DNS Extensions O. Kolkman
Internet-Draft RIPE NCC
Expires: July 2, 2003 J. Schlyter
Carlstedt Research &
Technology
E. Lewis
ARIN
January 2003
KEY RR Secure Entry Point (SEP) Flag
draft-ietf-dnsext-keyrr-key-signing-flag-07
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
With the DS resource record the concept of a key acting as a secure
entry point has been introduced. During key-exchanges with the
parent there is a need to differentiate secure entry point keys from
other keys in the KEY resource record set. A flag bit in the KEY RR
is defined to indicate that KEY is to be used as a secure entry
point.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The Secure Entry Point (SEP) Flag . . . . . . . . . . . . . . 4
3. DNSSEC Protocol Changes . . . . . . . . . . . . . . . . . . . 4
4. Operational Guidelines . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Internationalization Considerations . . . . . . . . . . . . . 6
8. Document Changes . . . . . . . . . . . . . . . . . . . . . . . 6
8.1 draft version 00 -> 01 . . . . . . . . . . . . . . . . . . . . 6
8.2 draft version 01 -> 02 . . . . . . . . . . . . . . . . . . . . 6
8.3 draft version 02 -> 03 . . . . . . . . . . . . . . . . . . . . 6
8.4 draft version 03 -> 04 . . . . . . . . . . . . . . . . . . . . 6
8.5 draft version 04 -> 05 . . . . . . . . . . . . . . . . . . . . 7
8.6 draft version 05 -> 06 . . . . . . . . . . . . . . . . . . . . 7
8.7 draft version 06 -> 07 . . . . . . . . . . . . . . . . . . . . 7
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
Normative References . . . . . . . . . . . . . . . . . . . . . 7
Informative References . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 8
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
"All keys are equal but some keys are more equal than others" [6]
With the definition of the DS Resource Record [5] it has become
important to differentiate between the zone keys that are (to be)
pointed to by parental DS RRs and other keys in the zone. We refer
to these keys as Secure Entry Point (SEP) keys. A SEP key is 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) keys in each
zone has been prevalent. One key is used to sign just the zone's KEY
RR set and is the key referenced by a DS RR at the parent or
configured statically in a resolver. Another key is used to sign the
rest of the zone's data sets. The former key 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, but there
will be multiples of each at times.
It should be noted that division of zone keys 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.
The reason for the term "SEP" is a result of the observation that the
distinction between KSK and ZSK is only significant to the signer
element of the DNS. Servers, resolvers and verifiers do not need to
make the distinction. Further, distinguishing between a KSK and ZSK
requires more than one bit, as a key could be fulfilling both roles.
Hence, there is no definition for a ZSK bit and another for a KSK
bit, just a single bit to assist operational procedures to correctly
generate DS RRs, or to indicate what keys are intended for static
configuration.
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 RFC2119.
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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 /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
KEY RR Format
The SEP bit (TBD) in the flags field is assigned to be the secure
entry point flag. If the 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. The document proposes
using the current 15th bit [4] as the SEP bit. This way 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 and resolution
protocol.
4. Operational Guidelines
The SEP bit is set by the key-generator and MAY be used by the zone
signer to decide whether the key is to be prepared for input to a DS
RR generation function. As the SEP bit is within the data that is
used to compute a KEY RR's footprint, changing the SEP bit will
change the identity of the key within DNS.
When a key pair is created, the operator needs to indicate whether
the SEP bit is to be set in the KEY RR. The SEP bit is recommended
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.
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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 KEY
RR set, such keys might be pending retirement or not yet in use.
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 key to be used as
trusted root, administrators can choose to ignore the fact that a KEY
has its SEP bit set or not when configuring a trusted root for their
resolvers.
Using the flag a key roll over can be automated. The parent can use
an existing trust relation to verify key sets in which a new key with
the SEP flag appears.
5. Security Considerations
As stated in Section 3 the flag is not to 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 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 key exchange
where a key set, containing two keys with the SEP flag set, is sent
to the parent. The parent verifies the key set with the existing
trust relation and creates the new DS RR from the key that the
current DS 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.
6. IANA Considerations
draft-ietf-dnsext-restrict-key-for-dnssec [4] eliminates all flags
field except for the zone key flag in the KEY RR. We propose to use
the 15'th bit as the SEP bit; the decimal representation of the
flagfield will then be odd for key-signing keys.
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7. Internationalization Considerations
Although SEP is a popular acronym in many different languages, there
are no internationalization considerations.
8. Document Changes
8.1 draft version 00 -> 01
Clean up of references and correction of typos;
modified Abstract text a little;
Added explicit warning for replay attacks to the security section;
Removed the text that hinted on a distinction between a key-
signing key configured in resolvers and in parent zones.
8.2 draft version 01 -> 02
Added IANA and Internationalization section.
Split references into informational and normative.
Spelling and style corrections.
8.3 draft version 02 -> 03
Changed the name from KS to KSK, this to prevent confusion with
NS, DS and other acronyms in DNS.
In the security section: Rewrote the section so that it does not
suggest to use a particular type of registry and that it is clear
that a key registry is only one of the defenses possible.
Spelling and style corrections.
8.4 draft version 03 -> 04
Text has been made consistent with the statement: ' No special
meaning should be assigned to the bit not being set.'
Made explicit that the key tag changes in SIG RR.
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8.5 draft version 04 -> 05
One occurrence of must and one occurrence of should uppercased
(RFC2119).
Reordering of sentences in section 3, so that the point of the bit
NOT being used in resolving is made directly.
To make explicit that the KSK is used at key generation and at
signing time I added the first sentence to section 4.
Some minor style and spelling corrections.
8.6 draft version 05 -> 06
References and acronyms where stripped from the Abstract. the
Introduction and the the Operational Guideline section were
rewritten in such a way that the draft does not suggest any use of
the bit in the verification process and that the draft does not
enforce, but suggests, the use of a key- and zone-signing key.
Added 'and verification' in the sentence "MUST NOT be used during
the resolving and verification process" (protocol changes
section).
8.7 draft version 06 -> 07
Based on comments during the last call we changed the name from
KSK-flag to SEP flag. The introduction was rewritten to reflect
the motivations of this name change and to stress that the SEP key
is not relevant to the signer process.
9. 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, Miek Gieben, Olafur Gudmundsson, Daniel
Karrenberg, Dan Massey, 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.
Normative References
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[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] Massey, D. and S. Rose, "Limiting the Scope of the KEY Resource
Record (RR)", RFC 3445, December 2002.
Informative References
[5] Gudmundsson, O., "Delegation Signer Resource Record", draft-
ietf-dnsext-delegation-signer-14 (work in progress), May 2003.
[6] Orwell, G. and R. Steadman (illustrator), "Animal Farm; a Fairy
Story"", ISBN 0151002177 (50th anniversery edition), April 1996.
Authors' Addresses
Olaf M. Kolkman
RIPE NCC
Singel 256
Amsterdam 1016 AB
NL
Phone: +31 20 535 4444
EMail: olaf@ripe.net
URI: http://www.ripe.net/
Jakob Schlyter
Carlstedt Research & Technology
Stora Badhusgatan 18-20
Goteborg SE-411 21
Sweden
EMail: jakob@crt.se
URI: http://www.crt.se/~jakob/
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Edward P. Lewis
ARIN
3635 Concorde Parkway Suite 200
Chantilly, VA 20151
US
Phone: +1 703 227 9854
EMail: edlewis@arin.net
URI: http://www.arin.net/
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