NSEC5, DNSSEC Authenticated Denial of Existence
draft-vcelak-nsec5-00
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| Last updated | 2015-03-23 | ||
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draft-vcelak-nsec5-00
Network Working Group J. Vcelak
Internet-Draft CZ.NIC
Intended status: Standards Track S. Goldberg
Expires: September 24, 2015 Boston University
March 23, 2015
NSEC5, DNSSEC Authenticated Denial of Existence
draft-vcelak-nsec5-00
Abstract
The Domain Name System Security (DNSSEC) Extensions introduced the
NSEC resource record (RR) for authenticated denial of existence and
the NSEC3 for hashed authenticated denial of existence. The NSEC RR
allows for the entire zone contents to be enumerated if a server is
queried for carefully chosen domain names; N queries suffice to
enumerate a zone containing N names. The NSEC3 RR adds domain-name
hashing, which makes the zone enumeration harder, but not impossible.
This document introduces NSEC5, which provides an cryptographically-
proven mechanism that prevents zone enumeration. NSEC5 has the
additional advantage of not requiring private zone-signing keys to be
present on all authoritative servers for the zone.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on September 24, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Rationale . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Backward Compatibility . . . . . . . . . . . . . . . . . . . 5
3. How NSEC5 Works . . . . . . . . . . . . . . . . . . . . . . . 5
4. NSEC5 Algorithms . . . . . . . . . . . . . . . . . . . . . . 6
5. The NSEC5KEY Resource Record . . . . . . . . . . . . . . . . 7
5.1. NSEC5KEY RDATA Wire Format . . . . . . . . . . . . . . . 7
5.2. NSEC5KEY RDATA Presentation Format . . . . . . . . . . . 7
6. The NSEC5 Resource Record . . . . . . . . . . . . . . . . . . 7
6.1. NSEC5 RDATA Wire Format . . . . . . . . . . . . . . . . . 7
6.2. NSEC5 Flags Field . . . . . . . . . . . . . . . . . . . . 8
6.3. NSEC5 RDATA Presentation Format . . . . . . . . . . . . . 9
7. The NSEC5PROOF Resource Record . . . . . . . . . . . . . . . 9
7.1. NSEC5PROOF RDATA Wire Format . . . . . . . . . . . . . . 9
7.2. NSEC5PROOF RDATA Presentation Format . . . . . . . . . . 9
8. NSEC5 Proofs . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Name Error Responses . . . . . . . . . . . . . . . . . . 10
8.2. No Data Responses . . . . . . . . . . . . . . . . . . . . 11
8.2.1. No Data Response, Opt-Out Not In Effect . . . . . . . 11
8.2.2. No Data Response, Opt-Out In Effect . . . . . . . . . 11
8.3. Wildcard Responses . . . . . . . . . . . . . . . . . . . 12
8.4. Wildcard No Data Responses . . . . . . . . . . . . . . . 12
9. Authoritative Server Considerations . . . . . . . . . . . . . 13
9.1. Zone Signing . . . . . . . . . . . . . . . . . . . . . . 13
9.2. Zone Serving . . . . . . . . . . . . . . . . . . . . . . 14
9.3. NSEC5KEY Rollover Mechanism . . . . . . . . . . . . . . . 15
9.4. Secondary Servers . . . . . . . . . . . . . . . . . . . . 15
9.5. Zones Using Unknown Hash Algorithms . . . . . . . . . . . 16
9.6. Dynamic Updates . . . . . . . . . . . . . . . . . . . . . 16
10. Resolver Considerations . . . . . . . . . . . . . . . . . . . 16
11. Validator Considerations . . . . . . . . . . . . . . . . . . 16
11.1. Validating Responses . . . . . . . . . . . . . . . . . . 16
11.2. Validating Referrals to Unsigned Subzones . . . . . . . 17
11.3. Responses With Unknown Hash Algorithms . . . . . . . . . 17
12. Special Considerations . . . . . . . . . . . . . . . . . . . 17
12.1. Transition Mechanism . . . . . . . . . . . . . . . . . . 17
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12.2. NSEC5 Private Keys . . . . . . . . . . . . . . . . . . . 18
12.3. Domain Name Length Restrictions . . . . . . . . . . . . 18
13. Performance Considerations . . . . . . . . . . . . . . . . . 19
14. Security Considerations . . . . . . . . . . . . . . . . . . . 19
14.1. Zone Enumeration Attacks . . . . . . . . . . . . . . . . 19
14.2. Hash Collisions . . . . . . . . . . . . . . . . . . . . 19
14.3. Compromise of the Private NSEC5 Key . . . . . . . . . . 19
14.4. Key Length Considerations . . . . . . . . . . . . . . . 20
14.5. Transitioning to a New NSEC5 Algorithm . . . . . . . . . 20
15. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
16. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 21
17. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
17.1. Normative References . . . . . . . . . . . . . . . . . . 21
17.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix A. Full Domain Hash Algorithm . . . . . . . . . . . . . 23
A.1. FDH signature . . . . . . . . . . . . . . . . . . . . . . 23
A.2. FDH verification . . . . . . . . . . . . . . . . . . . . 24
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 25
Appendix C. Open Issues . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
1.1. Rationale
The DNS Security Extensions (DNSSEC) provides data integrity
protection using public-key cryptography, while not requiring that
authoritative servers compute signatures on-the-fly. The content of
the zone is usually pre-computed and served as is. The evident
advantages of this approach are reduced performance requirements per
query, as well as not requiring private zone-signing keys to be
present on nameservers facing the network.
With DNSSEC, each resource record (RR) set in the zone is signed.
The signature is retained as an RRSIG RR directly in the zone. This
enables response authentication for data existing in the zone. To
ensure integrity of denying answers, an NSEC chain of all existing
domain names in the zone is constructed. The chain is made of RRs,
where each RR represents two consecutive domain names in canonical
order present in the zone. The NSEC RRs are signed the same way as
any other RRs in the zone, and each NSEC can be used to prove that a
given name does not existing in a part of the domain name space.
As side-effect, however, the NSEC chain existence allows for the
enumeration of the zone's contents by querying for names immediately
individual RRs in the chain; N queries suffice to enumerate a zone
containing N names. As such, the NSEC3 hashed denial of existence
was introduced to prevent zone enumeration. In NSEC3, the original
domain names in the NSEC chain are replaced by their cryptographic
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hashes. While NSEC3 makes zone enumeration more difficult, offline
dictionary attacks are still possible and have been demonstrated;
this is because hashes may be computed offline and the space of
possible domain names is restricted [nsec3walker][nsec3gpu].
Zone enumeration can be prevented with NSEC3 if having the
authoritative server compute NSEC3 RRs on-the-fly, in response to
queries with denying responses. Usually, this is done with Minimally
Covering NSEC Records or NSEC3 White Lies [RFC7129]. One of the most
significant disadvantage of this approach is a required presence of
the private key on all authoritative servers for the zone. This is
often undesirable, as the holder of the private key can tamper with
the zone content, and having private keys on many network-facing
servers increases the risk that keys can be compromised.
To prevent zone content enumeration without keeping private keys on
all authoritative servers, NSEC5 replaces the unkeyed cryptographic
hash function used in NSEC3 with a public-key hashing scheme.
Hashing in NSEC5 is performed with a separate NSEC5 key. The public
portion of this key is distributed in an NSEC5KEY RR, and is used to
validate NSEC5 hash values. The private portion of the NSEC5 key is
present on all authoritative servers for the zone, and is used to
compute hash values.
Importantly, the NSEC5KEY key cannot be used to modify the contents
of the zone. Thus, any compromise of the private NSEC5 key does not
lead to a compromise of zone contents; all that is lost is privacy
against zone enumeration, effectively downgrading the security of
NSEC5 to that of NSEC3. NSEC5 comes with a cryptographic proof of
security, available in [nsec5].
The NSEC5 is not intended to replace NSEC or NSEC3. It is designed
as an alternative mechanisms for authenticated denial of existence.
1.2. Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.3. Terminology
The reader is assumed to be familiar with the basic DNS and DNSSEC
concepts described in [RFC1034], [RFC1035], [RFC4033], [RFC4034],
[RFC4035], and subsequent RFCs that update them: [RFC2136],
[RFC2181], [RFC2308], and [RFC5155].
The following terminology is used through this document:
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Base32hex: The "Base 32 Encoding with Extended Hex Alphabet" as
specified in [RFC4648]. The padding characters ("=") are not used
in NSEC5 specification.
Base64: The "Base 64 Encoding" as specified in [RFC4648].
NSEC5 proof: A signed hash of a domain name (hash-and-sign
paradigm). A holder of the private key (e.g., authoritative
server) can compute the proof. Anyone knowing the public key
(e.g., client) can verify it's validity.
NSEC5 hash: A cryptographic hash (digest) of an NSEC5 proof. If the
NSEC5 proof is known, anyone can compute and verify it's NSEC5
hash.
NSEC5 algorithm: A pair of algorithms used to compute NSEC5 proofs
and NSEC5 hashes.
2. Backward Compatibility
The specification describes a protocol change that is not backward
compatible with [RFC4035] and [RFC5155]. NSEC5-unaware resolver will
fail to validate responses introduced by this document.
To prevent NSEC5-unaware resolvers from attempting to validate the
responses, new DNSSEC algorithms identifiers are introduced, the
identifiers alias with existing algorithm numbers. The zones signed
according to this specification MUST use only these algorithm
identifiers, thus NSEC5-unaware resolvers will treat the zone as
insecure.
The new algorithm identifiers defined by this document are listed in
Section 15.
3. How NSEC5 Works
To prove non-existence of a domain name in a zone, NSEC uses a chain
built from domain names present in the zone. NSEC3 replaces the
original domain names by their cryptographic hashes. NSEC5 is very
similar to NSEC3, however a public-key based hashing scheme is used.
In NSEC5, the original domain name is hashed twice:
1. First, the domain name is hashed using the NSEC5 private key; the
result is called the NSEC5 proof. Only an authoritative server
that knows the private NSEC5 key can compute the NSEC5 proof.
Any client that knows the public NSEC5 key can validate the NSEC5
proof.
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2. Second, the NSEC5 proof is hashed using a cryptographic hash
function; the result is called the NSEC5 hash. This hash can be
computed by any party that knows the input NSEC5 proof.
The NSEC5 hash determines the position of a domain name in an NSEC5
chain. That is, all the NSEC5 hashes for a zone are sorted in their
canonical order, and each consecutive pair forms an NSEC5 RR.
To prove an non-existence of a particular domain name in response to
a query, the server computes on the fly, the NSEC5 proof (using the
private NSEC5 key). Then it uses the NSEC5 proof to compute the
corresponding NSEC5 hash. It then identifies the NSEC5 RR that
covers the hash. In the response message, the server returns the
NSEC5 RR, it's corresponding signature (RRSIG RRset), and synthesized
NSEC5PROOF RR containing the NSEC5 proof it computed on the fly.
To validate the response, the client first uses the public NSEC5 key
(stored in the zone as an NSEC5KEY RR) to verify that the NSEC5 proof
corresponds with the domain name to be disproved. Then, the client
computes the NSEC5 hash from the NSEC5 proof and checks if the NSEC5
RR content and it's signature are valid.
4. NSEC5 Algorithms
The algorithms used for NSEC5 authenticated denial are independent on
the algorithms used for DNSSEC signing. An NSEC5 algorithm defines
how the NSEC5 proof and the NSEC5 hash is computed and validated.
The input for the NSEC5 proof computation is an RR owner name in the
canonical form in the wire format and an NSEC5 private key; the
output is an octet string.
The input for the NSEC5 hash computation is the corresponding NSEC5
proof; the output is an octet string.
This document defines FDH-SHA256-SHA256 NSEC5 algorithm as follows:
o NSEC5 proof is an RSA based Full Domain Hash (FDH) with SHA-256
hash function used internally for input preprocessing. The FDH
signature and verification is formally specified in Appendix A.
o NSEC5 hash is an SHA-256 hash function as specified in [RFC6234].
o The public key format to be used in NSEC5KEY RR is defined in
Section 2 of [RFC3110] and thus is the same as the format used to
store RSA public keys in DNSKEY RRs.
The NSEC5 algorithm identifier for FDH-SHA256-SHA256 is 1.
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5. The NSEC5KEY Resource Record
The NSEC5KEY RR stores an NSEC5 public key. The key allows clients
to verify a validity of NSEC5 proof sent by a server.
5.1. NSEC5KEY RDATA Wire Format
The RDATA for NSEC5KEY RR is as shown below:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm | Public Key /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Algorithm is a single octet identifying NSEC5 algorithm.
Public Key is a variable sized field holding public key material for
NSEC5 proof verification.
5.2. NSEC5KEY RDATA Presentation Format
The presentation format of the NSEC5KEY RDATA is as follows:
The Algorithm field is represented as an unsigned decimal integer.
The Public Key field is represented in Base64 encoding. Whitespace
is allowed within the Base64 text.
6. The NSEC5 Resource Record
The NSEC5 RR provides authenticated denial of existence for an RRset.
One NSEC5 RR represents one piece of an NSEC5 chain, proving
existence of RR types present at the original domain name and also
non-existence of other domain names in a part of the hashed domain
name space.
6.1. NSEC5 RDATA Wire Format
The RDATA for NSEC5 RR is as shown below:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Tag | Flags | Next Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hashed Owner Name /
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Type Bit Maps /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Key Tag field contains the key tag value of the NSEC5KEY RR that
validates the NSEC5 RR, in network byte order. The value is computed
from the NSEC5KEY RDATA using the same algorithm, which is used to
compute key tag values for DNSKEY RRs. The algorithm is defined in
[RFC4034].
Flags field is a single octet. The meaning of individual bits of the
field is defined in Section 6.2.
Next length is an unsigned single octet specifying the length of the
Next Hashed Owner Name field in octets.
Next Hashed Owner Name field is a sequence of binary octets. It
contains an NSEC5 hash of the next domain name in the NSEC5 chain.
Type Bit Maps is a variable sized field encoding RR types present at
the original owner name matching the NSEC5 RR. The format of the
field is equivalent to the format used in NSEC3 RR, described in
[RFC5155].
6.2. NSEC5 Flags Field
The following one-bit NSEC5 flags are defined:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| |W|O|
+-+-+-+-+-+-+-+-+
O - Opt-Out flag
W - Wildcard flag
All the other flags are reserved for future use and MUST be zero.
The Opt-Out flag has the same semantics as in NSEC3. The definition
and considerations in [RFC5155] are valid, except that NSEC3 is
replaced by NSEC5.
The Wildcard flag indicates that a wildcard synthesis is possible at
the original domain name level (i.e., there is a wildcard node
immediately descending from the immediate ancestor of the original
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domain name). The purpose of the Wildcard flag is to reduce a
maximum number of RRs required for authenticated denial of existence
proof.
6.3. NSEC5 RDATA Presentation Format
The presentation format of the NSEC5 RDATA is as follows:
The Key Tag field is represented as an unsigned decimal integer.
The Flags field is represented as an unsigned decimal integer.
The Next Length field is not represented.
The Next Hashed Owner Name field is represented as a sequence of
case-insensitive Base32hex digits without any whitespace and without
padding.
The Type Bit Maps representation is equivalent to the representation
used in NSEC3 RR, described in [RFC5155].
7. The NSEC5PROOF Resource Record
The NSEC5PROOF record is synthesized by the authoritative server on-
the-fly. The record contains the NSEC5 proof, proving a position of
the owner name in an NSEC5 chain.
7.1. NSEC5PROOF RDATA Wire Format
The RDATA for NSEC5PROOF is as as shown below:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Tag | Owner Name Hash /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Key Tag field contains the key tag value of the NSEC5KEY RR that
validates the NSEC5PROOF RR, in network byte order.
Owner Name Hash is a variable sized sequence of binary octets
encoding the NSEC5 proof of the owner name of the RR.
7.2. NSEC5PROOF RDATA Presentation Format
The presentation format of the NSEC5PROOF RDATA is as follows:
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The Key Tag field is represented as an unsigned decimal integer.
The Owner Name Hash is represented in Base64 encoding. Whitespace is
allowed within the Base64 text.
8. NSEC5 Proofs
This section summarizes all possible types of authenticated denial of
existence. For each type the following lists are included:
1. Facts to prove. The minimum amount of information an
authoritative server must provide to a client to assure the
client that the response content is valid.
2. Authoritative server proofs. NSEC5 RRs an authoritative server
must include in a response to prove the listed facts.
3. Validator checks. Individual checks a validating server is
required to perform on a response. The response content is
considered valid only if all the checks pass.
If NSEC5 is said to match a domain name, the owner name of the NSEC5
has to be equivalent to an NSEC5 hash of that domain name. If NSEC5
is said to cover a domain name, the NSEC5 hash of that name must lay
strictly between the NSEC5 owner name and the NSEC5 Next Hashed Owner
Name.
8.1. Name Error Responses
Facts to prove:
No RRset matching the QNAME exactly exists.
No RRset matching the QNAME via wildcard expansion exists.
The QNAME does not fall into a delegation.
The QNAME does not fall into a DNAME redirection.
Authoritative server proofs:
Closest encloser.
Next closer name.
Validator checks:
Closest encloser belongs to the zone.
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Closest encloser has the Wildcard flag cleared.
Closest encloser does not have NS without SOA in the Type Bit Map.
Closest encloser does not have DNAME in the Type Bit Maps.
Next closer name is derived correctly.
8.2. No Data Responses
The processing of a No Data response for DS QTYPE differs if the Opt-
Out is in effect. For DS QTYPE queries, the validator has two
possible checking paths. The correct path can be simply decided by
inspecting if the NSEC5 RR in the response matches the QNAME.
Note that the Opt-Out is valid only for DS QTYPE queries.
8.2.1. No Data Response, Opt-Out Not In Effect
Facts to prove:
An RRset matching the QNAME exists.
No QTYPE RRset matching the QNAME exists.
No CNAME RRset matching the QNAME exists.
Authoritative server proofs:
QNAME.
Validator checks:
The NSEC5 RR exactly matches the QNAME.
The NSEC5 RR does not have QTYPE in the Type Bit Map.
The NSEC5 RR does not have CNAME in the Type Bit Map.
8.2.2. No Data Response, Opt-Out In Effect
Facts to prove:
The delegation is not covered by the NSEC5 chain.
Authoritative server proofs:
Closest provable encloser.
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Validator checks:
Closest provable encloser is in zone.
Closest provable encloser covers (not matches) the QNAME.
Closest provable encloser has the Opt-Out flag set.
8.3. Wildcard Responses
Facts to prove:
No RRset matching the QNAME exactly exists.
No wildcard closer to the QNAME exists.
Authoritative server proofs:
Next closer name.
Validator checks:
Next closer name is derived correctly.
Next closer name covers (not matches).
8.4. Wildcard No Data Responses
Facts to prove:
No RRset matching the QNAME exactly exists.
No QTYPE RRset exists at the wildcard matching the QNAME.
No CNAME RRset exists at the wildcard matching the QNAME.
No wildcard closer to the QNAME exists.
Authoritative server proofs:
Source of synthesis (i.e., wildcard at closest encloser).
Next closer name.
Validator checks:
Source of synthesis matches exactly the QNAME.
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Source of synthesis does not have QTYPE in the Type Bit Map.
Source of synthesis does not have CNAME in the Type Bit Map.
Next closer name is derived correctly.
Next closer name covers (not matches).
9. Authoritative Server Considerations
9.1. Zone Signing
Zones using NSEC5 MUST satisfy the same properties as described in
Section 7.1 of [RFC5155], with NSEC3 replaced by NSEC5. In addition,
the following conditions MUST be satisfied as well:
o If the original owner name has a wildcard label immediately
descending from the original owner name, the corresponding NSEC5
RR MUST have the Wildcard flag set in the Flags field. Otherwise,
the flag MUST be cleared.
o The zone apex MUST include an NSEC5KEY RRset containing a NSEC5
public key allowing verification of the current NSEC5 chain.
The following steps describe one possible method to properly add
required NSEC5 related records into a zone. This is not the only
such existing method.
1. Select an algorithm for NSEC5. Generate the public and private
NSEC5 keys.
2. Add a NSEC5KEY RR into the zone apex containing the public NSEC5
key.
3. For each unique original domain name in the zone and each empty
non-terminal, add an NSEC5 RR. If Opt-Out is used, owner names
of unsigned delegations MAY be excluded.
a. The owner name of the NSEC5 RR is the NSEC5 hash of the
original owner name encoded in Base32hex without padding,
prepended as a single label to the zone name.
b. Set the Key Tag field to be the key tag corresponding to the
public NSEC5 key.
c. Clear the Flags field. If Opt-Out is being used, set the
Opt-Out flag. If there is a wildcard label directly
descending from the original domain name, set the Wildcard
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flag. Note that the wildcard can be an empty non-terminal
(i.e., the wildcard synthesis does not take effect and
therefore the flag is not to be set).
d. Set the Next Length field to a value determined by the used
NSEC5 algorithm. Leave the Next Hashed Owner Name field
blank.
e. Set the Type Bit Maps field based on the RRsets present at
the original owner name.
4. Sort the set of NSEC5 RRs into canonical order.
5. For each NSEC5 RR, set the Next Hashed Owner Name field by using
the owner name of the next NSEC5 RR in the canonical order. If
the updated NSEC5 is the last NSEC5 RR in the chain, the owner
name of the first NSEC5 RR in the chain is used instead.
The NSEC5KEY and NSEC5 RRs MUST have the same class as the zone SOA
RR. Also the NSEC5 RRs SHOULD have the same TTL value as the SOA
minimum TTL field.
Notice that a use of Opt-Out is not indicated in the zone. This does
not affect the ability of a server to prove insecure delegations.
The Opt-Out MAY be part of the zone-signing tool configuration.
9.2. Zone Serving
This specification modifies DNSSEC-enabled DNS responses generated by
authoritative servers. In particular, it replaces use of NSEC or
NSEC3 RRs in such responses with NSEC5 RRs and adds on-the-fly
computed NSEC5PROOF RRs.
The authenticated denial of existence proofs in NSEC5 are almost the
same as in NSEC3. However, due to introduction of Wildcard flag in
NSEC5 RRs, the NSEC5 proof consists from (up to) two NSEC5 RRs,
instead of (up to) three.
According to a type of a response, an authoritative server MUST
include NSEC5 RRs in a response as defined in Section 8. For each
NSEC5 RR in the response a matching RRSIG RRset and a synthesized
NSEC5PROOF MUST be added as well.
A synthesized NSEC5PROOF RR has the owner name set to a domain name
exactly matching the name required for the proof. The class and TTL
of the RR MUST be the same as the class and TTL value of the
corresponding NSEC5 RR. The RDATA are set according to the
description in Section 7.1.
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Notice, that the NSEC5PROOF owner name can be a wildcard (e.g.,
source of synthesis proof in wildcard No Data responses). The name
also always matches the domain name required for the proof while the
NSEC5 RR may only cover (not match) the name in the proof (e.g.,
closest encloser in Name Error responses).
If NSEC5 is used, an answering server MUST use exactly one NSEC5
chain for one signed zone.
NSEC5 MUST NOT be used in parallel with NSEC, NSEC3, or any other
authenticated denial of existence mechanism that allows for
enumeration of zone contents.
Similarly to NSEC3, the owner names of NSEC5 RRs are not represented
in the NSEC5 chain and therefore NSEC5 records deny their own
existence. The desired behavior caused by this paradox is the same
as described in Section 7.2.8 of [RFC5155].
9.3. NSEC5KEY Rollover Mechanism
Replacement of the NSEC5 key implies generating a new NSEC5 chain.
The NSEC5KEY rollover mechanism is similar to "Pre-Publish Zone
Signing Key Rollover" as specified in [RFC6781]. The NSEC5KEY
rollover MUST be performed as a sequence of the following steps:
1. A new public NSEC5 key is added into the NSEC5KEY RRset in the
zone apex.
2. The old NSEC5 chain is replaced by a new NSEC5 chain constructed
using the new key. This replacement MUST happen as a single
atomic operation; the server MUST NOT be responding with RRs from
both the new and old chain at the same time.
3. The old public key is removed from the NSEC5KEY RRset in the zone
apex.
The minimal delay between the steps 1. and 2. MUST be the time it
takes for the data to propagate to the authoritative servers, plus
the TTL value of the old NSEC5KEY RRset.
The minimal delay between the steps 2. and 3. MUST be the time it
takes for the data to propagate to the authoritative servers, plus
the maximum zone TTL value of any of the data in the previous version
of the zone.
9.4. Secondary Servers
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This document does not define mechanism to distribute NSEC5 private
keys.
9.5. Zones Using Unknown Hash Algorithms
Zones that are signed with unknown NSEC5 algorithm or by an
unavailable NSEC5 private key cannot be effectively served. Such
zones SHOULD be rejected when loading and servers SHOULD respond with
RCODE=2 (Server failure) when handling queries that would fall under
such zones.
9.6. Dynamic Updates
A zone signed using NSEC5 MAY accept dynamic updates. The changes to
the zone MUST be performed in a way, that the zone satisfies the
properties specified in Section 9.1 at any time.
It is RECOMMENDED that the server rejects all updates containing
changes to the NSEC5 chain (or related RRSIG RRs) and performs itself
any required alternations of the NSEC5 chain induced by the update.
Alternatively, the server MUST verify that all the properties are
satisfied prior to performing the update atomically.
10. Resolver Considerations
The same considerations as described in Section 9 of [RFC5155] for
NSEC3 apply to NSEC5. In addition, as NSEC5 RRs can be validated
only with appropriate NSEC5PROOF RRs, the NSEC5PROOF RRs MUST be all
together cached and included in responses with NSEC5 RRs.
11. Validator Considerations
11.1. Validating Responses
The validator MUST ignore NSEC5 RRs with Flags field values other
than the ones defined in Section 6.2.
The validator MAY treat responses as bogus if the response contains
NSEC5 RRs that refer to a different NSEC5KEY.
According to a type of a response, the validator MUST verify all
conditions defined in Section 8. Prior to making decision based on
the content of NSEC5 RRs in a response, the NSEC5 RRs MUST be
validated.
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To validate a denial of existence, zone NSEC5 public keys are
required in addition to DNSSEC public keys. Similarly to DNSKEY RRs,
the NSEC5KEY RRs are present in the zone apex.
The NSEC5 RR is validated as follows:
1. Select a correct NSEC5 public key to validate the NSEC5PROOF.
The Key Tag value of the NSEC5PROOF RR must match with the key
tag value computed from the NSEC5KEY RDATA.
2. Validate the NSEC5 proof present in the NSEC5PROOF Owner Name
Hash field using the NSEC5 public key. If there are multiple
NSEC5KEY RRs matching the key tag, at least one of the keys must
validate the NSEC5 proof.
3. Compute the NSEC5 hash value from the NSEC5 proof and check if
the response contains NSEC5 RR matching or covering the computed
NSEC5 hash. The TTL values of the NSEC5 and NSEC5PROOF RRs must
be the same.
4. Validate the signature of the NSEC5 RR.
If the NSEC5 RR fails to validate, it MUST be ignored. If some of
the conditions required for an NSEC5 proof is not satisfied, the
response MUST be treated as bogus.
Notice that determining closest encloser and next closer name in
NSEC5 is easier than in NSEC3. NSEC5 and NSEC5PROOF RRs are always
present in pairs in responses and the original owner name of the
NSEC5 RR matches the owner name of the NSEC5PROOF RR.
11.2. Validating Referrals to Unsigned Subzones
The same considerations as defined in Section 8.9 of [RFC5155] for
NSEC3 apply to NSEC5.
11.3. Responses With Unknown Hash Algorithms
A validator MUST ignore NSEC5KEY RRs with unknown NSEC5 algorithms.
The practical result of this is that zones sighed with unknown
algorithms will be considered bogus.
12. Special Considerations
12.1. Transition Mechanism
TODO: Not finished. Following information will be covered:
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o Transition from NSEC or NSEC3.
o Transition from NSEC5 to NSEC/NSEC3
o Transition to new algorithms within NSEC5
Quick notes on transition from NSEC/NSEC3 to NSEC5:
1. Publish NSEC5KEY RR.
2. Wait for data propagation to slaves and cache expiration.
3. Instantly switch answering from NSEC/NSEC3 to NSEC5.
Quick notes on transition from NSEC5 to NSEC/NSEC3:
1. Instantly switch answering from NSEC5 to NSEC/NSEC3.
2. Wait for NSEC5 RRs expiration in caches.
3. Remove NSEC5KEY RR from the zone.
12.2. NSEC5 Private Keys
This document does not define format to store NSEC5 private key. Use
of standardized and adopted format is RECOMMENDED.
The NSEC5 private key MAY be shared between multiple zones, however a
separate key is RECOMMENDED for each zone.
12.3. Domain Name Length Restrictions
The NSEC5 creates additional restrictions on domain name lengths. In
particular, zones with names that, when converted into hashed owner
names exceed the 255 octet length limit imposed by [RFC1035], cannot
use this specification.
The actual maximum length of a domain name depends on the length of
the zone name and used NSEC5 algorithm.
With FDH-SHA256-SHA256 defined in this document, the SHA-256 hash
function is used to construct NSEC5 hash values. SHA-256 produces a
hash of 256 bits, which can be encoded in 52 characters in Base32hex
without padding. The encoded string is prepended to the name of the
zone as a single label, which includes the length field of a single
octet. The maximal length of the zone name is therefore 202 octets
(255 - 53).
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13. Performance Considerations
TODO: Not finished. Following information will be covered:
o Size of the answers
o NSEC5 FDH-SHA256-SHA256 vs NSEC3 SHA1
o NSEC5 closest encloser name in NSEC5PROOF owner name vs NSEC5 SHA1
hashing
o Total number of crypto operations on server/client side
14. Security Considerations
14.1. Zone Enumeration Attacks
NSEC5 is robust to zone enumeration via offline dictionary attacks by
any attacker that does not know the NSEC5 private key. Without the
private NSEC5 key, that attacker cannot compute the NSEC5 proof that
corresponds to a given name; the only way it can learn the NSEC5
proof value for a given name is by sending a queries for that name to
the authoritative server. Without the NSEC5 proof value, the
attacker cannot learn the NSEC5 hash value. Thus, even an attacker
that collects the entire chain of NSEC5 RR for a zone cannot use
offline attacks to "reverse" that NSEC5 hash values in these NSEC5 RR
and thus learn which names are present in the zone. A formal
cryptographic proof of this property is in [nsec5].
14.2. Hash Collisions
Hash collisions between QNAME and the owner name of an NSEC5 RR may
occur. When they do, it will be impossible to prove the non-
existence of the colliding QNAME. However, with SHA-256, this is
highly unlikely (on the order of 1 in 2^128). Note that DNSSEC
already relies on the presumption that a cryptographic hash function
is collision resistant, since these hash functions are used for
generating and validating signatures and DS RRs. See also the
discussion on key lengths in [nsec5].
14.3. Compromise of the Private NSEC5 Key
NSEC5 requires authoritative servers to hold the private NSEC5 key,
but not the private zone-signing keys or the private key-signing keys
for the zone. The private NSEC5 key needs only be as secure as the
DNSSEC records whose the privacy (against zone-enumeration attacks)
that NSEC5 is protecting. This is because even an adversary that
knows the private NSEC5 key cannot modify the contents of the zone;
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this is because the zone contents are signed using the private zone-
signing key, while the private NSEC5 key is only used to compute
NSEC5 proof values. Thus, a compromise of the private NSEC5 keys
does not lead to a compromise of the integrity of the DNSSEC record
in the zone; instead, all that is lost is privacy against zone
enumeration, if the attacker that knows the private NSEC5 key can
compute NSEC5 hashes offline, and thus launch offline dictionary
attacks. Thus, a compromise of the private NSEC5 key effectively
downgrades the security of NSEC5 to that of NSEC3. A formal
cryptographic proof of this property is in [nsec5].
14.4. Key Length Considerations
The NSEC5 key must be long enough to withstand attacks for as long as
the privacy of the zone is important. Even if the NSEC5 key is
rolled frequently, its length cannot be too short, because zone
privacy may be important for a period of time longer than the
lifetime of the key. (For example, an attacker might collect the
entire chain of NSEC5 RR for the zone over one short period, and
then, later (even after the NSEC5 key expires) perform an offline
dictionary attack that attempt to "reverse" the NSEC5 hash values
present in the NSEC5 RRs.) This is in contrast to zone-signing and
key-signing keys used in DNSSEC; these keys, which ensure the
authenticity and integrity of the zone contents need to remain secure
only during their lifetime.
14.5. Transitioning to a New NSEC5 Algorithm
Although the NSEC5KEY RR formats include a hash algorithm parameter,
this document does not define a particular mechanism for safely
transitioning from one NSEC5 algorithm to another. When specifying a
new hash algorithm for use with NSEC5, a transition mechanism MUST
also be defined. It is possible that the only practical and
palatable transition mechanisms may require an intermediate
transition to an insecure state, or to a state that uses NSEC or
NSEC3 records instead of NSEC5.
15. IANA Considerations
This document updates the IANA registry "Domain Name System (DNS)
Parameters" in subregistry "Resource Record (RR) TYPEs", by defining
the following new RR types:
NSEC5KEY value XXX.
NSEC5 value XXX.
NSEC5PROOF value XXX.
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This document creates a new IANA registry for NSEC5 algorithms. This
registry is named "DNSSEC NSEC5 Algorithms". The initial content of
the registry is:
0 is Reserved.
1 is FDH-SHA256-SHA256.
2-255 is Available for assignment.
This document updates the IANA registry "DNS Security Algorithm
Numbers" by defining following aliases:
XXX is NSEC5-RSASHA256, alias for RSASHA256 (8).
XXX is NSEC5-RSASHA512, alias for RSASHA512 (10).
XXX is NSEC5-ECDSAP256SHA256 alias for ECDSAP256SHA256 (13).
XXX is NSEC5-ECDSAP384SHA384 alias for ECDSAP384SHA384 (14).
16. Contributors
This document would not be possible without help of Moni Naor
(Weizmann Institute), Dimitrios Papadopoulos (Boston University),
Sachin Vasant (Cisco Systems), Leonid Reyzin (Boston University), and
Asaf Ziv (Weizmann Institute) who contributed to the design of NSEC5,
and Ondrej Sury (CZ.NIC Labs) who provided advice on its
implementation.
17. References
17.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997.
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[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998.
[RFC3110] Eastlake, D., "RSA/SHA-1 SIGs and RSA KEYs in the Domain
Name System (DNS)", RFC 3110, May 2001.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, March 2008.
[RFC6234] Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234, May 2011.
17.2. Informative References
[nsec5] Goldberg, S., Naor, M., Papadopoulos, D., Reyzin, L.,
Vasant, S., and A. Ziv, "NSEC5: Provably Preventing DNSSEC
Zone Enumeration", July 2014.
[nsec3gpu]
Wander, M., Schwittmann, L., Boelmann, C., and T. Weis,
"GPU-Based NSEC3 Hash Breaking", in IEEE Symp. Network
Computing and Applications (NCA), 2014.
[nsec3walker]
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Bernstein, D., "Nsec3 walker", 2011,
<http://dnscurve.org/nsec3walker.html>.
[RFC6781] Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
Operational Practices, Version 2", RFC 6781, December
2012.
[RFC7129] Gieben, R. and W. Mekking, "Authenticated Denial of
Existence in the DNS", RFC 7129, February 2014.
Appendix A. Full Domain Hash Algorithm
The Full Domain Hash (FDH) is a RSA-based scheme that allows
authentication of hashes using public-key cryptography.
In this document, the notation from [RFC3447] is used.
Used parameters:
(n, e) - RSA public key
K - RSA private key
k - length of the RSA modulus n in octets
Fixed options:
Hash - hash function to be used with MGF1
Used primitives:
I2OSP - Coversion of a nonnegative integer to an octet string as
defined in Section 4.1 of [RFC3447]
OS2IP - Coversion of an octet string to a nonnegative integer as
defined in Section 4.2 of [RFC3447]
RSASP1 - RSA signature primitive as defined in Section 5.2.1 of
[RFC3447]
RSAVP1 - RSA verification primitive as defined in Section 5.2.2 of
[RFC3447]
MGF1 - Mask Generation Function based on a hash function as
defined in Section B.2.1 of [RFC3447]
A.1. FDH signature
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FDH_SIGN(K, M)
Input:
K - RSA private key
M - message to be signed, an octet string
Output:
S - signature, an octet string of length k
Steps:
1. EM = MGF1(M, k - 1)
2. m = OS2IP(EM)
3. s = RSASP1(K, m)
4. S = I2OSP(s, k)
5. Output S
A.2. FDH verification
FDH_VERIFY((n, e), M, S)
Input:
(n, e) - RSA public key
M - message whose signature is to be verified, an octet string
S - signature to be verified, an octet string of length k
Output:
"valid signature" or "invalid signature"
Steps:
1. s = OS2IP(S)
2. m = RSAVP1((n, e), s)
3. EM = I2OSP(m, k - 1)
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4. EM' = MGF1(M, k - 1)
5. If EM and EM' are the same, output "valid signature"; else output
"invalid signature".
Appendix B. Change Log
Note to RFC Editor: if this document does not obsolete an existing
RFC, please remove this appendix before publication as an RFC.
pre 00 - initial version of the document submitted to mailing list
only
00 - fix NSEC5KEY rollover mechanism, clarify NSEC5PROOF RDATA,
clarify inputs and outputs for NSEC5 proof and NSEC5 hash
computation
Appendix C. Open Issues
Note to RFC Editor: please remove this appendix before publication as
an RFC.
1. Consider alternative way to signalize NSEC5 support. The NSEC5
could use only one DNSSEC algorithm identifier, and the actual
algorithm to be used for signing can be the first byte in DNSKEY
public key field and RRSIG signature field. Similar approach is
used by PRIVATEDNS and PRIVATEOID defined in [RFC4034].
2. How to add new NSEC5 hashing algorithm. We will need to add new
DNSSEC algorithm identifiers again.
3. NSEC and NSEC3 define optional steps for hash collisions
detection. We don't have a way to avoid them if they really
appear (unlikely). We would have to drop the signing key and
generate a new one. Which cannot be done instantly.
4. Write Special Considerations section.
5. Write Performance Considerations section.
6. Contributor list has to be completed.
Authors' Addresses
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Jan Vcelak
CZ.NIC
Milesovska 1136/5
Praha 130 00
CZ
EMail: jan.vcelak@nic.cz
Sharon Goldberg
Boston University
111 Cummington St, MCS135
Boston, MA 02215
USA
EMail: goldbe@cs.bu.edu
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