interval and no more often than once per hour.


   This translates to:








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    activeRefresh = MAX(1 hour,
                        MIN(sigExpirationTime / 2,
                            MAX(TTL of K_old DNSKEY RRSet) / 2,
                            15 days)
                        )

6.1.6.  activeRefreshOffset

   The activeRefreshOffset term must be added for situations where the
   activeRefresh value is not a factor of the addHoldDownTime.
   Specifically, activeRefreshOffset will be "addHoldDownTime %
   activeRefresh", where % is the mathematical mod operator (calculating
   the remainder in a division problem).  This will frequently be zero,
   but could be nearly as large as activeRefresh itself.

   Note that later (in Section 6.1.8), when real-world scenerios will
   trump this value that is useful only in theoretical worlds with no
   network delays and other operational considerations.  We leave it
   here only as an important marker in the security analysis of the base
   RFC5011 protocol.

6.1.7.  driftSafetyMargin

   Moving past the theoretical model parameters above, we not that clock
   drift, network delays and implementation differences will result in
   the RFC5011 Resolver query times to drift over time.  Because of
   this, a driftSafetyMargin term must be introduce that accounts for
   these real world delays.  We set this value to be the same as the
   activeRefresh value, which will ensure that any timing drift in
   RFC5011 Resolver queries will be accounted for.

   Note: even a negative clock drift can actually cause RFC5011
   Resolvers to require up to an extra activeRefresh period before it
   will accept a new DNSKEY as a trust anchor.

6.1.8.  timingSafetyMargin

   Both of the activeRefreshOffset and driftSafetyMargin parameters deal
   with timing delays introduced by mathematical analysis of RFC5011
   (activeRefreshOffset) and by real world considerations
   (driftSafetyMargin).  To find a safe value to extend timing, we
   define a timingSafetyMargin that is the maximum of these two values.
   Since the driftSafetyMargin is set to activeRefresh, and
   activeRefreshOffset is always less than an activeRefresh, the final
   timingSafetyMargin value will be activeRefresh.

   Explicitly expanding out the math:




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        timingSafetyMargin = min(activeRefreshOffset, driftSafetyMargin)

        timingSafetyMargin = min(addHoldDownTime % activeRefresh,
                                 activeRefresh)

        timingSafetyMargin = activeRefresh

6.1.9.  retrySafetyMargin

   The retrySafetyMargin is an extra period of time to account for
   caching, network delays, dropped packets, and other operational
   concerns otherwise beyond the scope of this document.  The value
   operators should chose is highly dependent on the deployment
   situation associated with their zone.  Note that no value of a
   retrySafetyMargin can protect against resolvers that are "down".
   None the less, we do offer the following as one method considering
   reasonable values to select from.

   The following list of variables need to be considered when selecting
   an appropriate retrySafetyMargin value:

   successRate:  A likely success rate for client queries and retries

   numResolvers:  The number of client RFC5011 Resolvers

   Note that RFC5011 defines retryTime as:

         If the query fails, the resolver MUST repeat the query until
         satisfied no more often than once an hour and no less often
         than the lesser of 1 day, 10% of the original TTL, or 10% of
         the original expiration interval.  That is,
         retryTime = MAX (1 hour, MIN (1 day, .1 * origTTL,
                                       .1 * expireInterval)).

   With the successRate and numResolvers values selected and the
   definition of retryTime from RFC5011, one method for determining how
   many retryTime intervals to wait in order to reduce the set of
   uncompleted servers to 0 assuming normal probability is thus:

                         x = (1/(1 - successRate))

            retryCountWait = Log_base_x(numResolvers)

   To reduce the need for readers to pull out a scientific calculator,
   we offer the following lookup table based on successRate and
   numResolvers:





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                          retryCountWait lookup table
                        ---------------------------

                       Number of client RFC5011 Resolvers (numResolvers)
                       -------------------------------------------------
                        10,000  100,000 1,000,000 10,000,000 100,000,000
                 0.01      917     1146      1375       1604        1833
   Probability   0.05      180      225       270        315         360
   of Success    0.10       88      110       132        153         175
   Per Retry     0.15       57       71        86        100         114
   Interval      0.25       33       41        49         57          65
   (successRate) 0.50       14       17        20         24          27
                 0.90        4        5         6          7           8
                 0.95        4        4         5          6           7
                 0.99        2        3         3          4           4
                 0.999       2        2         2          3           3

   Finally, a suggested value of retrySafetyMargin can then be this
   retryCountWait number multiplied by the retryTime from RFC5011:

                 retrySafetyMargin = retryCountWait * retryTime

6.2.  Timing Requirements For Adding a New KSK

   Section 6.2.1 defines a method for calculating the amount of time to
   wait until it is safe to start signing exclusively with a new DNSKEY
   (especially useful for writing code involving sleep based timers),
   and Section 6.2.2 defines a method for calculating a wall-clock value
   after which it is safe to start signing exclusively with a new DNSKEY
   (especially useful for writing code based on clock-based event
   triggers).

6.2.1.  Wait Timer Based Calculation

   Given the attack description in Section 5, the correct minimum length
   of time required for the Zone Signer to wait after publishing K_new
   but before exclusively using it and newer keys is:

      addWaitTime = addHoldDownTime
                    + sigExpirationTimeRemaining
                    + activeRefresh
                    + timingSafetyMargin
                    + retrySafetyMargin








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6.2.1.1.  Fully expanded equation

   Given the equation components defined in Section 6.1, the full
   expanded equation is:

      addWaitTime = addHoldDownTime
                    + sigExpirationTimeRemaining
                    + MAX(1 hour,
                          MIN(sigExpirationTime / 2,
                              MAX(TTL of K_old DNSKEY RRSet) / 2,
                              15 days)
                          )
                    + activeRefresh
                    + retrySafetyMargin

6.2.2.  Wall-Clock Based Calculation

   The equations in Section 6.2.1 are defined based upon how long to
   wait from a particular moment in time.  An alternative, but
   equivalent, method is to calculate the date and time before which it
   is unsafe to use a key for signing.  This calculation thus becomes:

      addWallClockTime = lastSigExpirationTime
                       + addHoldDownTime
                       + activeRefresh
                       + timingSafetyMargin
                       + retrySafetyMargin

   where lastSigExpirationTime is the latest value of any
   sigExpirationTime for which RRSIGs were created that could
   potentially be replayed.  Fully expanded, this becomes:

    addWallClockTime = lastSigExpirationTime
                       + addHoldDownTime
                       + 2 * MAX(1 hour,
                                 MIN(sigExpirationTime / 2,
                                     MAX(TTL of K_old DNSKEY RRSet) / 2,
                                     15 days)
                                 )
                       + activeRefresh
                       + retrySafetyMargin

6.2.3.  Timing Constraint Summary

   The important timing constraint introduced by this memo relates to
   the last point at which a RFC5011 Resolver may have received a
   replayed original DNSKEY set, containing K_old and not K_new.  The
   next query of the RFC5011 validator at which K_new will be seen



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   without the potential for a replay attack will occur after the old
   DNSKEY RRSIG's Signature Expriation Time.  Thus, the latest time that
   a RFC5011 Validator may begin their hold down timer is an "Active
   Refresh" period after the last point that an attacker can replay the
   K_old DNSKEY set.  The worst case scenario of this attack is if the
   attacker can replay K_old just seconds before the (DNSKEY RRSIG
   Signature Validity) field of the last K_old only RRSIG.

6.2.4.  Additional Considerations for RFC7583

   Note: our notion of addWaitTime is called "Itrp" in Section 3.3.4.1
   of [RFC7583].  The equation for Itrp in RFC7583 is insecure as it
   does not include the sigExpirationTime listed above.  The Itrp
   equation in RFC7583 also does not include the 2*TTL safety margin,
   though that is an operational consideration.

6.2.5.  Example Scenario Calculations

   For the parameters listed in Section 5.1, our resulting addWaitTime
   is:

     addWaitTime = 30
                   + 10
                   + 1 / 2
                   + 1 / 2          (days)

     addWaitTime = 43               (days)

   This addWaitTime of 42.5 days is 12.5 days longer than just the hold
   down timer, even with the needed retrySafetyMargin value being left
   out (which we exclude due to the lack of necessary operational
   parameters).

6.3.  Timing Requirements For Revoking an Old KSK

   This issue affects not just the publication of new DNSKEYs intended
   to be used as trust anchors, but also the length of time required to
   continuously publish a DNSKEY with the revoke bit set.

   Section 6.2.1 defines a method for calculating the amount of time
   operators need to wait until it is safe to cease publishing a DNSKEY
   (especially useful for writing code involving sleep based timers),
   and Section 6.2.2 defines a method for calculating a minimal wall-
   clock value after which it is safe to cease publishing a DNSKEY
   (especially useful for writing code based on clock-based event
   triggers).





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6.3.1.  Wait Timer Based Calculation

   Both of these publication timing requirements are affected by the
   attacks described in this document, but with revocation the key is
   revoked immediately and the addHoldDown timer does not apply.  Thus
   the minimum amount of time that a SEP Publisher must wait before
   removing a revoked key from publication is:

     remWaitTime = sigExpirationTimeRemaining
                   + activeRefresh
                   + timingSafetyMargin
                   + retrySafetyMargin

     remWaitTime = sigExpirationTimeRemaining
                   + MAX(1 hour,
                         MIN((sigExpirationTime) / 2,
                             MAX(TTL of K_old DNSKEY RRSet) / 2,
                             15 days))
                   + activeRefresh
                   + retrySafetyMargin

   Note also that adding retryTime intervals to the remWaitTime may be
   wise, just as it was for addWaitTime in Section 6.

6.3.2.  Wall-Clock Based Calculation

   Like before, the above equations are defined based upon how long to
   wait from a particular moment in time.  An alternative, but
   equivalent, method is to calculate the date and time before which it
   is unsafe to cease publishing a revoked key.  This calculation thus
   becomes:

      remWallClockTime = lastSigExpirationTime
                       + activeRefresh
                       + timingSafetyMargin
                       + retrySafetyMargin

      remWallClockTime = lastSigExpirationTime
                       + MAX(1 hour,
                             MIN((sigExpirationTime) / 2,
                                 MAX(TTL of K_old DNSKEY RRSet) / 2,
                                 15 days))
                       + timingSafetyMargin
                       + retrySafetyMargin

   where lastSigExpirationTime is the latest value of any
   sigExpirationTime for which RRSIGs were created that could
   potentially be replayed.  Fully expanded, this becomes:



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6.3.3.  Additional Considerations for RFC7583

   Note that our notion of remWaitTime is called "Irev" in
   Section 3.3.4.2 of [RFC7583].  The equation for Irev in RFC7583 is
   insecure as it does not include the sigExpirationTime listed above.
   The Irev equation in RFC7583 also does not include a safety margin,
   though that is an operational consideration.

6.3.4.  Example Scenario Calculations

   For the parameters listed in Section 5.1, our example:

     remwaitTime = 10
                   + 1 / 2          (days)

     remwaitTime = 10.5             (days)

   Note that for the values in this example produce a length shorter
   than the recommended 30 days in RFC5011's section 6.6, step 3.  Other
   values of sigExpirationTime and the original TTL of the K_old DNSKEY
   RRSet, however, can produce values longer than 30 days.

   Note that because revocation happens immediately, an attacker has a
   much harder job tricking a RFC5011 Resolver into leaving a trust
   anchor in place, as the attacker must successfully replay the old
   data for every query a RFC5011 Resolver sends, not just one.

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 practice considerations from the
   perspective of a zone publisher and SEP Publisher.  However, this
   companion document has yet to be published.  The authors of this
   document hope that it will at some point in the future, as RFC5011
   timing can be tricky as we have shown, and a BCP is clearly
   warranted.  This document is intended only to fill a single
   operational void which, when left misunderstood, can result in
   serious security ramifications.  This document does not attempt to
   document any other missing operational guidance for zone publishers.








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9.  Security Considerations

   This document, is solely about the security considerations with
   respect to the SEP Publisher's ability to advertise new DNSKEYs via
   the RFC5011 automated trust anchor update process.  Thus the entire
   document is a discussion of Security Considerations when adding or
   removing DNSKEYs from trust anchor storage using the RFC5011 process.

   For simplicity, this document assumes that the SEP Publisher will use
   a consistent RRSIG validity period.  SEP Publishers that vary the
   length of RRSIG validity periods will need to adjust the
   sigExpirationTime value accordingly so that the equations in
   Section 6 and Section 6.3 use a value that coincides with the last
   time a replay of older RRSIGs will no longer succeed.

10.  Acknowledgements

   The authors would like to especially thank to Michael StJohns for his
   help and advice and the care and thought he put into RFC5011 itself
   and his continued reviews and suggestions for this document.  He also
   designed the suggested math behind the suggested retrySafetyMargin
   values in Section 6.1.9.

   We would also like to thank Bob Harold, Shane Kerr, Matthijs Mekking,
   Duane Wessels, Petr Petr Spacek, Ed Lewis, and the dnsop working
   group who have assisted with this document.

11.  Normative References

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

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <http://www.rfc-editor.org/info/rfc4033>.

   [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>.

   [RFC7583]  Morris, S., Ihren, J., Dickinson, J., and W. Mekking,
              "DNSSEC Key Rollover Timing Considerations", RFC 7583,
              DOI 10.17487/RFC7583, October 2015, <https://www.rfc-
              editor.org/info/rfc7583>.






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   [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", RFC 7719, DOI 10.17487/RFC7719, December
              2015, <http://www.rfc-editor.org/info/rfc7719>.

Appendix A.  Real World Example: The 2017 Root KSK Key Roll

   In 2017 and 2018, ICANN expects to (or has, depending on when you're
   reading this) roll the key signing key (KSK) for the root zone.  The
   relevant parameters associated with the root zone at the time of this
   writing is as follows:

         addHoldDownTime:                      30 days
         Old DNSKEY sigExpirationTime:         21 days
         Old DNSKEY TTL:                        2 days

   Thus, sticking this information into the equation in
   Section Section 6 yields (in days from publication time):

     addWaitTime = 30
                   + 21
                   + MAX(1 hour,
                         MIN(21 / 2,     # activeRefresh
                             MAX(2) / 2,
                             15 days),
                         )
                   + activeRefresh

     addWaitTime = 30 + 21 + 1 + 1

     addWaitTime = 53 days

   Also note that we exclude the retrySafetyMargin value, which is
   calculated based on the expected client deployment size.

   Thus, ICANN must wait a minimum of 52 days before switching to the
   newly published KSK (and 26 days before removing the old revoked key
   once it is published as revoked).  ICANN's current plans involve
   waiting over 3 months before using the new KEY and 69 days before
   removing the old, revoked key.  Thus, their current rollover plans
   are sufficiently secure from the attack discussed in this memo.

Authors' Addresses









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   Wes Hardaker
   USC/ISI
   P.O. Box 382
   Davis, CA  95617
   US

   Email: ietf@hardakers.net


   Warren Kumari
   Google
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   US

   Email: warren@kumari.net



































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