DNS Extensions O. Kolkman
Internet-Draft RIPE NCC
Expires: March 2, 2003 J. Schlyter
Carlstedt Research &
Technology
September 2002
KEY RR Key-Signing Key (KSK) Flag
draft-ietf-dnsext-keyrr-key-signing-flag-02
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
With the DS record [1] the concept of key-signing and zone-signing
keys has been introduced. Key-signing keys are the keys that sign
the keyset only. In general, key-signing keys are the keys that are
pointed to by DS records and are the first keys to be used when
following a chain of trust into the zone. The key-signing keys only
sign the KEY RRset at the apex of a zone, zone- signing keys sign all
other data in a zone. We propose a flag to distinguish the key-
signing key from other keys in the KEY RR set during DNSSEC
operations.
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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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The Key-Signing Key (KSK) Flag . . . . . . . . . . . . . . . . 3
3. DNSSEC Protocol Changes . . . . . . . . . . . . . . . . . . . 4
4. Operational Guidelines . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Internationalization Considerations . . . . . . . . . . . . . 5
8. Document Changes . . . . . . . . . . . . . . . . . . . . . . . 5
8.1 draft version 00 -> 01 . . . . . . . . . . . . . . . . . . . . 5
8.2 draft version 01 -> 02 . . . . . . . . . . . . . . . . . . . . 5
8.3 draft version 02 -> 03 . . . . . . . . . . . . . . . . . . . . 5
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
Normative References . . . . . . . . . . . . . . . . . . . . . 6
Informative References . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 6
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
"All keys are equal but some keys are more equal than others" [5]
With the DS record [1] the concept of key-signing and zone-signing
keys has been introduced into DNSSEC[2]. In general these are the
keys that are pointed to by DS records and are the first keys to be
used when following the chain of trust into a zone ( secure entry
points of the zone). These key-signing keys may also be configured
in resolver systems that use zones as a trusted root[3] for a secure
island.
Early deployment tests have shown that during the key-exchange
between the parent and the child it is useful to highlight which keys
are to be used as the secure entry point to a zone. We introduce the
Key-Signing Key flag to indicate this special 'administrative' status
of the key. The availability of the flag allows the key exchange to
be automated where, without the flag, some additional out-of-band
communication is needed.
2. The Key-Signing Key (KSK) 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 |K| protocol | algorithm |
| |S| | |
| |K| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| /
/ public key /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
KEY RR Format
The KSK bit (TBD) in the flags field is assigned to be the key-
signing flag. If set the key is intended to be used as key-signing
key. No special meaning should be assigned to the bit not being set.
The draft proposes using the current 15'th bit [4] as the KSK bit.
This way operators can tell the difference between key and zone-
signing keys from the decimal representation of the flag field; it is
odd or even respectively.
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3. DNSSEC Protocol Changes
The use of the KSK flag does not change the DNS resolution and
resolution protocol. The KSK flag is only used to provide a hint
about the different administrative properties and MUST NOT be used
during the resolving process.
4. Operational Guidelines
By setting the KSK flag on a particular key, zone administrators
indicate that that key SHOULD be used as the secure entry point for
their zone. Therefore zone administrators SHOULD set the bit only
for zone keys that are used to sign the KEY RRset and are intended to
act as the first link in the chain of trust for their zone.
Parent zone administrators and resolver administrators that want to
configure a key-signing key as their 'trusted key' MAY choose to
ignore the flag.
Using the flag a key roll over can be automated. The parent can use
an existing trust relation to verify keysets in which a new key with
the KSK flag appears.
If the bit is modified during the lifetime of the key then this would
have impact on the keytag and on the hash data in the DS RRs
intending to point to this key. The bit SHOULD NOT be modified once
the key has been put into use.
5. Security Considerations
The flag MUST NOT 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 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 keyset, containing two keys with the KSK flag set, is sent to
the parent. The parent verifies the keyset 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 defence. A simple defence can
be based on a registry of keys that have been used to generate DS RRs
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during the most recent roll over.
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 KSK bit; the decimal representation of the
flagfield will then be odd for key-signing keys and even for zone-
signing keys.
7. Internationalization Considerations
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 defences possible.
Spelling and style corrections
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9. Acknowledgments
The ideas documented in this draft are inspired by communications we
had with numerous people and ideas published by other folk, Olafur
Gudmundsson, Daniel Karrenberg, Ed Lewis, Dan Massey and Sam Weiler
have been helping with providing ideas and feedback.
This document saw the light during a workshop on DNSSEC operations
hosted by USC/ISI.
Normative References
[1] Gudmundsson, O., "Delegation Signer Resource Record", draft-
ietf-dnsext-delegation-signer-10 (work in progress), October
2002.
[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.
Informative References
[4] Massey, D. and S. Rose, "Limiting the Scope of the KEY Resource
Record out", draft-ietf-dnsext-restrict-key-for-dnssec-04 (work
in progress), September 2002.
[5] Orwell, "Animal Farm; a Fairy Story"", 1945, <http://
www.ddc.net/ygg/etext/animal.htm#10>.
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/
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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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