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This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.
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This Internet-Draft will expire on July 4, 2003.
Copyright (C) The Internet Society (2003). All Rights Reserved.
With the DS resource record the concept of key-signing and zone-signing keys has been introduced. During key-exchanges with the parent there is a need to differentiate between these zone- and key-signing keys. We propose a flag to indicate which key is used as key-signing key.
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"All keys are equal but some keys are more equal than others" [6]
With the definition of the DS Resource Record[5] the concept of a key being either a key-signing key (KSK) or zone-signing key(ZSK) has been introduced into DNSSEC[3]. A KSK is one that signs the zone's KEY RR set, and is a key that is either used to generate a DS RR or is distributed to resolvers that use the key as the root of a trusted subtree[4].
In early deployment tests, the use of two keys has been prevalent, one key for exchange with delegating zone and the other key to sign the zone. These dual roles were defined to allow a zone to more rapidly change the ZSK without a high volume of traffic needed to make new DS RRs. Because of this, participants have had to manage two keys at all times, one acting as a KSK and the other ZSK (per cryptographic algorithm). In practice, participants used a longer key for the KSK or resorted to writing the footprints on paper.
There is a need to differentiate between a KSK and a ZSK by the zone administrator. This need is driven by knowing which keys are to be sent for DS RRs, which keys are to be distributed to resolvers, and which keys are fed to the signer application at the appropriate time.
While addressing this need it is important that the distinction is made in a way compatible with single key zone, those whose KSK and ZSK is one in the same. The best way to address this is to define a bit setting in the KEY RR flags field that is ignored in the resolver. This allows for both dual key and single key management to be workable.
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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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 key flag. If the the bit is set to 1 the key is intended to be used as key-signing key. No special meaning should be assigned to the bit is set to 0. The draft proposes using the current 15th bit [1] as the KSK bit. This way operators can recognize the key-signing by the even or odd-ness of the decimal representation of the flag field.
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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 and verification process.
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The KSK bit is used to indicate that the key represented in the KEY RR is intended to sign the KEY RR set of the zone. As the KSK bit is within the data that is used to compute a KEY RR's footprint, changing the KSK bit will change the identity of the key within DNS.
When a key pair is created, the operator needs to indicate whether the KSK bit is to be set in the KEY RR. The KSK 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.
When signing a zone, it is intended that a key with the KSK bit set be 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 KSK bit set will sign the KEY RR set, such keys might be pending retirement or not yet in use.
When verifying an RR set, the KSK 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 KSK flag provides a hint on which key to be used as trusted root, administrators can choose to ignore the flag 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 KSK flag appears.
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As stated in DNSSEC Protocol Changes 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 KSK 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 defence. A simple defence can be based on a registry of keys that have been used to generate DS RRs during the most recent roll over.
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draft-ietf-dnsext-restrict-key-for-dnssec[1] 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.
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There are no internationalization considerations
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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.
Added IANA and Internationalization section.
Split references into informational and normative.
Spelling and style corrections.
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
Text has been made consistent with the statement: ' No special meaning should be assigned to the bit not being set.'
Made explicit that the keytag changes in SIG RR.
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).
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The ideas documented in this draft are inspired by communications we had with numerous people and ideas published by other folk. Among others Mark Andrews, 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.
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| [1] | 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. |
| [2] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997 (TXT, HTML, XML). |
| [3] | Eastlake, D., "Domain Name System Security Extensions", RFC 2535, March 1999. |
| [4] | Lewis, E., "DNS Security Extension Clarification on Zone Status", RFC 3090, March 2001. |
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| [5] | Gudmundsson, O., "Delegation Signer Resource Record", draft-ietf-dnsext-delegation-signer-12 (work in progress), December 2002. |
| [6] | Orwell, G. and R. Steadman (illustrator), "Animal Farm; a Fairy Story"", ISBN 0151002177 (50th anniversery edition), April 1996. |
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| Olaf M. Kolkman | |
| RIPE NCC | |
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| Jakob Schlyter | |
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| Edward P. Lewis | |
| ARIN | |
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| EMail: | edlewis@arin.net |
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