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Versions: 00 01

Network Working Group                                           J. Ihren
Internet-Draft                                             Autonomica AB
Expires: April 23, 2006                                       O. Kolkman
                                                                RIPE NCC
                                                              B. Manning
                                                                  EP.net
                                                        October 23, 2005



  An In-Band Rollover Mechanism and an Out-Of-Band Priming Method for
                         DNSSEC Trust Anchors.
               draft-ietf-dnsext-trustupdate-threshold-01


Status of this Memo

   By submitting this Internet-Draft, each author represents
   that any applicable patent or other IPR claims of which he
   or she is aware have been or will be disclosed, and any of
   which he or she becomes aware will be disclosed, in
   accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on April 23, 2006.


Copyright Notice


   Copyright (C) The Internet Society (2005).  All Rights Reserved.


Abstract


   The DNS Security Extensions (DNSSEC) works by validating so called
   chains of authority.  The start of these chains of authority are
   usually public keys that are anchored in the DNS clients.  These keys
   are known as the so called trust anchors.





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   This memo describes a method how these client trust anchors can be
   replaced using the DNS validation and querying mechanisms (in-band)
   when the key pairs used for signing by zone owner are rolled.


   This memo also describes a method to establish the validity of trust
   anchors for initial configuration, or priming, using out of band
   mechanisms.


Table of Contents


   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1   Key Signing Keys, Zone Signing Keys and Secure Entry
           Points . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction and Background  . . . . . . . . . . . . . . . . .  5
     2.1   Dangers of Stale Trust Anchors . . . . . . . . . . . . . .  5
   3.  Threshold-based Trust Anchor Rollover  . . . . . . . . . . . .  7
     3.1   Why Rollover?. . . . . . . . . . . . . . . . . . . . . . .  7
     3.2   The Rollover . . . . . . . . . . . . . . . . . . . . . . .  7
     3.3   Threshold-based Trust Update . . . . . . . . . . . . . . .  8
     3.4   Possible Trust Update States . . . . . . . . . . . . . . .  9
     3.5   Implementation notes . . . . . . . . . . . . . . . . . . . 10
     3.6   Possible transactions  . . . . . . . . . . . . . . . . . . 11
       3.6.1   Single DNSKEY replaced . . . . . . . . . . . . . . . . 12
       3.6.2   Addition of a new DNSKEY (no removal)  . . . . . . . . 12
       3.6.3   Removal of old DNSKEY (no addition)  . . . . . . . . . 12
       3.6.4   Multiple DNSKEYs replaced  . . . . . . . . . . . . . . 12
     3.7   Removal of trust anchors for a trust point . . . . . . . . 12
     3.8   No need for resolver-side overlap of old and new keys  . . 13
   4.  Bootstrapping automatic rollovers  . . . . . . . . . . . . . . 14
     4.1   Priming Keys . . . . . . . . . . . . . . . . . . . . . . . 14
       4.1.1   Bootstrapping trust anchors using a priming key  . . . 14
       4.1.2   Distribution of priming keys . . . . . . . . . . . . . 15
   5.  The Threshold Rollover Mechanism vs Priming  . . . . . . . . . 16
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
     6.1   Threshold-based Trust Update Security Considerations . . . 17
     6.2   Priming Key Security Considerations  . . . . . . . . . . . 17
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
   8.1   Normative References . . . . . . . . . . . . . . . . . . . . 20
   8.2   Informative References . . . . . . . . . . . . . . . . . . . 20
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 20
   A.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 22
   B.  Document History . . . . . . . . . . . . . . . . . . . . . . . 23
     B.1   prior to version 00  . . . . . . . . . . . . . . . . . . . 23
     B.2   version 00 . . . . . . . . . . . . . . . . . . . . . . . . 23
       Intellectual Property and Copyright Statements . . . . . . . . 24







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1.  Terminology


   The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED",
   and "MAY" in this document are to be interpreted as described in
   RFC2119 [1].


   The term "zone" refers to the unit of administrative control in the
   Domain Name System.  In this document "name server" denotes a DNS
   name server that is authoritative (i.e.  knows all there is to know)
   for a DNS zone.  A "zone owner" is the entity responsible for signing
   and publishing a zone on a name server.  The terms "authentication
   chain", "bogus", "trust anchors" and "Island of Security" are defined
   in [4].  Throughout this document we use the term "resolver" to mean
   "Validating Stub Resolvers" as defined in [4].


   We use the term "security apex" as the zone for which a trust anchor
   has been configured (by validating clients) and which is therefore,
   by definition, at the root of an island of security.  The
   configuration of trust anchors is a client side issue.  Therefore a
   zone owner may not always know if their zone has become a security
   apex.


   A "stale anchor" is a trust anchor (a public key) that relates to a
   key that is not used for signing.  Since trust anchors indicate that
   a zone is supposed to be secure a validator will mark the all data in
   an island of security as bogus when all trust anchors become stale.


   It is assumed that the reader is familiar with public key
   cryptography concepts [REF: Schneier Applied Cryptography] and is
   able to distinguish between the private and public parts of a key
   based on the context in which we use the term "key".  If there is a
   possible ambiguity we will explicitly mention if a private or a
   public part of a key is used.


   The term "administrator" is used loosely throughout the text.  In
   some cases an administrator is meant to be a person, in other cases
   the administrator may be a process that has been delegated certain
   responsibilities.


1.1  Key Signing Keys, Zone Signing Keys and Secure Entry Points


   Although the DNSSEC protocol does not make a distinction between
   different keys the operational practice is that a distinction is made
   between zone signing keys and key signing keys.  A key signing key is
   used to exclusively sign the DNSKEY Resource Record (RR) set at the
   apex of a zone and the zone signing keys sign all the data in the
   zone (including the DNSKEY RRset at the apex).





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   Keys that are intended to be used as the start of the authentication
   chain for a particular zone, either because they are pointed to by a
   parental DS RR or because they are configured as a trust anchor, are
   called Secure Entry Point (SEP) keys.  In practice these SEP keys
   will be key signing keys.


   In order for the mechanism described herein to work the keys that are
   intended to be used as secure entry points MUST have the SEP [2] flag
   set.  In the examples it is assumed that keys with the SEP flag set
   are used as key signing keys and thus exclusively sign the DNSKEY
   RRset published at the apex of the zone.









































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2.  Introduction and Background


   When DNSSEC signatures are validated the resolver constructs a chain
   of authority from a pre-configured trust anchor to the DNSKEY
   Resource Record (RR), which contains the public key that validates
   the signature stored in an RRSIG RR.  DNSSEC is designed so that the
   administrator of a resolver can validate data in multiple islands of
   security by configuring multiple trust anchors.


   It is expected that resolvers will have more than one trust anchor
   configured.  Although there is no deployment experience it is not
   unreasonable to expect resolvers to be configured with a number of
   trust anchors that varies between order 1 and order 1000.  Because
   zone owners are expected to roll their keys, trust anchors will have
   to be maintained (in the resolver end) in order not to become stale.


   Since there is no global key maintenance policy for zone owners and
   there are no mechanisms in the DNS to signal the key maintenance
   policy it may be very hard for resolvers administrators to keep their
   set of trust anchors up to date.  For instance, if there is only one
   trust anchor configured and the key maintenance policy is clearly
   published, through some out of band trusted channel, then a resolver
   administrator can probably keep track of key rollovers and update the
   trust anchor manually.  However, with an increasing number of trust
   anchors all rolled according to individual policies that are all
   published through different channels this soon becomes an
   unmanageable problem.


2.1  Dangers of Stale Trust Anchors


   Whenever a SEP key at a security apex is rolled there exists a danger
   that "stale anchors" are created.  A stale anchor is a trust anchor
   (i.e.  a public key configured in a validating resolver) that relates
   to a private key that is no longer used for signing.


   The problem with a stale anchors is that they will (from the
   validating resolvers point of view) prove data to be false even
   though it is actually correct.  This is because the data is either
   signed by a new key or is no longer signed and the resolver expects
   data to be signed by the old (now stale) key.


   This situation is arguably worse than not having a trusted key
   configured for the secure entry point, since with a stale key no
   lookup is typically possible (presuming that the default
   configuration of a validating recursive nameserver is to not give out
   data that is signed but failed to verify.


   The danger of making configured trust anchors become stale anchors




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   may be a reason for zone owners not to roll their keys.  If a
   resolver is configured with many trust anchors that need manual
   maintenance it may be easy to not notice a key rollover at a security
   apex, resulting in a stale anchor.


   In Section 3 this memo sets out a lightweight, in-DNS, mechanism to
   track key rollovers and modify the configured trust anchors
   accordingly.  The mechanism is stateless and does not need protocol
   extensions.  The proposed design is that this mechanism is
   implemented as a "trust updating machine" that is run entirely
   separate from the validating resolver except that the trust updater
   will have influence over the trust anchors used by the latter.


   In Section 4 we describe a method [Editors note: for now only the
   frame work and a set of requirements] to install trust anchors.  This
   method can be used at first configuration or when the trust anchors
   became stale (typically due to a failure to track several rollover
   events).


   The choice for which domains trust anchors are to be configured is a
   local policy issue.  So is the choice which trust anchors has
   prevalence if there are multiple chains of trust to a given piece of
   DNS data (e.g.  when a parent zone and its child both have trust
   anchors configured).  Both issues are out of the scope of this
   document.



























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3.  Threshold-based Trust Anchor Rollover

3.1  Why Rollover?

   Any cryptographic system must be able to periodically replace the
   secret keys used. Reasons for this include everything from accidental
   compromise to cryptographic exposure through prolonged use. In this
   case we argue that the compromise case is the most crucial and difficult
   one, as the exposure can be managed through controlled, periodic
   rollovers while the compromise case causes either no rollover (because
   it isn't detected in time) or immediate emergency rollover.

   Also it is crucial to note that for the compromise case a compromised
   key must not enable the attacker to do his own rollover into new keys
   under attacker control. Especially in the case of a compromise that isn't
   immediately detected this is important.

3.2  The Rollover

   When a key pair is replaced all signatures (in DNSSEC these are the
   RRSIG records) created with the old key will be replaced by new
   signatures created by the new key.  Access to the new public key is
   needed to verify these signatures.

   Since zone signing keys are in "the middle" of a chain of authority
   they can be verified using the signature made by a key signing key.
   Rollover of zone signing keys is therefore transparent to validators
   and requires no action in the validator end.

   But if a key signing key is rolled a resolver can determine its
   authenticity by either following the authorization chain from the
   parents DS record, an out-of-DNS authentication mechanism or by
   relying on other trust anchors known for the zone in which the key is
   rolled.

   The threshold trust anchor rollover mechanism (or trust update),
   described below, is based on using existing trust anchors to verify a
   subset of the available signatures.  This is then used as the basis
   for a decision to accept the new keys as valid trust anchors.

   Our example pseudo zone below contains a number of key signing keys
   numbered 1 through Y and two zone signing keys A and B.  During a key
   rollover key 2 is replaced by key Y+1.  The zone content changes
   from:

          example.com.  DNSKEY key1
          example.com.  DNSKEY key2
          example.com.  DNSKEY key3
          ...
          example.com.  DNSKEY keyY

          example.com.  DNSKEY keyA
          example.com.  DNSKEY keyB

          example.com.  RRSIG DNSKEY ... (key1)
          example.com.  RRSIG DNSKEY ... (key2)
          example.com.  RRSIG DNSKEY ... (key3)
          ...
          example.com.  RRSIG DNSKEY ... (keyY)
          example.com.  RRSIG DNSKEY ... (keyA)
          example.com.  RRSIG DNSKEY ... (keyB)

    to:



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          example.com.  DNSKEY key1
          example.com.  DNSKEY key3
          ...
          example.com.  DNSKEY keyY
          example.com.  DNSKEY keyY+1


          example.com.  RRSIG DNSKEY ... (key1)
          example.com.  RRSIG DNSKEY ... (key3)
          ...
          example.com.  RRSIG DNSKEY ... (keyY)
          example.com.  RRSIG DNSKEY ... (keyY+1)
          example.com.  RRSIG DNSKEY ... (keyA)
          example.com.  RRSIG DNSKEY ... (keyB)


   When the rollover becomes visible to the verifying stub resolver it
   will be able to verify the RRSIGs associated with key1, key3 ...
   keyY.  There will be no RRSIG by key2 and the RRSIG by keyY+1 will
   not be used for validation, since that key is previously unknown and
   therefore not trusted.

   Note that this example is simplified.  Because of operational
   considerations described in [5] having a period during which the two
   key signing keys are both available is necessary.


3.3  Threshold-based Trust Update


   The threshold-based trust update algorithm applies as follows.  If
   for a particular secure entry point
   o  if the DNSKEY RRset in the zone has been replaced by a more recent
      one (as determined by comparing the RRSIG inception dates)
   and
   o  if at least M configured trust anchors directly verify the related
      RRSIGs over the new DNSKEY RRset
   and
   o  the number of configured trust anchors that verify the related
      RRSIGs over the new DNSKEY RRset exceed a locally defined minimum
      number that should be greater than one
   then all the trust anchors for the particular secure entry point are
   replaced by the set of keys from the zones DNSKEY RRset that have the
   SEP flag set.


   The choices for the rollover acceptance policy parameter M is left to
   the administrator of the resolver.  To be certain that a rollover is
   accepted up by resolvers using this mechanism zone owners should roll
   as few SEP keys at a time as possible (preferably just one).  That
   way they comply to the most strict rollover acceptance policy of
   M=N-1.





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   The value of M has an upper bound, limited by the number of of SEP
   keys a zone owner publishes (i.e.  N).  But there is also a lower
   bound, since it will not be safe to base the trust in too few
   signatures.  The corner case is M=1 when any validating RRSIG will be
   sufficient for a complete replacement of the trust anchors for that
   secure entry point.  This is not a recommended configuration, since
   that will allow an attacker to initiate rollover of the trust anchors
   himself given access to just one compromised key.  Hence M should in
   be strictly larger than 1 as shown by the third requirement above.


   If the rollover acceptance policy is M=1 then the result for the
   rollover in our example above should be that the local database of
   trust anchors is updated by removing key "key2" from and adding key
   "keyY+1" to the key store.


3.4  Possible Trust Update States


   We define five states for trust anchor configuration at the client
   side.
   PRIMING: There are no trust anchors configured.  There may be priming
      keys available for initial priming of trust anchors.
   IN-SYNC: The set of trust anchors configured exactly matches the set
      of SEP keys used by the zone owner to sign the zone.
   OUT-OF-SYNC: The set of trust anchors is not exactly the same as the
      set of SEP keys used by the zone owner to sign the zone but there
      are enough SEP key in use by the zone owner that is also in the
      trust anchor configuration.
   UNSYNCABLE: There is not enough overlap between the configured trust
      anchors and the set of SEP keys used to sign the zone for the new
      set to be accepted by the validator (i.e.  the number of
      signatures that verify is not sufficient).
   STALE: There is no overlap between the configured trust anchors and
      the set of SEP keys used to sign the zone.  Here validation of
      data is no longer possible and hence we are in a situation where
      the trust anchors are stale.


   Of these five states only two (IN-SYNC and OUT-OF-SYNC) are part of
   the automatic trust update mechanism.  The PRIMING state is where a
   validator is located before acquiring an up-to-date set of trust
   anchors.  The transition from PRIMING to IN-SYNC is manual (see
   Section 4 below).


   Example: assume a secure entry point with four SEP keys and a
   validator with the policy that it will accept any update to the set
   of trust anchors as long as no more than two signatures fail to
   validate (i.e.  M >= N-2) and at least two signature does validate
   (i.e.  M >= 2).  In this case the rollover of a single key will move
   the validator from IN-SYNC to OUT-OF-SYNC.  When the trust update




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   state machine updates the trust anchors it returns to state IN-SYNC.


   If if for some reason it fails to update the trust anchors then the
   next rollover (of a different key) will move the validator from
   OUT-OF-SYNC to OUT-OF-SYNC (again), since there are still two keys
   that are configured as trust anchors and that is sufficient to accpt
   an automatic update of the trust anchors.


   The UNSYNCABLE state is where a validator is located if it for some
   reason fails to incorporate enough updates to the trust anchors to be
   able to accept new updates according to its local policy.  In this
   example (i.e.  with the policy specified above) this will either be
   because M < N-2 or M < 2, which does not suffice to authenticate a
   successful update of trust anchors.


   Continuing with the previous example where two of the four SEP keys
   have already rolled, but the validator has failed to update the set
   of trust anchors.  When the third key rolls over there will only be
   one trust anchor left that can do successful validation.  This is not
   sufficient to enable automatic update of the trust anchors, hence the
   new state is UNSYNCABLE.  Note, however, that the remaining
   up-to-date trust anchor is still enough to do successful validation
   so the validator is still "working" from a DNSSEC point of view.


   The STALE state, finally, is where a validator ends up when it has
   zero remaining current trust anchors.  This is a dangerous state,
   since the stale trust anchors will cause all validation to fail.  The
   escape is to remove the stale trust anchors and thereby revert to the
   PRIMING state.


3.5  Implementation notes


   The DNSSEC protocol specification ordains that a DNSKEY to which a DS
   record points should be self-signed.  Since the keys that serve as
   trust anchors and the keys that are pointed to by DS records serve
   the same purpose, they are both secure entry points, we RECOMMEND
   that zone owners who want to facilitate the automated rollover scheme
   documented herein self-sign DNSKEYs with the SEP bit set and that
   implementation check that DNSKEYs with the SEP bit set are
   self-signed.


   In order to maintain a uniform way of determining that a keyset in
   the zone has been replaced by a more recent set the automatic trust
   update machine SHOULD only accept new DNSKEY RRsets if the
   accompanying RRSIGs show a more recent inception date than the
   present set of trust anchors.  This is also needed as a safe guard
   against possible replay attacks where old updates are replayed
   "backwards" (i.e.  one change at a time, but going in the wrong




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   direction, thereby luring the validator into the UNSYNCABLE and
   finally STALE states).


   In order to be resilient against failures the implementation should
   collect the DNSKEY RRsets from (other) authoritative servers if
   verification of the self signatures fails.


   The threshold-based trust update mechanism SHOULD only be applied to
   algorithms, as represented in the algorithm field in the DNSKEY/RRSIG
   [3], that the resolver is aware of.  In other words the SEP keys of
   unknown algorithms should not be used when counting the number of
   available signatures (the N constant) and the SEP keys of unknown
   algorithm should not be entered as trust anchors.


   When in state UNSYNCABLE or STALE manual intervention will be needed
   to return to the IN-SYNC state.  These states should be flagged.  The
   most appropriate action is human audit possibly followed by
   re-priming (Section 4) the keyset (i.e.  manual transfer to the
   PRIMING state through removal of the configured trust anchors).


   An implementation should regularly probe the the authoritative
   nameservers for new keys.  Since there is no mechanism to publish
   rollover frequencies this document RECOMMENDS zone owners not to roll
   their key signing keys more often than once per month and resolver
   administrators to probe for key rollsovers (and apply the threshold
   criterion for acceptance of trust update) not less often than once
   per month.  If the rollover frequency is higher than the probing
   frequency then trust anchors may become stale.  The exact relation
   between the frequencies depends on the number of SEP keys rolled by
   the zone owner and the value M configured by the resolver
   administrator.


   In all the cases below a transaction where the threshold criterion is
   not satisfied should be considered bad (i.e.  possibly spoofed or
   otherwise corrupted data).  The most appropriate action is human
   audit.


   There is one case where a "bad" state may be escaped from in an
   automated fashion.  This is when entering the STALE state where all
   DNSSEC validation starts to fail.  If this happens it is concievable
   that it is better to completely discard the stale trust anchors
   (thereby reverting to the PRIMING state where validation is not
   possible).  A local policy that automates removal of stale trust
   anchors is therefore suggested.


3.6  Possible transactions






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3.6.1  Single DNSKEY replaced


   This is probably the most typical transaction on the zone owners
   part.  The result should be that if the threshold criterion is
   satisfied then the key store is updated by removal of the old trust
   anchor and addition of the new key as a new trust anchor.  Note that
   if the DNSKEY RRset contains exactly M keys replacement of keys is
   not possible, i.e.  for automatic rollover to work M must be stricly
   less than N.


3.6.2  Addition of a new DNSKEY (no removal)


   If the threshold criterion is satisfied then the new key is added as
   a configured trust anchor.  Not more than N-M keys can be added at
   once, since otherwise the algorithm will fail.


3.6.3  Removal of old DNSKEY (no addition)


   If the threshold criterion is satisfied then the old key is removed
   from being a configured trust anchor.  Note that it is not possible
   to reduce the size of the DNSKEY RRset to a size smaller than the
   minimum required value for M.


3.6.4  Multiple DNSKEYs replaced


   Arguably it is not a good idea for the zone administrator to replace
   several keys at the same time, but from the resolver point of view
   this is exactly what will happen if the validating resolver for some
   reason failed to notice a previous rollover event.


   Not more than N-M keys can be replaced at one time or the threshold
   criterion will not be satisfied.  Or, expressed another way: as long
   as the number of changed keys is less than or equal to N-M the
   validator is in state OUT-OF-SYNC.  When the number of changed keys
   becomes greater than N-M the state changes to UNSYNCABLE and manual
   action is needed.


3.7  Removal of trust anchors for a trust point


   If the parent of a secure entry point gets signed and it's trusted
   keys get configured in the key store of the validating resolver then
   the configured trust anchors for the child should be removed entirely
   unless explicitly configured (in the utility configuration) to be an
   exception.


   The reason for such a configuration would be that the resolver has a
   local policy that requires maintenance of trusted keys further down
   the tree hierarchy than strictly needed from the point of view.




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   The default action when the parent zone changes from unsigned to
   signed should be to remove the configured trust anchors for the
   child.  This form of "garbage collect" will ensure that the automatic
   rollover machinery scales as DNSSEC deployment progresses.


3.8  No need for resolver-side overlap of old and new keys


   It is worth pointing out that there is no need for the resolver to
   keep state about old keys versus new keys, beyond the requirement of
   tracking signature inception time for the covering RRSIGs as
   described in Section 3.4.


   From the resolver point of view there are only trusted and not
   trusted keys.  The reason is that the zone owner needs to do proper
   maintenance of RRSIGs regardless of the resolver rollover mechanism
   and hence must ensure that no key rolled out out the DNSKEY set until
   there cannot be any RRSIGs created by this key still legally cached.


   Hence the rollover mechanism is entirely stateless with regard to the
   keys involved: as soon as the resolver (or in this case the rollover
   tracking utility) detects a change in the DNSKEY RRset (i.e.  it is
   now in the state OUT-OF-SYNC) with a sufficient number of matching
   RRSIGs the configured trust anchors are immediately updated (and
   thereby the machine return to state IN-SYNC).  I.e.  the rollover
   machine changes states (mostly oscillating between IN-SYNC and
   OUT-OF-SYNC), but the status of the DNSSEC keys is stateless.


























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4.  Bootstrapping automatic rollovers


   It is expected that with the ability to automatically roll trust
   anchors at trust points will follow a diminished unwillingness to
   roll these keys, since the risks associated with stale keys are
   minimized.


   The problem of "priming" the trust anchors, or bringing them into
   sync (which could happen if a resolver is off line for a long period
   in which a set of SEP keys in a zone 'evolve' away from its trust
   anchor configuration) remains.


   For (re)priming we can rely on out of band technology and we propose
   the following framework.


4.1  Priming Keys


   If all the trust anchors roll somewhat frequently (on the order of
   months or at most about a year) then it will not be possible to
   design a device, or a software distribution that includes trust
   anchors, that after being manufactured is put on a shelf for several
   key rollover periods before being brought into use (since no trust
   anchors that were known at the time of manufacture remain active).


   To alleviate this we propose the concept of "priming keys".  Priming
   keys are ordinary DNSSEC Key Signing Keys with the characteristic
   that
   o  The private part of a priming key signs the DNSKEY RRset at the
      security apex, i.e.  at least one RRSIG DNSKEY is created by a
      priming key rather than by an "ordinary" trust anchor
   o  the public parts of priming keys are not included in the DNSKEY
      RRset.  Instead the public parts of priming keys are only
      available out-of-band.
   o  The public parts of the priming keys have a validity period.
      Within this period they can be used to obtain trust anchors.
   o  The priming key pairs are long lived (relative to the key rollover
      period.)


4.1.1  Bootstrapping trust anchors using a priming key


   To install the trust anchors for a particular security apex an
   administrator of a validating resolver will need to:
   o  query for the DNSKEY RRset of the zone at the security apex;
   o  verify the self signatures of all DNSKEYs in the RRset;
   o  verify the signature of the RRSIG made with a priming key --
      verification using one of the public priming keys that is valid at
      that moment is sufficient;





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   o  create the trust anchors by extracting the DNSKEY RRs with the SEP
      flag set.
   The SEP keys with algorithms unknown to the validating resolver
   SHOULD be ignored during the creation of the trust anchors.


4.1.2  Distribution of priming keys


   The public parts of the priming keys SHOULD be distributed
   exclusively through out-of-DNS mechanisms.  The requirements for a
   distribution mechanism are:
   o  it can carry the "validity" period for the priming keys;
   o  it can carry the self-signature of the priming keys;
   o  and it allows for verification using trust relations outside the
      DNS.
   A distribution mechanism would benefit from:
   o  the availability of revocation lists;
   o  the ability of carrying zone owners policy information such as
      recommended values for "M" and "N" and a rollover frequency;
   o  and the technology on which is based is readily available.

































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5.  The Threshold Rollover Mechanism vs Priming


   There is overlap between the threshold-based trust updater and the
   Priming method.  One could exclusively use the Priming method for
   maintaining the trust anchors.  However the priming method probably
   relies on "non-DNS' technology and may therefore not be available for
   all devices that have a resolver.













































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6.  Security Considerations


6.1  Threshold-based Trust Update Security Considerations


   A clear issue for resolvers will be how to ensure that they track all
   rollover events for the zones they have configure trust anchors for.
   Because of temporary outages validating resolvers may have missed a
   rollover of a KSK.  The parameters that determine the robustness
   against failures are: the length of the period between rollovers
   during which the KSK set is stable and validating resolvers can
   actually notice the change; the number of available KSKs (i.e.  N)
   and the number of signatures that may fail to validate (i.e.  N-M).


   With a large N (i.e.  many KSKs) and a small value of M this
   operation becomes more robust since losing one key, for whatever
   reason, will not be crucial.  Unfortunately the choice for the number
   of KSKs is a local policy issue for the zone owner while the choice
   for the parameter M is a local policy issue for the resolver
   administrator.


   Higher values of M increase the resilience against attacks somewhat;
   more signatures need to verify for a rollover to be approved.  On the
   other hand the number of rollover events that may pass unnoticed
   before the resolver reaches the UNSYNCABLE state goes down.


   The threshold-based trust update intentionally does not provide a
   revocation mechanism.  In the case that a sufficient number of
   private keys of a zone owner are simultaneously compromised the the
   attacker may use these private keys to roll the trust anchors of (a
   subset of) the resolvers.  This is obviously a bad situation but it
   is not different from most other public keys systems.


   However, it is important to point out that since any reasonable trust
   anchor rollover policy (in validating resolvers) will require more
   than one RRSIG to validate this proposal does provide security
   concious zone administrators with the option of not storing the
   individual private keys in the same location and thereby decreasing
   the likelihood of simultaneous compromise.


6.2  Priming Key Security Considerations


   Since priming keys are not included in the DNSKEY RR set they are
   less sensitive to packet size constraints and can be chosen
   relatively large.  The private parts are only needed to sign the
   DNSKEY RR set during the validity period of the particular priming
   key pair.  Note that the private part of the priming key is used each
   time when a DNSKEY RRset has to be resigned.  In practice there is
   therefore little difference between the usage pattern of the private




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   part of key signing keys and priming keys.



















































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7.  IANA Considerations


   NONE.

















































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8.  References


8.1  Normative References


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


   [2]  Kolkman, O., Schlyter, J. and E. Lewis, "Domain Name System KEY
        (DNSKEY) Resource Record (RR) Secure Entry Point (SEP) Flag",
        RFC 3757, May 2004.


   [3]  Arends, R., Austein, R., Massey, D., Larson, M. and S. Rose,
        "Resource Records for the DNS Security Extensions", RFC 4034,
        March 2005.


   [4]  Arends, R., Austein, R., Massey, D., Larson, M. and S. Rose,
        "DNS Security Introduction and Requirements", RFC 4033,
        March 2005.


8.2  Informative References


   [5]  Kolkman, O., "DNSSEC Operational Practices",
        draft-ietf-dnsop-dnssec-operational-practices-01 (work in
        progress), May 2004.


   [6]  Housley, R., Ford, W., Polk, T. and D. Solo, "Internet X.509
        Public Key Infrastructure Certificate and CRL Profile", RFC
        2459, January 1999.



Authors' Addresses


   Johan Ihren
   Autonomica AB
   Bellmansgatan 30
   Stockholm  SE-118 47
   Sweden


   EMail: johani@autonomica.se












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   Olaf M. Kolkman
   NLnet Labs
   Kruislaan 419
   1098 VA Amsterdam
   NL


   EMail: olaf@nlnetlabs.nl
   URI:   http://www.nlnetlabs.nl/



   Bill Manning
   EP.net
   Marina del Rey, CA  90295
   USA





































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Appendix A.  Acknowledgments


   The present design for in-band automatic rollovers of DNSSEC trust
   anchors is the result of many conversations and it is no longer
   possible to remember exactly who contributed what.


   In addition we've also had appreciated help from (in no particular
   order) Paul Vixie, Sam Weiler, Suzanne Woolf, Steve Crocker, Matt
   Larson and Mark Kosters.











































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Appendix B.  Document History


   This appendix will be removed if and when the document is submitted
   to the RFC editor.


   The version you are reading is tagged as $Revision: 1.2 $.


   Text between square brackets, other than references, are editorial
   comments and will be removed.


B.1  prior to version 00


   This draft was initially published as a personal submission under the
   name draft-kolkman-dnsext-dnssec-in-band-rollover-00.txt.


   Kolkman documented the ideas provided by Ihren and Manning.  In the
   process of documenting (and prototyping) Kolkman changed some of the
   details of the M-N algorithms working.  Ihren did not have a chance
   to review the draft before Kolkman posted;


   Kolkman takes responsibilities for omissions, fuzzy definitions and
   mistakes.


B.2  version 00
   o  The name of the draft was changed as a result of the draft being
      adopted as a working group document.
   o  A small section on the concept of stale trust anchors was added.
   o  The different possible states are more clearly defined, including
      examples of transitions between states.
   o  The terminology is changed throughout the document.  The old term
      "M-N" is replaced by "threshold" (more or less).  Also the
      interpretation of the constants M and N is significantly
      simplified to bring the usage more in line with "standard"
      threshold terminlogy.

B.3  version 01
   o  This is a notice that the draft temporarily expired.

B.4  version 02
   o  Resurrected the draft
   o  Added new section on why rollovers are needed with particular
      empathatis on the compromise case.
   o  Updated references and author affiliations.


















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