DNS Extensions O. Kolkman
Internet-Draft RIPE NCC
Expires: March 2, 2003 J. Schlyter
Carlstedt Research &
Technology
September 2002
KEY RR Key Signing (KS) Flag
draft-ietf-dnsext-keyrr-key-signing-flag-01
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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
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 Flag . . . . . . . . . . . . . . . . . . . . . 3
3. DNSSEC Protocol Changes . . . . . . . . . . . . . . . . . . . 3
4. Operational Guidelines . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. Document Changes . . . . . . . . . . . . . . . . . . . . . . . 4
6.1 draft version 00 -> 01 . . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5
References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 6
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 7
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1. Introduction
"All keys are equal but some keys are more equal than others" [2]
With the DS record [1] the concept of key signing and zone signing
keys has been introduced. 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[4] for a secure island.
Early deployment tests have shown that during the key-exchange
between the parent and the child it is useful to indicate 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 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| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| /
/ public key /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
KEY RR Format
The bit 15th 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. If the bit is not set, no special meaning should be assigned.
The 15th bit is currently reserved [3].
3. DNSSEC Protocol Changes
The use of the KS flag does not change the DNS resolution and
resolution protocol. The KS flag is only used to provide a hint
about the different administrative properties and MUST NOT be used
during the resolving process.
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4. Operational Guidelines
By setting the KS 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 keysigning key as their 'trusted key' MAY choose to
ignore the flag.
Using the flag a key rollover can be automated. The parent can use
an existing trust relation to verify keysets in which a new key with
the KS 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 MAY be used for automating key exchanges, we think
the following consideration is in place.
Automated mechanisms for rollover of the DS RR may be vulnerable to a
class of replay attacks. This may happen after a key exchange where
a keyset, containing two keys with the KS 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 may be replayed, if the parent
does not maintain state of which DS RRs where used previously so that
the new DS RR is replaced by the old DS RR again. These kinds of
attacks can be prevented by maintaining a registry of keys that have
been used to generate DS RRs from previously.
6. Document Changes
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6.1 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 keysigning
key configured in resolvers and in parent zones.
7. 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, 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.
References
[1] Gudmundsson, "Delegation Signer Resource Record", work in
progress draft-ietf-dnsext-delegation-signer-08.txt, June 2002.
[2] Orwell, "Animal Farm; a Fairy Story"", 1945, <http://
www.ddc.net/ygg/etext/animal.htm#10>.
[3] Massey and Rose, "Limiting the Scope of the KEY Resource
Record", work in progress draft-ietf-dnsext-restrict-key-for-
dnssec-03, June 28 2002.
[4] Lewis, E., "DNS Security Extension Clarification on Zone
Status", RFC 3090, March 2001.
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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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