Network Working Group J. Vcelak
Internet-Draft CZ.NIC
Intended status: Standards Track S. Goldberg
Expires: March 24, 2016 Boston University
September 21, 2015

NSEC5, DNSSEC Authenticated Denial of Existence


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

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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

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This Internet-Draft will expire on March 24, 2016.

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Table of Contents

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 allows for enumeration of the zone's contents by sequentially querying for the names immediately following those in the most-recently retrieved NSEC record; 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 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 mechanism 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:

The "Base 32 Encoding with Extended Hex Alphabet" as specified in [RFC4648]. The padding characters ("=") are not used in NSEC5 specification.
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.
  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 of 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:

The NSEC5 algorithm identifier for FDH-SHA256-SHA256 is 1.

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                    /
/                         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|

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 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:

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:

Authoritative server proofs:

Validator checks:

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:

Authoritative server proofs:

Validator checks:

8.2.2. No Data Response, Opt-Out In Effect

Facts to prove:

Authoritative server proofs:

Validator checks:

8.3. Wildcard Responses

Facts to prove:

Authoritative server proofs:

Validator checks:

8.4. Wildcard No Data Responses

Facts to prove:

Authoritative server proofs:

Validator checks:

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:

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.
    1. 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.
    2. Set the Key Tag field to be the key tag corresponding to the public NSEC5 key.
    3. 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 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).
    4. Set the Next Length field to a value determined by the used NSEC5 algorithm. Leave the Next Hashed Owner Name field blank.
    5. 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.

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

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.

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:

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).

13. Performance Considerations

This section compares NSEC, NSEC3, and NSEC5 with regards to the size of denial-of-existence responses and performance impact on the DNS components.

13.1. Performance of Cryptographic Operations

Additional performance costs depend on the costs of cryptographic operations to a great extent. The following results were retrieved with OpenSSL 1.0.1k, running an ordinary laptop on a single-core of a CPU manufactured in 2011. The parameters of cryptographic operations were chosen to reflect the parameters used in the real-world application of DNSSEC.

Comparison of NSEC3 and NSEC5 hashing performance:

According to the results, the NSEC5 hashing is about three orders of magnitude slower than NSEC3 hashing and the NSEC5 hash verification is about one order of magnitude slower than NSEC3 hash verification.

Comparison of signing and verification performance of different DNSSEC signing algorithms:

Algorithm Key size (bits) Signature size (octets) Performance (sign/s) Performance (verify/s)
RSASHA256 1024 128 2000 40000
RSASHA256 2048 256 400 10000
RSASHA256 4096 512 50 3000
ECDSAP256SHA256 256 64 5000 1000
ECDSAP384SHA384 384 96 3000 600

The retrieved values are important primarily for the purpose of evaluating performance of response validation. The signing performance is not that important because the zone is usually signed offline.

13.2. Authoritative Server Startup

This section compares additional server startup cost based on the used authenticated denial mechanism.

There are no special requirements on processing of a NSEC-signed zone during an authoritative server startup. The server handles the NSEC RRs the same way as any other records in the zone.
The authoritative server can precompute NSEC3 hashes for all domain names in the NSEC3-signed zone on startup. With respect to query answering, this can speed up inclusion of NSEC3 RRs for existing domain names (i.e., Closest provable encloser and QNAME for No Data response).
Very similar considerations apply for NSEC3 and NSEC5. There is a strong motivation to precompute the NSEC5 proofs because they are costly to compute. A possible solution to reduce the startup time is to store the precomputed NSEC5 proofs and NSEC5 hashes in a persistent storage.

The impact of NSEC3 and NSEC5 on the authoritative server startup performance is greatly implementation specific. The server software vendor has to seek balance between answering performance, startup slowdown, and additional storage cost.

13.3. NSEC5 Answer Generating and Processing

An authenticated denial proof is required for No Data, Name Error, Wildcard Match, and Wildcard No Data answer. The number of required records depends on used authenticated denial mechanism. Their size, generation cost, and validation cost depend on various zone and signing parameters.

In the worst case, the following additional records authenticating the denial will be included into the response:

The following table compares the size of NSEC, NSEC3, and NSEC5 records. We assume the SHA-1 hash algorithm for NSEC3 and the FDH-SHA256-SHA256 algorithm for NSEC5.

Fixed part Common part RR type-specific part
NSEC 10 octets Type Bit Maps Owner Name, Next Name
NSEC3 64 octets Type Bit Maps Salt
NSEC5 101 octets Type Bit Maps

The size covers complete RR representation in wire format:

The size of RRSIG RR is a sum of length of possibly compressed RR owner name, 28 octets for fixed-size fields, the length of uncompressed signer name, and the length of the signature.

The size of NSEC5PROOF RR is a sum of length of possibly compressed RR owner name, 2 octets for fixed-size fields, and the length of the proof.

The following list summarizes the increase of the DNS response size for authenticated denial worst-case scenario. As a significant part of the increase is caused by the used DNSSEC signing algorithm, the summary includes two variants: RSA stands for the RSASHA256 algorithm with 2048-bit key and ECDSA stands for the ECDSAP256SHA256 algorithm. For NSEC5, FDH-SHA256-SHA256 with 2048-bit key is used.

The following list summarizes additional cryptographic operations performed by the authoritative server for authenticated denial worst-case scenario:

As anticipated, NSEC is the most efficient authenticated denial mechanism as for response size and performance cost. The bottleneck of NSEC5 is the NSEC5 proof computation. If the proofs are precomputed for domain names in the zone, the server has to compute just one NSEC5 proof per answer. But it will still be more expensive than NSEC3 and the same applies for the response size.

As for the response processing, the validation cost reflects from the records present in the response. Again, the NSEC validation seems to be the most inexpensive. However the difference between RSA and ECDSA verification performance is huge and for some parameters, NSEC3 is faster to validate than NSEC5 and vice versa.

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; 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:

This document creates a new IANA registry for NSEC5 algorithms. This registry is named "DNSSEC NSEC5 Algorithms". The initial content of the registry is:

This document updates the IANA registry "DNS Security Algorithm Numbers" by defining following aliases:

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, DOI 10.17487/RFC1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y. and J. Bound, "Dynamic Updates in the Domain Name System (DNS UPDATE)", RFC 2136, DOI 10.17487/RFC2136, April 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998.
[RFC3110] Eastlake 3rd, D., "RSA/SHA-1 SIGs and RSA KEYs in the Domain Name System (DNS)", RFC 3110, DOI 10.17487/RFC3110, May 2001.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February 2003.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D. and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, DOI 10.17487/RFC4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D. and S. Rose, "Resource Records for the DNS Security Extensions", RFC 4034, DOI 10.17487/RFC4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D. and S. Rose, "Protocol Modifications for the DNS Security Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006.
[RFC5155] Laurie, B., Sisson, G., Arends, R. and D. Blacka, "DNS Security (DNSSEC) Hashed Authenticated Denial of Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI 10.17487/RFC6234, 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] Bernstein, D., "Nsec3 walker", 2011.
[RFC6781] Kolkman, O., Mekking, W. and R. Gieben, "DNSSEC Operational Practices, Version 2", RFC 6781, DOI 10.17487/RFC6781, December 2012.
[RFC7129] Gieben, R. and W. Mekking, "Authenticated Denial of Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129, 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:

Fixed options:

Used primitives:

A.1. FDH signature





  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)




  1. s = OS2IP(S)
  2. m = RSAVP1((n, e), s)
  3. EM = I2OSP(m, k - 1)
  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.

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 octet 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. Contributor list has to be completed.

Authors' Addresses

Jan Vcelak CZ.NIC Milesovska 1136/5 Praha, 130 00 CZ EMail:
Sharon Goldberg Boston University 111 Cummington St, MCS135 Boston, MA 02215 USA EMail: