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diff --git a/doc/draft/draft-ihren-dnsext-threshold-validation-00.txt b/doc/draft/draft-ihren-dnsext-threshold-validation-00.txt new file mode 100644 index 0000000..3578d2a --- /dev/null +++ b/doc/draft/draft-ihren-dnsext-threshold-validation-00.txt @@ -0,0 +1,519 @@ + +Internet Draft Johan Ihren +draft-ihren-dnsext-threshold-validation-00.txt Autonomica +February 2003 +Expires in six months + + + Threshold Validation: + + A Mechanism for Improved Trust and Redundancy for DNSSEC Keys + + +Status of this Memo + + 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. + + Internet-Drafts are draft documents valid for a maximum of six + months and may be updated, replaced, or obsoleted by other + documents at any time. It is inappropriate to use Internet-Drafts + as reference material or to cite them other than as "work in + progress." + + The list of current Internet-Drafts can be accessed at + http://www.ietf.org/ietf/1id-abstracts.txt + + The list of Internet-Draft Shadow Directories can be accessed at + http://www.ietf.org/shadow.html. + + +Abstract + + This memo documents a proposal for a different method of validation + for DNSSEC aware resolvers. The key change is that by changing from + a model of one Key Signing Key, KSK, at a time to multiple KSKs it + will be possible to increase the aggregated trust in the signed + keys by leveraging from the trust associated with the different + signees. + + By having multiple keys to chose from validating resolvers get the + opportunity to use local policy to reflect actual trust in + different keys. For instance, it is possible to trust a single, + particular key ultimately, while requiring multiple valid + signatures by less trusted keys for validation to succeed. + Furthermore, with multiple KSKs there are additional redundancy + benefits available since it is possible to roll over different KSKs + at different times which may make rollover scenarios easier to + manage. + +Contents + + 1. Terminology + 2. Introduction and Background + + 3. Trust in DNSSEC Keys + 3.1. Key Management, Split Keys and Trust Models + 3.2. Trust Expansion: Authentication versus Authorization + + 4. Proposed Semantics for Signing the KEY Resource Record + Set + 4.1. Packet Size Considerations + + 5. Proposed Use of Multiple "Trusted Keys" in a Validating + Resolver + 5.1. Not All Possible KSKs Need to Be Trusted + 5.2. Possible to do Threshold Validation + 5.3. Not All Trusted Keys Will Be Available + + 6. Additional Benefits from Having Multiple KSKs + 6.1. More Robust Key Rollovers + 6.2. Evaluation of Multiple Key Distribution Mechanisms + + 7. Security Considerations + 8. IANA Considerations. + 9. References + 9.1. Normative. + 9.2. Informative. + 10. Acknowledgments. + 11. Authors' Address + + +1. Terminology + + The key words "MUST", "SHALL", "REQUIRED", "SHOULD", "RECOMMENDED", + and "MAY" in this document are to be interpreted as described in + RFC 2119. + + The term "zone" refers to the unit of administrative control in the + Domain Name System. "Name server" denotes a DNS name server that is + authoritative (i.e. knows all there is to know) for a DNS zone, + typically the root zone. A "resolver", is a DNS "client", i.e. an + entity that sends DNS queries to authoritative nameservers and + interpret the results. A "validating resolver" is a resolver that + attempts to perform DNSSEC validation on data it retrieves by doing + DNS lookups. + + +2. Introduction and Background + + From a protocol perspective there is no real difference between + different keys in DNSSEC. They are all just keys. However, in + actual use there is lots of difference. First and foremost, most + DNSSEC keys have in-band verification. I.e. the keys are signed by + some other key, and this other key is in its turn also signed by + yet another key. This way a "chain of trust" is created. Such + chains have to end in what is referred to as a "trusted key" for + validation of DNS lookups to be possible. + + A "trusted key" is a the public part of a key that the resolver + acquired by some other means than by looking it up in DNS. The + trusted key has to be explicitly configured. + + A node in the DNS hierarchy that issues such out-of-band "trusted + keys" is called a "security apex" and the trusted key for that apex + is the ultimate source of trust for all DNS lookups within that + entire subtree. + + DNSSEC is designed to be able to work with more than on security + apex. These apexes will all share the problem of how to distribute + their "trusted keys" in a way that provides validating resolvers + confidence in the distributed keys. + + Maximizing that confidence is crucial to the usefulness of DNSSEC + and this document tries to address this issue. + + +3. Trust in DNSSEC Keys + + In the end the trust that a validating resolver will be able to put + in a key that it cannot validate within DNSSEC will have to be a + function of + + * trust in the key issuer, aka the KSK holder + + * trust in the distribution method + + * trust in extra, out-of-band verification + + The KSK holder needs to be trusted not to accidentally lose private + keys in public places. Furthermore it needs to be trusted to + perform correct identification of the ZSK holders in case they are + separate from the KSK holder itself. + + The distribution mechanism can be more or less tamper-proof. If the + key holder publishes the public key, or perhaps just a secure + fingerprint of the key in a major newspaper it may be rather + difficult to tamper with. A key acquired that way may be easier to + trust than if it had just been downloaded from a web page. + + Out-of-band verification can for instance be the key being signed + by a certificate issued by a known Certificate Authority that the + resolver has reason to trust. + +3.1. Simplicity vs Trust + + The fewer keys that are in use the simpler the key management + becomes. Therefore increasing the number of keys should only be + considered when the complexity is not the major concern. A perfect + example of this is the distinction between so called Key Signing + Keys, KSK, and Zone Signing Keys, ZSK. This distinction adds + overall complexity but simplifies real life operations and was an + overall gain since operational simplification was considered to be + a more crucial issue than the added complexity. + + In the case of a security apex there are additional issues to + consider, among them + + * maximizing trust in the KSK received out-of-band + + * authenticating the legitimacy of the ZSKs used + + In some cases this will be easy, since the same entity will manage + both ZSKs and KSKs (i.e. it will authenticate itself, somewhat + similar to a self-signed certificate). In some environments it will + be possible to get the trusted key installed in the resolver end by + decree (this would seem to be a likely method within corporate and + government environments). + + In other cases, however, this will possibly not be sufficient. In + the case of the root zone this is obvious, but there may well be + other cases. + +3.2. Expanding the "Trust Base" + + For a security apex where the ZSKs and KSK are not held by the same + entity the KSK will effectively authenticate the identity of + whoever does real operational zone signing. The amount of trust + that the data signed by a ZSK will get is directly dependent on + whether the end resolver trusts the KSK or not, since the resolver + has no OOB access to the public part of the ZSKs (for practical + reasons). + + Since the KSK holder is distinct from the ZSK holder the obvious + question is whether it would then be possible to further improve + the situation by using multiple KSK holders and thereby expanding + the trust base to the union of that available to each individual + KSK holder. "Trust base" is an invented term intended to signify + the aggregate of Internet resolvers that will eventually choose to + trust a key issued by a particular KSK holder. + + A crucial issue when considering trust expansion through addition + of multiple KSK holders is that the KSK holders are only used to + authenticate the ZSKs used for signing the zone. I.e. the function + performed by the KSK is basically: + + "This is indeed the official ZSK holder for this zone, + I've verified this fact to the best of my abilitites." + + Which can be thought of as similar to the service of a public + notary. I.e. the point with adding more KSK holders is to improve + the public trust in data signed by the ZSK holders by improving the + strength of available authentication. + + Therefore adding more KSK holders, each with their own trust base, + is by definition a good thing. More authentication is not + controversial. On the contrary, when it comes to authentication, + the more the merrier. + + +4. Proposed Semantics for Signing the KEY Resource Record Set + + In DNSSEC according to RFC2535 all KEY Resource Records are used to + sign all authoritative data in the zone, including the KEY RRset + itself, since RFC2535 makes no distinction between Key Signing + Keys, KSK, and Zone Signing Keys, ZSK. With Delegation Signer [DS] + it is possible to change this to the KEY RRset being signed with + all KSKs and ZSKs but the rest of the zone only being signed by the + ZSKs. + + This proposal changes this one step further, by recommending that + the KEY RRset is only signed by the Key Signing Keys, KSK, and + explicitly not by the Zone Signing Keys, ZSK. The reason for this + is to maximize the amount of space in the DNS response packet that + is available for additional KSKs and signatures thereof. The rest + of the authoritative zone contents are as previously signed by only + the ZSKs. + +4.1. Packet Size Considerations + + The reason for the change is to keep down the size of the aggregate + of KEY RRset plus SIG(KEY) that resolvers will need to acquire to + perform validation of data below a security apex. For DNSSEC data + to be returned the DNSSEC OK bit in the EDNS0 OPT Record has to be + set, and therefore the allowed packet size can be assumed to be at + least the EDNS0 minimum of 4000 bytes. + + When querying for KEY + SIG(KEY) for "." (the case that is assumed + to be most crucial) the size of the response packet after the + change to only sign the KEY RR with the KSKs break down into a + rather large space of possibilities. Here are a few examples for + the possible alternatives for different numbers of KSKs and ZSKs + for some different key lengths (all RSA keys, with a public + exponent that is < 254). This is all based upon the size of the + response for the particular example of querying for + + ". KEY IN" + + with a response of entire KEY + SIG(KEY) with the authority and + additional sections empty: + + ZSK/768 and KSK/1024 (real small) + Max 12 KSK + 3 ZSK at 3975 + 10 KSK + 8 ZSK at 3934 + 8 KSK + 13 ZSK at 3893 + + ZSK/768 + KSK/1280 + MAX 10 KSK + 2 ZSK at 3913 + 8 KSK + 9 ZSK at 3970 + 6 KSK + 15 ZSK at 3914 + + ZSK/768 + KSK/1536 + MAX 8 KSK + 4 ZSK at 3917 + 7 KSK + 8 ZSK at 3938 + 6 KSK + 12 ZSK at 3959 + + ZSK/768 + KSK/2048 + MAX 6 KSK + 5 ZSK at 3936 + 5 KSK + 10 ZSK at 3942 + + ZSK/1024 + KSK/1024 + MAX 12 KSK + 2 ZSK at 3943 + 11 KSK + 4 ZSK at 3930 + 10 KSK + 6 ZSK at 3917 + 8 KSK + 10 ZSK at 3891 + + ZSK/1024 + KSK/1536 + MAX 8 KSK + 3 ZSK at 3900 + 7 KSK + 6 ZSK at 3904 + 6 KSK + 9 ZSK at 3908 + + ZSK/1024 + KSK/2048 + MAX 6 KSK + 4 ZSK at 3951 + 5 KSK + 8 ZSK at 3972 + 4 KSK + 12 ZSK at 3993 + + Note that these are just examples and this document is not making + any recommendations on suitable choices of either key lengths nor + number of different keys employed at a security apex. + + This document does however, based upon the above figures, make the + recommendation that at a security apex that expects to distribute + "trusted keys" the KEY RRset should only be signed with the KSKs + and not with the ZSKs to keep the size of the response packets + down. + + +5. Proposed Use of Multiple "Trusted Keys" in a Validating Resolver + + In DNSSEC according to RFC2535[RFC2535] validation is the process + of tracing a chain of signatures (and keys) upwards through the DNS + hierarchy until a "trusted key" is reached. If there is a known + trusted key present at a security apex above the starting point + validation becomes an exercise with a binary outcome: either the + validation succeeds or it fails. No intermediate states are + possible. + + With multiple "trusted keys" (i.e. the KEY RRset for the security + apex signed by multiple KSKs) this changes into a more complicated + space of alternatives. From the perspective of complexity that may + be regarded as a change for the worse. However, from a perspective + of maximizing available trust the multiple KSKs add value to the + system. + +5.1. Possible to do Threshold Validation + + With multiple KSKs a new option that opens for the security + concious resolver is to not trust a key individually. Instead the + resolver may decide to require the validated signatures to exceed a + threshold. For instance, given M trusted keys it is possible for + the resolver to require N-of-M signatures to treat the data as + validated. + + I.e. with the following pseudo-configuration in a validating + resolver + + security-apex "." IN { + keys { ksk-1 .... ; + ksk-2 .... ; + ksk-3 .... ; + ksk-4 .... ; + ksk-5 .... ; + }; + validation { + # Note that ksk-4 is not present below + keys { ksk-1; ksk-2; ksk-3; ksk-5; }; + # 3 signatures needed with 4 possible keys, aka 75% + needed-signatures 3; + }; + }; + + we configure five trusted keys for the root zone, but require two + valid signatures for the top-most KEY for validation to + succeed. I.e. threshold validation does not force multiple + signatures on the entire signature chain, only on the top-most + signature, closest to the security apex for which the resolver has + trusted keys. + +5.2. Not All Trusted Keys Will Be Available + + With multiple KSKs held and managed by separate entities the end + resolvers will not always manage to get access to all possible + trusted keys. In the case of just a single KSK this would be fatal + to validation and necessary to avoid at whatever cost. But with + several fully trusted keys available the resolver can decide to + trust several of them individually. An example based upon more + pseudo-configuration: + + security-apex "." IN { + keys { ksk-1 .... ; + ksk-2 .... ; + ksk-3 .... ; + ksk-4 .... ; + ksk-5 .... ; + }; + validation { + # Only these two keys are trusted independently + keys { ksk-1; ksk-4; }; + # With these keys a single signature is sufficient + needed-signatures 1; + }; + }; + + Here we have the same five keys and instruct the validating + resolver to fully trust data that ends up with just one signature + from by a fully trusted key. + + The typical case where this will be useful is for the case where + there is a risk of the resolver not catching a rollover event by + one of the KSKs. By doing rollovers of different KSKs with + different schedules it is possible for a resolver to "survive" + missing a rollover without validation breaking. This improves + overall robustness from a management point of view. + +5.3. Not All Possible KSKs Need to Be Trusted + + With just one key available it simply has to be trusted, since that + is the only option available. With multiple KSKs the validating + resolver immediately get the option of implementing a local policy + of only trusting some of the possible keys. + + This local policy can be implemented either by simply not + configuring keys that are not trusted or, possibly, configure them + but specify to the resolver that certain keys are not to be + ultimately trusted alone. + + +6. Additional Benefits from Having Multiple KSKs + +6.1. More Robust Key Rollovers + + With only one KSK the rollover operation will be a delicate + operation since the new trusted key needs to reach every validating + resolver before the old key is retired. For this reason it is + expected that long periods of overlap will be needed. + + With multiple KSKs this changes into a system where different + "series" of KSKs can have different rollover schedules, thereby + changing from one "big" rollover to several "smaller" rollovers. + + If the resolver trusts several of the available keys individually + then even a failure to track a certain rollover operation within + the overlap period will not be fatal to validation since the other + available trusted keys will be sufficient. + +6.2. Evaluation of Multiple Key Distribution Mechanisms + + Distribution of the trusted keys for the DNS root zone is + recognized to be a difficult problem that ... + + With only one trusted key, from one single "source" to distribute + it will be difficult to evaluate what distribution mechanism works + best. With multiple KSKs, held by separate entitites it will be + possible to measure how large fraction of the resolver population + that is trusting what subsets of KSKs. + + +7. Security Considerations + + From a systems perspective the simplest design is arguably the + best, i.e. one single holder of both KSK and ZSKs. However, if that + is not possible in all cases a more complex scheme is needed where + additional trust is injected by using multiple KSK holders, each + contributing trust, then there are only two alternatives + available. The first is so called "split keys", where a single key + is split up among KSK holders, each contributing trust. The second + is the multiple KSK design outlined in this proposal. + + Both these alternatives provide for threshold mechanisms. However + split keys makes the threshold integral to the key generating + mechanism (i.e. it will be a property of the keys how many + signatures are needed). In the case of multiple KSKs the threshold + validation is not a property of the keys but rather local policy in + the validating resolver. A benefit from this is that it is possible + for different resolvers to use different trust policies. Some may + configure threshold validation requiring multiple signatures and + specific keys (optimizing for security) while others may choose to + accept a single signature from a larger set of keys (optimizing for + redundancy). Since the security requirements are different it would + seem to be a good idea to make this choice local policy rather than + global policy. + + Furthermore, a clear issue for validating resolvers will be how to + ensure that they track all rollover events for keys they + trust. Even with operlap during the rollover (which is clearly + needed) there is still a need to be exceedingly careful not to miss + any rollovers (or fail to acquire a new key) since without this + single key validation will fail. With multiple KSKs this operation + becomes more robust, since different KSKs may roll at different + times according to different rollover schedules and losing one key, + for whatever reason, will not be crucial unless the resolver + intentionally chooses to be completely dependent on that exact key. + +8. IANA Considerations. + + NONE. + + +9. References + +9.1. Normative. + + [RFC2535] Domain Name System Security Extensions. D. Eastlake. + March 1999. + + [RFC3090] DNS Security Extension Clarification on Zone Status. + E. Lewis. March 2001. + + +9.2. Informative. + + [RFC3110] RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System + (DNS). D. Eastlake 3rd. May 2001. + + [RFC3225] Indicating Resolver Support of DNSSEC. D. Conrad. + December 2001. + + [DS] Delegation Signer Resource Record. + O. Gudmundsson. October 2002. Work In Progress. + +10. Acknowledgments. + + Bill Manning came up with the original idea of moving complexity + from the signing side down to the resolver in the form of threshold + validation. I've also had much appreciated help from (in no + particular order) Jakob Schlyter, Paul Vixie, Olafur Gudmundson and + Olaf Kolkman. + + +11. Authors' Address +Johan Ihren +Autonomica AB +Bellmansgatan 30 +SE-118 47 Stockholm, Sweden +johani@autonomica.se |