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DNSOP                                                         O. Kolkman
Internet-Draft                                                  RIPE NCC
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                                                              NLnet Labs
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                                                              March 2005
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                      DNSSEC Operational Practices
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          draft-ietf-dnsop-dnssec-operational-practices-04.txt
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Status of this Memo

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   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.
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   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   other groups may also distribute working documents as Internet-
   Drafts.
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on September 2, 2005.
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Copyright Notice

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   Copyright (C) The Internet Society (2005).
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Abstract

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   This document describes a set of practices for operating the DNS with
   security extensions (DNSSEC).  The target audience is zone
   administrators deploying DNSSEC.

   The document discusses operational aspects of using keys and
   signatures in the DNS.  It discusses issues as key generation, key
   storage, signature generation, key rollover and related policies.
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Table of Contents

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   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1   The Use of the Term 'key'  . . . . . . . . . . . . . . . .  4
     1.2   Time Definitions . . . . . . . . . . . . . . . . . . . . .  5
   2.  Keeping the Chain of Trust Intact  . . . . . . . . . . . . . .  5
   3.  Keys Generation and Storage  . . . . . . . . . . . . . . . . .  6
     3.1   Zone and Key Signing Keys  . . . . . . . . . . . . . . . .  6
       3.1.1   Motivations for the KSK and ZSK Separation . . . . . .  6
       3.1.2   KSKs for high level zones  . . . . . . . . . . . . . .  7
     3.2   Randomness . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.3   Key Effectivity Period . . . . . . . . . . . . . . . . . .  8
     3.4   Key Algorithm  . . . . . . . . . . . . . . . . . . . . . .  9
     3.5   Key Sizes  . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.6   Private Key Storage  . . . . . . . . . . . . . . . . . . . 10
   4.  Signature generation, Key Rollover and Related Policies  . . . 11
     4.1   Time in DNSSEC . . . . . . . . . . . . . . . . . . . . . . 11
       4.1.1   Time Considerations  . . . . . . . . . . . . . . . . . 11
     4.2   Key Rollovers  . . . . . . . . . . . . . . . . . . . . . . 13
       4.2.1   Zone-signing Key Rollovers . . . . . . . . . . . . . . 13
       4.2.2   Key-signing Key Rollovers  . . . . . . . . . . . . . . 17
       4.2.3   Difference Between ZSK and KSK Rollovers . . . . . . . 18
       4.2.4   Automated Key Rollovers  . . . . . . . . . . . . . . . 19
     4.3   Planning for Emergency Key Rollover  . . . . . . . . . . . 19
       4.3.1   KSK Compromise . . . . . . . . . . . . . . . . . . . . 20
       4.3.2   ZSK Compromise . . . . . . . . . . . . . . . . . . . . 20
       4.3.3   Compromises of Keys Anchored in Resolvers  . . . . . . 20
     4.4   Parental Policies  . . . . . . . . . . . . . . . . . . . . 21
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       4.4.1   Initial Key Exchanges and Parental Policies
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               Considerations . . . . . . . . . . . . . . . . . . . . 21
       4.4.2   Storing Keys or Hashes?  . . . . . . . . . . . . . . . 21
       4.4.3   Security Lameness  . . . . . . . . . . . . . . . . . . 22
       4.4.4   DS Signature Validity Period . . . . . . . . . . . . . 22
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 23
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     7.1   Normative References . . . . . . . . . . . . . . . . . . . 24
     7.2   Informative References . . . . . . . . . . . . . . . . . . 24
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 25
   A.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . . 25
   B.  Zone-signing Key Rollover Howto  . . . . . . . . . . . . . . . 26
   C.  Typographic Conventions  . . . . . . . . . . . . . . . . . . . 26
   D.  Document Details and Changes . . . . . . . . . . . . . . . . . 29
     D.1   draft-ietf-dnsop-dnssec-operational-practices-00 . . . . . 29
     D.2   draft-ietf-dnsop-dnssec-operational-practices-01 . . . . . 29
     D.3   draft-ietf-dnsop-dnssec-operational-practices-02 . . . . . 29
     D.4   draft-ietf-dnsop-dnssec-operational-practices-03 . . . . . 29
     D.5   draft-ietf-dnsop-dnssec-operational-practices-04 . . . . . 30



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       Intellectual Property and Copyright Statements . . . . . . . . 31


















































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1.  Introduction

   During workshops and early operational deployment tests, operators
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   and system administrators gained experience about operating the DNS
   with security extensions (DNSSEC).  This document translates these
   experiences into a set of practices for zone administrators.  At the
   time of writing, there exists very little experience with DNSSEC in
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   production environments; this document should therefore explicitly
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   not be seen as representing 'Best Current Practices'.
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   The procedures herein are focused on the maintenance of signed zones
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   (i.e. signing and publishing zones on authoritative servers).  It is
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   intended that maintenance of zones such as resigning or key rollovers
   be transparent to any verifying clients on the Internet.

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   The structure of this document is as follows.  In Section 2 we
   discuss the importance of keeping the "chain of trust" intact.
   Aspects of key generation and storage of private keys are discussed
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   in Section 3; the focus in this section is mainly on the private part
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   of the key(s).  Section 4 describes considerations concerning the
   public part of the keys.  Since these public keys appear in the DNS
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   one has to take into account all kinds of timing issues, which are
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   discussed in Section 4.1.  Section 4.2 and Section 4.3 deal with the
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   rollover, or which, of keys.  Finally Section 4.4 discusses
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   considerations on how parents deal with their children's public keys
   in order to maintain chains of trust.

   The typographic conventions used in this document are explained in
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   Appendix C.

   Since this is a document with operational suggestions and there are
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   no protocol specifications, the RFC2119 [4] language does not apply.
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   This document obsoletes RFC2541 [7]
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1.1  The Use of the Term 'key'

   It is assumed that the reader is familiar with the concept of
   asymmetric keys on which DNSSEC is based (Public Key Cryptography
   [11]).  Therefore, this document will use the term 'key' rather
   loosely.  Where it is written that 'a key is used to sign data' it is
   assumed that the reader understands that it is the private part of
   the key-pair that is used for signing.  It is also assumed that the
   reader understands that the public part of the key-pair is published
   in the DNSKEY resource record and that it is the public part that is
   used in key-exchanges.





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1.2  Time Definitions
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   In this document we will be using a number of time related terms.
   The following definitions apply:
   o  "Signature validity period"
         The period that a signature is valid.  It starts at the time
         specified in the signature inception field of the RRSIG RR and
         ends at the time specified in the expiration field of the RRSIG
         RR.
   o  "Signature publication period"
         Time after which a signature (made with a specific key) is
         replaced with a new signature (made with the same key).  This
         replacement takes place by publishing the relevant RRSIG in the
         master zone file.
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         After one stopped publishing an RRSIG in a zone it may take a
         while before the RRSIG has expired from caches and has actually
         been removed from the DNS.
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   o  "Key effectivity period"
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         The period which a key pair is expected to be effective.  This
         period is defined as the time between the first inception time
         stamp and the last expiration date of any signature made with
         this key.
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         The key effectivity period can span multiple signature validity
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         periods.
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   o  "Maximum/Minimum Zone TTL"
         The maximum or minimum value of the TTLs from the complete set
         of RRs in a zone.

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2.  Keeping the Chain of Trust Intact
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   Maintaining a valid chain of trust is important because broken chains
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   of trust will result in data being marked as Bogus (as defined in [2]
   section 5), which may cause entire (sub)domains to become invisible
   to verifying clients.  The administrators of secured zones have to
   realize that their zone is, to their clients, part of a chain of
   trust.
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   As mentioned in the introduction, the procedures herein are intended
   to ensure maintenance of zones, such as resigning or key rollovers,
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   will be transparent to the verifying clients on the Internet.
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   Administrators of secured zones will have to keep in mind that data
   published on an authoritative primary server will not be immediately
   seen by verifying clients; it may take some time for the data to be
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   transfered to other secondary authoritative nameservers and clients
   may be fetching data from caching non-authoritative servers.
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   For the verifying clients it is important that data from secured
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   zones can be used to build chains of trust regardless of whether the
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   data came directly from an authoritative server, a caching nameserver
   or some middle box.  Only by carefully using the available timing
   parameters can a zone administrator assure that the data necessary
   for verification can be obtained.
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   The responsibility for maintaining the chain of trust is shared by
   administrators of secured zones in the chain of trust.  This is most
   obvious in the case of a 'key compromise' when a trade off between
   maintaining a valid chain of trust and replacing the compromised keys
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   as soon as possible must be made.  Then zone administrators will have
   to make a trade off, between keeping the chain of trust intact -
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   thereby allowing for attacks with the compromised key - or to
   deliberately break the chain of trust and making secured sub domains
   invisible to security aware resolvers.  Also see Section 4.3.
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3.  Keys Generation and Storage
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   This section describes a number of considerations with respect to the
   security of keys.  It deals with the generation, effectivity period,
   size and storage of private keys.

3.1  Zone and Key Signing Keys
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   The DNSSEC validation protocol does not distinguish between DNSKEYs.
   All DNSKEYs can be used during the validation.  In practice operators
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   use Key Signing and Zone Signing Keys and use the so-called (Secure
   Entry Point) SEP flag to distinguish between them during operations.
   The dynamics and considerations are discussed below.
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   To make zone resigning and key rollover procedures easier to
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   implement, it is possible to use one or more keys as Key Signing Keys
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   (KSK).  These keys will only sign the apex DNSKEY RR set in a zone.
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   Other keys can be used to sign all the RRsets in a zone and are
   referred to as Zone Signing Keys (ZSK).  In this document we assume
   that KSKs are the subset of keys that are used for key exchanges with
   the parent and potentially for configuration as trusted anchors - the
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   SEP keys.  In this document we assume a one-to-one mapping between
   KSK and SEP keys and we assume the SEP flag [1] to be set on all
   KSKs.
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3.1.1  Motivations for the KSK and ZSK Separation
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   Differentiating between the KSK and ZSK functions has several
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   advantages:

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   o  No parent/child interaction is required when ZSKs are updated.
   o  The KSK can be made stronger (i.e. using more bits in the key
      material).  This has little operational impact since it is only
      used to sign a small fraction of the zone data.  Also when
      verifying the KSK is only used to verify the zone's keyset.
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   o  As the KSK is only used to sign a key set, which is most probably
      updated less frequently than other data in the zone, it can be
      stored separately from and in a safer location than the ZSK.
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   o  A KSK can have a longer key effectivity period.
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   For almost any method of key management and zone signing the KSK is
   used less frequently than the ZSK.  Once a key set is signed with the
   KSK all the keys in the key set can be used as ZSK.  If a ZSK is
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   compromised, it can be simply dropped from the key set.  The new key
   set is then resigned with the KSK.

   Given the assumption that for KSKs the SEP flag is set, the KSK can
   be distinguished from a ZSK by examining the flag field in the DNSKEY
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   RR.  If the flag field is an odd number it is a KSK.  If it is an
   even number it is a ZSK.
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   The zone-signing key can be used to sign all the data in a zone on a
   regular basis.  When a zone-signing key is to be rolled, no
   interaction with the parent is needed.  This allows for "Signature
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   Validity Periods" on the order of days.
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   The key-signing key is only to be used to sign the DNSKEY RRs in a
   zone.  If a key-signing key is to be rolled over, there will be
   interactions with parties other than the zone administrator.  These
   can include the registry of the parent zone or administrators of
   verifying resolvers that have the particular key configured as
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   trusted entry points.  Hence, the key effectivity period of these
   keys can and should be made much longer.  Although, given a long
   enough key, the Key Usage Time can be on the order of years we
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   suggest planning for a key effectivity of the order of a few months
   so that a key rollover remains an operational routine.
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3.1.2  KSKs for high level zones
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   Higher level zones are generally more sensitive than lower level
   zones.  Anyone controlling or breaking the security of a zone thereby
   obtains authority over all of its sub domains (except in the case of
   resolvers that have locally configured the public key of a sub
   domain).  Therefore, extra care should be taken with high level zones
   and strong keys used.
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   The root zone is the most critical of all zones.  Someone controlling
   or compromising the security of the root zone would control the
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   entire DNS name space of all resolvers using that root zone (except
   in the case of resolvers that have locally configured the public key
   of a sub domain).  Therefore, the utmost care must be taken in the
   securing of the root zone.  The strongest and most carefully handled
   keys should be used.  The root zone private key should always be kept
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   off line.

   Many resolvers will start at a root server for their access to and
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   authentication of DNS data.  Securely updating the trust anchors in
   an enormous population of resolvers around the world will be
   extremely difficult.
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3.2  Randomness

   Careful generation of all keys is a sometimes overlooked but
   absolutely essential element in any cryptographically secure system.
   The strongest algorithms used with the longest keys are still of no
   use if an adversary can guess enough to lower the size of the likely
   key space so that it can be exhaustively searched.  Technical
   suggestions for the generation of random keys will be found in
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   RFC1750 [3].  One should carefully assess if the random number
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   generator used during key generation adheres to these suggestions.

   Keys with a long effectivity period are particularly sensitive as
   they will represent a more valuable target and be subject to attack
   for a longer time than short period keys.  It is strongly recommended
   that long term key generation occur off-line in a manner isolated
   from the network via an air gap or, at a minimum, high level secure
   hardware.

3.3  Key Effectivity Period

   For various reasons keys in DNSSEC need to be changed once in a
   while.  The longer a key is in use, the greater the probability that
   it will have been compromised through carelessness, accident,
   espionage, or cryptanalysis.  Furthermore when key rollovers are too
   rare an event, they will not become part of the operational habit and
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   there is risk that nobody on-site will remember the procedure for
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   rollover when the need is there.

   For Key Signing Keys a reasonable key effectivity period is 13
   months, with the intent to replace them after 12 months.  An intended
   key effectivity period of a month is reasonable for Zone Signing
   Keys.

   Using these recommendations will lead to rollovers occurring
   frequently enough to become part of 'operational habits'; the
   procedure does not have to be reinvented every time a key is
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   replaced.
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   Key effectivity periods can be made very short, as in the order of a
   few minutes.  But when replacing keys one has to take the
   considerations from Section 4.1 and Section 4.2 into account.
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3.4  Key Algorithm
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   There are currently three different types of algorithms that can be
   used in DNSSEC: RSA, DSA and elliptic curve cryptography.  The latter
   is fairly new and still needs to be standardized for usage in DNSSEC.

   RSA has been developed in an open and transparent manner.  As the
   patent on RSA expired in 2000, its use is now also free.

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   DSA has been developed by NIST.  The creation of signatures is
   roughly done at the same speed as with RSA, but is 10 to 40 times as
   slow for verification [11].
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   We suggest the use of RSA/SHA-1 as the preferred algorithm for the
   key.  The current known attacks on RSA can be defeated by making your
   key longer.  As the MD5 hashing algorithm is showing (theoretical)
   cracks, we recommend the usage of SHA1.

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   In 2005 some discoveries were made that SHA-1 also has some
   weaknesses.  Currently SHA-1 is strong enough for DNSSEC.  It is
   expected that a new hashing algorithm is rolled out, before any
   attack becomes practical.

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3.5  Key Sizes
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   When choosing key sizes, zone administrators will need to take into
   account how long a key will be used and how much data will be signed
   during the key publication period.  It is hard to give precise
   recommendations but Lenstra and Verheul [10] supplied the following
   table with lower bound estimates for cryptographic key sizes.  Their
   recommendations are based on a set of explicitly formulated parameter
   settings, combined with existing data points about cryptographic
   systems.  For details we refer to the original paper.


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       Year    RSA Key Sizes       Year    RSA Key Sizes

       2000        952             2015        1613
       2001        990             2016        1664
       2002        1028            2017        1717
       2003        1068            2018        1771
       2004        1108            2019        1825


       2005        1149            2020        1881
       2006        1191            2021        1937
       2007        1235            2022        1995
       2008        1279            2023        2054
       2009        1323            2024        2113


       2026        2236            2025        2174
       2010        1369            2027        2299
       2011        1416            2028        2362
       2012        1464            2029        2427
       2013        1513
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       2014        1562
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   For example, should you wish your key to last three years from 2003,
   check the RSA key size values for 2006 in this table.  In this case
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   it should be at least 1191 bits.

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   Please keep in mind that nobody can see into the future, and that
   these key lengths are only provided here as a guide.

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   When determining a key size one should take into account that a large
   key will be slower during generation and verification.  For RSA,
   verification, the most common operation, will vary roughly with the
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   square of the key size; signing will vary with the cube of the key
   size length; and key generation will vary with the fourth power of
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   the modulus length.  Besides larger keys will increase the sizes of
   the RRSIG and DNSKEY records and will therefore increase the chance
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   of DNS UDP packet overflow.  Also see Section 3.1.1 for a discussion
   of how keys serving different roles (ZSK v.  KSK) may need different
   key strengths.
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3.6  Private Key Storage

   It is recommended that, where possible, zone private keys and the
   zone file master copy be kept and used in off-line, non-network
   connected, physically secure machines only.  Periodically an
   application can be run to add authentication to a zone by adding
   RRSIG and NSEC RRs.  Then the augmented file can be transferred,



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   perhaps by sneaker-net, to the networked zone primary server machine.

   The ideal situation is to have a one way information flow to the
   network to avoid the possibility of tampering from the network.
   Keeping the zone master file on-line on the network and simply
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   cycling it through an off-line signer does not do this.  The on-line
   version could still be tampered with if the host it resides on is
   compromised.  For maximum security, the master copy of the zone file
   should be off net and should not be updated based on an unsecured
   network mediated communication.
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   In general keeping a zone-file off-line will not be practical and the
   machines on which zone files are maintained will be connected to a
   network.  Operators are advised to take security measures to shield
   unauthorized access to the master copy.
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   For dynamically updated secured zones [5] both the master copy and
   the private key that is used to update signatures on updated RRs will
   need to be on line.
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4.  Signature generation, Key Rollover and Related Policies
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4.1  Time in DNSSEC

   Without DNSSEC all times in DNS are relative.  The SOA RR's refresh,
   retry and expiration timers are counters that are used to determine
   the time elapsed after a slave server synchronized (or tried to
   synchronize) with a master server.  The Time to Live (TTL) value and
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   the SOA RR minimum TTL parameter [6] are used to determine how long a
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   forwarder should cache data after it has been fetched from an
   authoritative server.  By using a signature validity period, DNSSEC
   introduces the notion of an absolute time in the DNS.  Signatures in
   DNSSEC have an expiration date after which the signature is marked as
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   invalid and the signed data is to be considered Bogus.
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4.1.1  Time Considerations

   Because of the expiration of signatures, one should consider the
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   following:
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   o  We suggest the Maximum Zone TTL of your zone data to be a fraction
      of your signature validity period.
         If the TTL would be of similar order as the signature validity
         period, then all RRsets fetched during the validity period
         would be cached until the signature expiration time.  Section
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         7.1 of [2] suggests that "the resolver may use the time
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         remaining before expiration of the signature validity period of
         a signed RRset as an upper bound for the TTL".  As a result
         query load on authoritative servers would peak at signature



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         expiration time, as this is also the time at which records
         simultaneously expire from caches.
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         To avoid query load peaks we suggest the TTL on all the RRs in
         your zone to be at least a few times smaller than your
         signature validity period.
   o  We suggest the signature publication period to be at least one
      maximum TTL smaller than the signature validity period.
         Resigning a zone shortly before the end of the signature
         validity period may cause simultaneous expiration of data from
         caches.  This in turn may lead to peaks in the load on
         authoritative servers.
   o  We suggest the minimum zone TTL to be long enough to both fetch
      and verify all the RRs in the authentication chain.  A low TTL
      could cause two problems:
         1.  During validation, some data may expire before the
         validation is complete.  The validator should be able to keep
         all data, until is completed.  This applies to all RRs needed
         to complete the chain of trust: DSs, DNSKEYs, RRSIGs, and the
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         final answers i.e. the RR set that is returned for the initial
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         query.
         2.  Frequent verification causes load on recursive nameservers.
         Data at delegation points, DSs, DNSKEYs and RRSIGs benefit from
         caching.  The TTL on those should be relatively long.
   o  Slave servers will need to be able to fetch newly signed zones
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      well before the RRSIGs in the zone served by the slave server pass
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      their signature expiration time.
         When a slave server is out of sync with its master and data in
         a zone is signed by expired signatures it may be better for the
         slave server not to give out any answer.
         Normally a slave server that is not able to contact a master
         server for an extended period will expire a zone.  When that
         happens the zone will not respond on queries.  The time of
         expiration is set in the SOA record and is relative to the last
         successful refresh between the master and the slave server.
         There exists no coupling between the signature expiration of
         RRSIGs in the zone and the expire parameter in the SOA.
         If the server serves a DNSSEC zone than it may well happen that
         the signatures expire well before the SOA expiration timer
         counts down to zero.  It is not possible to completely prevent
         this from happening by tweaking the SOA parameters.
         However, the effects can be minimized where the SOA expiration
         time is equal or smaller than the signature validity period.
         The consequence of an authoritative server not being able to
         update a zone, whilst that zone includes expired signatures, is
         that non-secure resolvers will continue to be able to resolve
         data served by the particular slave servers while security
         aware resolvers will experience problems because of answers
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         being marked as Bogus.
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         We suggest the SOA expiration timer being approximately one
         third or one fourth of the signature validity period.  It will
         allow problems with transfers from the master server to be
         noticed before the actual signature time out.
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         We also suggest that operators of nameservers that supply
         secondary services develop 'watch dogs' to spot upcoming
         signature expirations in zones they slave, and take appropriate
         action.
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         When determining the value for the expiration parameter one has
         to take the following into account: What are the chances that
         all my secondary zones expire; How quickly can I reach an
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         administrator of secondary servers to load a valid zone?  All
         these arguments are not DNSSEC specific but may influence the
         choice of your signature validity intervals.
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4.2  Key Rollovers

   A DNSSEC key cannot be used forever (see Section 3.3).  So key
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   rollovers -- or supercessions, as they are sometimes called -- are a
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   fact of life when using DNSSEC.  Zone administrators who are in the
   process of rolling their keys have to take into account that data
   published in previous versions of their zone still lives in caches.
   When deploying DNSSEC, this becomes an important consideration;
   ignoring data that may be in caches may lead to loss of service for
   clients.

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   The most pressing example of this is when zone material signed with
   an old key is being validated by a resolver which does not have the
   old zone key cached.  If the old key is no longer present in the
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   current zone, this validation fails, marking the data Bogus.
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   Alternatively, an attempt could be made to validate data which is
   signed with a new key against an old key that lives in a local cache,
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   also resulting in data being marked Bogus.
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4.2.1  Zone-signing Key Rollovers
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   For zone-signing key rollovers there are two ways to make sure that
   during the rollover data still cached can be verified with the new
   key sets or newly generated signatures can be verified with the keys
   still in caches.  One schema, described in Section 4.2.1.2, uses
   double signatures; the other uses key pre-publication
   (Section 4.2.1.1).  The pros, cons and recommendations are described
   in Section 4.2.1.3.
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4.2.1.1  Pre-publish key set Rollover
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   This section shows how to perform a ZSK rollover without the need to
   sign all the data in a zone twice - the so-called "pre-publish
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   rollover".This method has advantages in the case of a key compromise.
   If the old key is compromised, the new key has already been
   distributed in the DNS.  The zone administrator is then able to
   quickly switch to the new key and remove the compromised key from the
   zone.  Another major advantage is that the zone size does not double,
   as is the case with the double signature ZSK rollover.  A small
   "HOWTO" for this kind of rollover can be found in Appendix B.

    normal          pre-roll         roll            after

    SOA0            SOA1             SOA2            SOA3
    RRSIG10(SOA0)   RRSIG10(SOA1)    RRSIG11(SOA2)   RRSIG11(SOA3)

    DNSKEY1         DNSKEY1          DNSKEY1         DNSKEY1
    DNSKEY10        DNSKEY10         DNSKEY10        DNSKEY11
                    DNSKEY11         DNSKEY11
    RRSIG1 (DNSKEY) RRSIG1 (DNSKEY)  RRSIG1(DNSKEY)  RRSIG1 (DNSKEY)
    RRSIG10(DNSKEY) RRSIG10(DNSKEY)  RRSIG11(DNSKEY) RRSIG11(DNSKEY)


   normal: Version 0 of the zone: DNSKEY 1 is the key-signing key.
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      DNSKEY 10 is used to sign all the data of the zone, the zone-
      signing key.
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   pre-roll: DNSKEY 11 is introduced into the key set.  Note that no
      signatures are generated with this key yet, but this does not
      secure against brute force attacks on the public key.  The minimum
      duration of this pre-roll phase is the time it takes for the data
      to propagate to the authoritative servers plus TTL value of the
      key set.  This equates to two times the Maximum Zone TTL.
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   roll: At the rollover stage (SOA serial 2) DNSKEY 11 is used to sign
      the data in the zone exclusively  (i.e. all the signatures from
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      DNSKEY 10 are removed from the zone).  DNSKEY 10 remains published
      in the key set.  This way data that was loaded into caches from
      version 1 of the zone can still be verified with key sets fetched
      from version 2 of the zone.
      The minimum time that the key set including DNSKEY 10 is to be
      published is the time that it takes for zone data from the
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      previous version of the zone to expire from old caches i.e. the
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      time it takes for this zone to propagate to all authoritative
      servers plus the Maximum Zone TTL value of any of the data in the
      previous version of the zone.
   after: DNSKEY 10 is removed from the zone.  The key set, now only
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      containing DNSKEY 1 and DNSKEY 11 is resigned with the DNSKEY 1.
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   The above scheme can be simplified by always publishing the "future"
   key immediately after the rollover.  The scheme would look as follows
   (we show two rollovers); the future key is introduced in "after" as
   DNSKEY 12 and again a newer one, numbered 13, in "2nd after":


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       normal              roll                after

       SOA0                SOA2                SOA3
       RRSIG10(SOA0)       RRSIG11(SOA2)       RRSIG11(SOA3)

       DNSKEY1             DNSKEY1             DNSKEY1
       DNSKEY10            DNSKEY10            DNSKEY11
       DNSKEY11            DNSKEY11            DNSKEY12
       RRSIG1(DNSKEY)      RRSIG1 (DNSKEY)     RRSIG1(DNSKEY)
       RRSIG10(DNSKEY)     RRSIG11(DNSKEY)     RRSIG11(DNSKEY)


       2nd roll            2nd after

       SOA4                SOA5
       RRSIG12(SOA4)       RRSIG12(SOA5)

       DNSKEY1             DNSKEY1
       DNSKEY11            DNSKEY12
       DNSKEY12            DNSKEY13
       RRSIG1(DNSKEY)      RRSIG1(DNSKEY)
       RRSIG12(DNSKEY)     RRSIG12(DNSKEY)


   Note that the key introduced after the rollover is not used for
   production yet; the private key can thus be stored in a physically
   secure manner and does not need to be 'fetched' every time a zone
   needs to be signed.

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4.2.1.2  Double Signature Zone-signing Key Rollover
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   This section shows how to perform a ZSK key rollover using the double
   zone data signature scheme, aptly named "double sig rollover".

   During the rollover stage the new version of the zone file will need
   to propagate to all authoritative servers and the data that exists in
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   (distant) caches will need to expire, requiring at least the maximum
   Zone TTL.
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       normal              roll               after

       SOA0                SOA1               SOA2
       RRSIG10(SOA0)       RRSIG10(SOA1)      RRSIG11(SOA2)
                           RRSIG11(SOA1)

       DNSKEY1             DNSKEY1            DNSKEY1
       DNSKEY10            DNSKEY10           DNSKEY11
                           DNSKEY11
       RRSIG1(DNSKEY)      RRSIG1(DNSKEY)     RRSIG1(DNSKEY)
       RRSIG10(DNSKEY)     RRSIG10(DNSKEY)    RRSIG11(DNSKEY)
                           RRSIG11(DNSKEY)

   normal: Version 0 of the zone: DNSKEY 1 is the key-signing key.
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      DNSKEY 10 is used to sign all the data of the zone, the zone-
      signing key.
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   roll: At the rollover stage (SOA serial 1) DNSKEY 11 is introduced
      into the key set and all the data in the zone is signed with
      DNSKEY 10 and DNSKEY 11.  The rollover period will need to exist
      until all data from version 0 of the zone has expired from remote
      caches.  This will take at least the maximum Zone TTL of version 0
      of the zone.
   after: DNSKEY 10 is removed from the zone.  All the signatures from
      DNSKEY 10 are removed from the zone.  The key set, now only
      containing DNSKEY 11, is resigned with DNSKEY 1.

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   At every instance, RRSIGs from the previous version of the zone can
   be verified with the DNSKEY RRset from the current version and the
   other way around.  The data from the current version can be verified
   with the data from the previous version of the zone.  The duration of
   the rollover phase and the period between rollovers should be at
   least the "Maximum Zone TTL".
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   Making sure that the rollover phase lasts until the signature
   expiration time of the data in version 0 of the zone is recommended.
   This way all caches are cleared of the old signatures.  However, this
   date could be considerably longer than the Maximum Zone TTL, making
   the rollover a lengthy procedure.

   Note that in this example we assumed that the zone was not modified
   during the rollover.  New data can be introduced in the zone as long
   as it is signed with both keys.

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4.2.1.3  Pros and Cons of the Schemes
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   Pre-publish-key set rollover: This rollover does not involve signing
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      the zone data twice.  Instead, before the actual rollover, the new
      key is published in the key set and thus available for
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      cryptanalysis attacks.  A small disadvantage is that this process
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      requires four steps.  Also the pre-publish scheme involves more
      parental work when used for KSK rollovers as explained in
      Section 4.2.
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   Double signature rollover: The drawback of this signing scheme is
      that during the rollover the number of signatures in your zone
      doubles, this may be prohibitive if you have very big zones.  An
      advantage is that it only requires three steps.

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4.2.2  Key-signing Key Rollovers
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   For the rollover of a key-signing key the same considerations as for
   the rollover of a zone-signing key apply.  However we can use a
   double signature scheme to guarantee that old data (only the apex key
   set) in caches can be verified with a new key set and vice versa.

   Since only the key set is signed with a KSK, zone size considerations
   do not apply.


       normal          roll            after

       SOA0            SOA1            SOA2
       RRSIG10(SOA0)   RRSIG10(SOA1)   RRSIG10(SOA2)

       DNSKEY1         DNSKEY1         DNSKEY2
                       DNSKEY2
       DNSKEY10        DNSKEY10        DNSKEY10
       RRSIG1 (DNSKEY) RRSIG1 (DNSKEY) RRSIG2(DNSKEY)
                       RRSIG2 (DNSKEY)
       RRSIG10(DNSKEY) RRSIG10(DNSKEY) RRSIG10(DNSKEY)

   normal: Version 0 of the zone.  The parental DS points to DNSKEY1.
      Before the rollover starts the child will have to verify what the
      TTL is of the DS RR that points to DNSKEY1 - it is needed during
      the rollover and we refer to the value as TTL_DS.
   roll: During the rollover phase the zone administrator generates a
      second KSK, DNSKEY2.  The key is provided to the parent and the
      child will have to wait until a new DS RR has been generated that
      points to DNSKEY2.  After that DS RR has been published on all
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      servers authoritative for the parent's zone, the zone
      administrator has to wait at least TTL_DS to make sure that the
      old DS RR has expired from caches.
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   after: DNSKEY1 has been removed.

   The scenario above puts the responsibility for maintaining a valid
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   chain of trust with the child.  It also is based on the premises that
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   the parent only has one DS RR (per algorithm) per zone.  An
   alternative mechanism has been considered.  Using an established
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   trust relation, the interaction can be performed in-band, and the
   removal of the keys by the child can possibly be signaled by the
   parent.  In this mechanism there are periods where there are two DS
   RRs at the parent.  Since at the moment of writing the protocol for
   this interaction has not been developed further discussion is out of
   scope for this document.

4.2.3  Difference Between ZSK and KSK Rollovers

   Note that KSK rollovers and ZSK rollovers are different.  A zone-key
   rollover can be handled in two different ways: pre-publish (Section
   Section 4.2.1.1) and double signature (Section Section 4.2.1.2).

   As the KSK is used to validate the key set and because the KSK is not
   changed during a ZSK rollover, a cache is able to validate the new
   key set of the zone.  The pre-publish method would work for a KSK
   rollover.  The record that are to be pre-published are the parental
   DS RRs.

   The pre-publish method has some drawbacks.  We first describe the
   rollover scheme and then indicate these drawbacks.

     normal          pre-roll         roll            after
   Parent:
     SOA0            SOA1             SOA2            SOA3
     RRSIGpar(SOA0)  RRSIGpar(SOA1)   RRSIGpar(SOA2)  RRSIGpar(SOA3)
     DS1             DS1              DS1             DS2
                     DS2              DS2
     RRSIGpar(DS)    RRSIGpar(DS)     RRSIGpar(DS)    RRSIGpar(DS)



   Child:
     SOA0            SOA0             SOA1            SOA1
     RRSIG10(SOA0)   RRSIG10(SOA0)    RRSIG10(SOA1)   RRSIG10(SOA1)

     DNSKEY1         DNSKEY1          DNSKEY2         DNSKEY2

     DNSKEY10        DNSKEY10         DNSKEY10        DNSKEY10
     RRSIG1 (DNSKEY) RRSIG1 (DNSKEY)  RRSIG2(DNSKEY)  RRSIG2 (DNSKEY)
     RRSIG10(DNSKEY) RRSIG10(DNSKEY)  RRSIG10(DNSKEY) RRSIG10(DNSKEY)




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   When the child zone wants to roll it notifies the parent during the
   pre-roll phase and submits the new key to the parent.  The parent
   publishes DS1 and DS2, pointing to DNSKEY1 and DNSKEY2 respectively.
   During the rollover, which can take place as soon as the new DS set
   propagated through the DNS, the child replaces DNSKEY1 with DNSKEY2.
   Immediately after that it can notify the parent that the old DS
   record can be deleted.

   The drawbacks of these scheme are that during the pre-roll phase the
   parent cannot verify the match between the DS RR and DNSKEY2 using
   the DNS.  Besides, we introduce a "security lame" DS record
   Section 4.4.3.  Finally the child-parent interaction consists of two
   steps.  The "double signature" method only needs one interaction.
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4.2.4  Automated Key Rollovers
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   As keys must be renewed periodically, there is some motivation to
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   automate the rollover process.  Consider that:
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   o  ZSK rollovers are easy to automate as only the local zone is
      involved.
   o  A KSK rollover needs interaction between the parent and child.
      Data exchange is needed to provide the new keys to the parent,
      consequently, this data must be authenticated and integrity must
      be guaranteed in order to avoid attacks on the rollover.
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   o  All time and TTL considerations presented in Section 4.2 apply to
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      an automated rollover.

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4.3  Planning for Emergency Key Rollover
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   This section deals with preparation for a possible key compromise.
   Our advice is to have a documented procedure ready for when a key
   compromise is suspected or confirmed.

   When the private material of one of your keys is compromised it can
   be used for as long as a valid authentication chain exists.  An
   authentication chain remains intact for:
   o  as long as a signature over the compromised key in the
      authentication chain is valid,
   o  as long as a parental DS RR (and signature) points to the
      compromised key,
   o  as long as the key is anchored in a resolver and is used as a
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      starting point for validation.  (This is generally the hardest to
      update.)
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   While an authentication chain to your compromised key exists, your
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   name-space is vulnerable to abuse by anyone who has obtained
   illegitimate possession of the key.Zone operators have to make a
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   trade off if the abuse of the compromised key is worse than having
   data in caches that cannot be validated.  If the zone operator
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   chooses to break the authentication chain to the compromised key,
   data in caches signed with this key cannot be validated.  However, if
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   the zone administrator chooses to take the path of a regular roll-
   over, the malicious key holder can spoof data so that it appears to
   be valid.  Note that this kind of attack is more likely to occur in a
   localized part of the network topology i.e. downstream from where the
   spoof takes place.
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4.3.1  KSK Compromise
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   When the KSK has been compromised the parent must be notified as soon
   as possible using secure means.  The key set of the zone should be
   resigned as soon as possible.  Care must be taken to not break the
   authentication chain.  The local zone can only be resigned with the
   new KSK after the parent's zone has created and reloaded its zone
   with the DS created from the new KSK.  Before this update takes place
   it would be best to drop the security status of a zone all together:
   the parent removes the DS of the child at the next zone update.
   After that the child can be made secure again.

   An additional danger of a key compromise is that the compromised key
   can be used to facilitate a legitimate DNSKEY/DS and/or nameserver
   rollover at the parent.  When that happens the domain can be in
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   dispute.  An authenticated out of band and secure notify mechanism to
   contact a parent is needed in this case.
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4.3.2  ZSK Compromise
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   Primarily because there is no parental interaction required when a
   ZSK is compromised, the situation is less severe than with with a KSK
   compromise.  The zone must still be resigned with a new ZSK as soon
   as possible.  As this is a local operation and requires no
   communication between the parent and child this can be achieved
   fairly quickly.  However, one has to take into account that just as
   with a normal rollover the immediate disappearance from the old
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   compromised key may lead to verification problems.  The pre-
   publication scheme as discussed above minimizes such problems.
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4.3.3  Compromises of Keys Anchored in Resolvers
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   A key can also be pre-configured in resolvers.  For instance, if
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   DNSSEC is successfully deployed the root key may be pre-configured in
   most security aware resolvers.
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   If trust-anchor keys are compromised, the resolvers using these keys



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   should be notified of this fact.  Zone administrators may consider
   setting up a mailing list to communicate the fact that a SEP key is
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   about to be rolled over.  This communication will of course need to
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   be authenticated e.g. by using digital signatures.
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   End-users faced with the task of updating an anchored key should
   always validate the new key.  New keys should be authenticated out of
   the DNS, for example, looking them up on an SSL secured announcement
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   website.

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4.4  Parental Policies
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4.4.1  Initial Key Exchanges and Parental Policies Considerations
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   The initial key exchange is always subject to the policies set by the
   parent (or its registry).  When designing a key exchange policy one
   should take into account that the authentication and authorization
   mechanisms used during a key exchange should be as strong as the
   authentication and authorization mechanisms used for the exchange of
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   delegation information between parent and child.  I.e. there is no
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   implicit need in DNSSEC to make the authentication process stronger
   than it was in DNS.

   Using the DNS itself as the source for the actual DNSKEY material,
   with an off-band check on the validity of the DNSKEY, has the benefit
   that it reduces the chances of user error.  A parental DNSKEY
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   download tool can make use of the SEP bit [1] to select the proper
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   key from a DNSSEC key set; thereby reducing the chance that the wrong
   DNSKEY is sent.  It can validate the self-signature over a key;
   thereby verifying the ownership of the private key material.
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   Fetching the DNSKEY from the DNS ensures that the chain of trust
   remains intact once the parent publishes the DS RR indicating the
   child is secure.
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   Note: the off-band verification is still needed when the key-material
   is fetched via the DNS.  The parent can never be sure whether the
   DNSKEY RRs have been spoofed or not.

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4.4.2  Storing Keys or Hashes?
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   When designing a registry system one should consider which of the
   DNSKEYs and/or the corresponding DSs to store.  Since a child zone
   might wish to have a DS published using a message digest algorithm
   not yet understood by the registry, the registry can't count on being
   able to generate the DS record from a raw DNSKEY.  Thus, we recommend
   that registry system at least support storing DS records.
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   It may also be useful to store DNSKEYs, since having them may help
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   during troubleshooting and, so long as the child's chosen message
   digest is supported, the overhead of generating DS records from them
   is minimal.  Having an out-of-band mechanism, such as a Whois
   database, to find out which keys are used to generate DS Resource
   Records for specific owners and/or zones may also help with
   troubleshooting.
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   The storage considerations also relate the design of the customer
   interface and the method by which data is transfered between
   registrant and registry; Will the child zone owner be able to upload
   DS RRs with unknown hash algorithms or does the interface only allows
   DNSKEYs?  In the registry-registrar model one can use the DNSSEC EPP
   protocol extensions [9] which allows transfer of DS RRs and
   optionally DNSKEY RRs.
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4.4.3  Security Lameness
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   Security Lameness is defined as what happens when a parent has a DS
   RR pointing to a non-existing DNSKEY RR.  During key exchange a
   parent should make sure that the child's key is actually configured
   in the DNS before publishing a DS RR in its zone.  Failure to do so
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   could cause the child's zone being marked as Bogus.
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   Child zones should be very careful removing DNSKEY material,
   specifically SEP keys, for which a DS RR exists.

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   Once a zone is "security lame", a fix (e.g. removing a DS RR) will
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   take time to propagate through the DNS.

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4.4.4  DS Signature Validity Period
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   Since the DS can be replayed as long as it has a valid signature, a
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   short signature validity period over the DS minimizes the time a
   child is vulnerable in the case of a compromise of the child's
   KSK(s).  A signature validity period that is too short introduces the
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   possibility that a zone is marked Bogus in case of a configuration
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   error in the signer.  There may not be enough time to fix the
   problems before signatures expire.  Something as mundane as operator
   unavailability during weekends shows the need for DS signature
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   validity periods longer than 2 days.  We recommend the minimum for a
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   DS signature validity period of a few days.
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   The maximum signature validity period of the DS record depends on how
   long child zones are willing to be vulnerable after a key compromise.
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