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Network Working Group O. Kolkman Request for Comments: 3757 RIPE NCC Updates: 3755, 2535 J. Schlyter Category: Standards Track NIC-SE

                                                              E. Lewis
                                                            April 2004
       Domain Name System KEY (DNSKEY) Resource Record (RR)
                   Secure Entry Point (SEP) Flag

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2004).  All Rights Reserved.


 With the Delegation Signer (DS) resource record (RR), the concept of
 a public key acting as a secure entry point (SEP) has been
 introduced.  During exchanges of public keys with the parent there is
 a need to differentiate SEP keys from other public keys in the Domain
 Name System KEY (DNSKEY) resource record set.  A flag bit in the
 DNSKEY RR is defined to indicate that DNSKEY is to be used as a SEP.
 The flag bit is intended to assist in operational procedures to
 correctly generate DS resource records, or to indicate what DNSKEYs
 are intended for static configuration.  The flag bit is not to be
 used in the DNS verification protocol.  This document updates RFC
 2535 and RFC 3755.

Kolkman, et al. Standard Track [Page 1] RFC 3757 DNSKEY RR SEP Flag April 2004

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
 2.  The Secure Entry Point (SEP) Flag. . . . . . . . . . . . . . .  4
 3.  DNSSEC Protocol Changes. . . . . . . . . . . . . . . . . . . .  4
 4.  Operational Guidelines . . . . . . . . . . . . . . . . . . . .  4
 5.  Security Considerations. . . . . . . . . . . . . . . . . . . .  5
 6.  IANA Considerations. . . . . . . . . . . . . . . . . . . . . .  6
 7.  Internationalization Considerations. . . . . . . . . . . . . .  6
 8.  Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . .  6
 9.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     9.1.  Normative References . . . . . . . . . . . . . . . . . .  6
     9.2.  Informative References . . . . . . . . . . . . . . . . .  6
 10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .  7
 11. Full Copyright Statement . . . . . . . . . . . . . . . . . . .  8

1. Introduction

 "All keys are equal but some keys are more equal than others" [6].
 With the definition of the Delegation Signer Resource Record (DS RR)
 [5], it has become important to differentiate between the keys in the
 DNSKEY RR set that are (to be) pointed to by parental DS RRs and the
 other keys in the DNSKEY RR set.  We refer to these public keys as
 Secure Entry Point (SEP) keys.  A SEP key either used to generate a
 DS RR or is distributed to resolvers that use the key as the root of
 a trusted subtree [3].
 In early deployment tests, the use of two (kinds of) key pairs for
 each zone has been prevalent.  For one kind of key pair the private
 key is used to sign just the zone's DNSKEY resource record (RR) set.
 Its public key is intended to be referenced by a DS RR at the parent
 or configured statically in a resolver.  The private key of the other
 kind of key pair is used to sign the rest of the zone's data sets.
 The former key pair is called a key-signing key (KSK) and the latter
 is called a zone-signing key (ZSK).  In practice there have been
 usually one of each kind of key pair, but there will be multiples of
 each at times.
 It should be noted that division of keys pairs into KSK's and ZSK's
 is not mandatory in any definition of DNSSEC, not even with the
 introduction of the DS RR.  But, in testing, this distinction has
 been helpful when designing key roll over (key super-cession)
 schemes.  Given that the distinction has proven helpful, the labels
 KSK and ZSK have begun to stick.

Kolkman, et al. Standard Track [Page 2] RFC 3757 DNSKEY RR SEP Flag April 2004

 There is a need to differentiate the public keys for the key pairs
 that are used for key signing from keys that are not used key signing
 (KSKs vs ZSKs).  This need is driven by knowing which DNSKEYs are to
 be sent for generating DS RRs, which DNSKEYs are to be distributed to
 resolvers, and which keys are fed to the signer application at the
 appropriate time.
 In other words, the SEP bit provides an in-band method to communicate
 a DNSKEY RR's intended use to third parties.  As an example we
 present 3 use cases in which the bit is useful:
    The parent is a registry, the parent and the child use secured DNS
    queries and responses, with a preexisting trust-relation, or plain
    DNS over a secured channel to exchange the child's DNSKEY RR sets.
    Since a DNSKEY RR set will contain a complete DNSKEY RRset the SEP
    bit can be used to isolate the DNSKEYs for which a DS RR needs to
    be created.
    An administrator has configured a DNSKEY as root for a trusted
    subtree into security aware resolver.  Using a special purpose
    tool that queries for the KEY RRs from that domain's apex, the
    administrator will be able to notice the roll over of the trusted
    anchor by a change of the subset of KEY RRs with the DS flag set.
    A signer might use the SEP bit on the public key to determine
    which private key to use to exclusively sign the DNSKEY RRset and
    which private key to use to sign the other RRsets in the zone.
 As demonstrated in the above examples it is important to be able to
 differentiate the SEP keys from the other keys in a DNSKEY RR set in
 the flow between signer and (parental) key-collector and in the flow
 between the signer and the resolver configuration.  The SEP flag is
 to be of no interest to the flow between the verifier and the
 authoritative data store.
 The reason for the term "SEP" is a result of the observation that the
 distinction between KSK and ZSK key pairs is made by the signer, a
 key pair could be used as both a KSK and a ZSK at the same time.  To
 be clear, the term SEP was coined to lessen the confusion caused by
 the overlap.  (Once this label was applied, it had the side effect of
 removing the temptation to have both a KSK flag bit and a ZSK flag
 The key words "MAY","MAY NOT", "MUST", "MUST NOT", "REQUIRED",
 "RECOMMENDED", "SHOULD", and "SHOULD NOT" in this document are to be
 interpreted as described in BCP 14, RFC 2119 [1].

Kolkman, et al. Standard Track [Page 3] RFC 3757 DNSKEY RR SEP Flag April 2004

2. The Secure Entry Point (SEP) Flag

                      1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 |              flags          |S|   protocol    |   algorithm   |
 |                             |E|               |               |
 |                             |P|               |               |
 |                                                               /
 /                        public key                             /
 /                                                               /
                        DNSKEY RR Format
 This document assigns the 15th bit in the flags field as the secure
 entry point (SEP) bit.  If the bit is set to 1 the key is intended to
 be used as secure entry point key.  One SHOULD NOT assign special
 meaning to the key if the bit is set to 0.  Operators can recognize
 the secure entry point key by the even or odd-ness of the decimal
 representation of the flag field.

3. DNSSEC Protocol Changes

 The bit MUST NOT be used during the resolving and verification
 process.  The SEP flag is only used to provide a hint about the
 different administrative properties of the key and therefore the use
 of the SEP flag does not change the DNS resolution protocol or the
 resolution process.

4. Operational Guidelines

 The SEP bit is set by the key-pair-generator and MAY be used by the
 zone signer to decide whether the public part of the key pair is to
 be prepared for input to a DS RR generation function.  The SEP bit is
 recommended to be set (to 1) whenever the public key of the key pair
 will be distributed to the parent zone to build the authentication
 chain or if the public key is to be distributed for static
 configuration in verifiers.
 When a key pair is created, the operator needs to indicate whether
 the SEP bit is to be set in the DNSKEY RR.  As the SEP bit is within
 the data that is used to compute the 'key tag field' in the SIG RR,
 changing the SEP bit will change the identity of the key within DNS.
 In other words, once a key is used to generate signatures, the
 setting of the SEP bit is to remain constant.  If not, a verifier
 will not be able to find the relevant KEY RR.

Kolkman, et al. Standard Track [Page 4] RFC 3757 DNSKEY RR SEP Flag April 2004

 When signing a zone, it is intended that the key(s) with the SEP bit
 set (if such keys exist) are used to sign the KEY RR set of the zone.
 The same key can be used to sign the rest of the zone data too.  It
 is conceivable that not all keys with a SEP bit set will sign the
 DNSKEY RR set, such keys might be pending retirement or not yet in
 When verifying a RR set, the SEP bit is not intended to play a role.
 How the key is used by the verifier is not intended to be a
 consideration at key creation time.
 Although the SEP flag provides a hint on which public key is to be
 used as trusted root, administrators can choose to ignore the fact
 that a DNSKEY has its SEP bit set or not when configuring a trusted
 root for their resolvers.
 Using the SEP flag a key roll over can be automated.  The parent can
 use an existing trust relation to verify DNSKEY RR sets in which a
 new DNSKEY RR with the SEP flag appears.

5. Security Considerations

 As stated in Section 3 the flag is not to be used in the resolution
 protocol or to determine the security status of a key.  The flag is
 to be used for administrative purposes only.
 No trust in a key should be inferred from this flag - trust MUST be
 inferred from an existing chain of trust or an out-of-band exchange.
 Since this flag might be used for automating public key exchanges, we
 think the following consideration is in place.
 Automated mechanisms for roll over of the DS RR might be vulnerable
 to a class of replay attacks.  This might happen after a public key
 exchange where a DNSKEY RR set, containing two DNSKEY RRs with the
 SEP flag set, is sent to the parent.  The parent verifies the DNSKEY
 RR set with the existing trust relation and creates the new DS RR
 from the DNSKEY RR that the current DS RR is not pointing to.  This
 key exchange might be replayed.  Parents are encouraged to implement
 a replay defense.  A simple defense can be based on a registry of
 keys that have been used to generate DS RRs during the most recent
 roll over.  These same considerations apply to entities that
 configure keys in resolvers.

Kolkman, et al. Standard Track [Page 5] RFC 3757 DNSKEY RR SEP Flag April 2004

6. IANA Considerations

 IANA has assigned the 15th bit in the DNSKEY Flags Registry (see
 Section 4.3 of [4]) as the Secure Entry Point (SEP) bit.

7. Internationalization Considerations

 Although SEP is a popular acronym in many different languages, there
 are no internationalization considerations.

8. Acknowledgments

 The ideas documented in this document are inspired by communications
 we had with numerous people and ideas published by other folk.  Among
 others Mark Andrews, Rob Austein, Miek Gieben, Olafur Gudmundsson,
 Daniel Karrenberg, Dan Massey, Scott Rose, Marcos Sanz and Sam Weiler
 have contributed ideas and provided feedback.
 This document saw the light during a workshop on DNSSEC operations
 hosted by USC/ISI in August 2002.

9. References

9.1. Normative References

 [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [2]  Eastlake, D., "Domain Name System Security Extensions", RFC
      2535, March 1999.
 [3]  Lewis, E., "DNS Security Extension Clarification on Zone
      Status", RFC 3090, March 2001.
 [4]  Weiler, S., "Legacy Resolver Compatibility for Delegation Signer
      (DS)", RFC 3755, April 2004.

9.2. Informative References

 [5]  Gudmundsson, O., "Delegation Signer (DS) Resource Record (RR)",
      RFC 3658, December 2003.
 [6]  Orwell, G. and R. Steadman (illustrator), "Animal Farm; a Fairy
      Story", ISBN 0151002177 (50th anniversary edition), April 1996.

Kolkman, et al. Standard Track [Page 6] RFC 3757 DNSKEY RR SEP Flag April 2004

10. Authors' Addresses

 Olaf M. Kolkman
 Singel 256
 Amsterdam  1016 AB
 Phone: +31 20 535 4444
 Jakob Schlyter
 Box 5774
 SE-114 87 Stockholm
 Edward P. Lewis
 3635 Concorde Parkway Suite 200
 Chantilly, VA  20151
 Phone: +1 703 227 9854

Kolkman, et al. Standard Track [Page 7] RFC 3757 DNSKEY RR SEP Flag April 2004

11. Full Copyright Statement

 Copyright (C) The Internet Society (2004).  This document is subject
 to the rights, licenses and restrictions contained in BCP 78 and
 except as set forth therein, the authors retain all their rights.
 This document and the information contained herein are provided on an

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Kolkman, et al. Standard Track [Page 8]

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