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Internet DNSOP Working Group Signature DNSSEC Algorithm This document describes how to apply the Module-Lattice-Based Digital Signature Algorithm (ML-DSA) and Merkle Tree Ladders (MTL) as a conservative post-quantum cryptographic algorithm for DNS Security Extensions (DNSSEC). This combination is referred to as the ML-DSA-MTL Signature scheme. This document describes how to specify ML-DSA-MTL keys and signatures in DNSSEC, specifically for ML-DSA-44 with SHAKE-128.

Introduction The Domain Name System Security Extensions (DNSSEC), which are broadly defined in , and , use cryptographic keys and digital signatures to provide data origin authentication and data integrity in the DNS. This document describes the application of Merkle Tree Ladders (MTL) to the Module-Lattice-Based Digital Signature Algorithm (ML-DSA) , referred to generally as ML-DSA-MTL. This document specifically defines the use of ML-DSA-44 with SHAKE-128 as defined in as the ML-DSA-44-MTL-SHAKE-128 signature scheme for DNSSEC. As described herein, a DNSKEY resource record (RR) for an ML-DSA-MTL key contains a ML-DSA key. The ML-DSA key is used for verifying signatures on MTL authentication data. An RRSIG resource record for an ML-DSA-MTL Signature contains a Merkle proof (authentication path) and signed MTL. A signed MTL with a Merkle proof can be verified in a single response, similar to how existing DNSSEC algorithms operate today. This draft focuses on the code-points applicable to DNSKEY and RRSIG formulation. Later versions or other drafts may describe DNSSEC protocol and/or operational guidance for zone signing, zone composition, zone updates, zone transfer, name server processing, resolver signature processing, resolver caching, or other potential optimizations related to MTL.

Motivation This document provides a conservatively designed PQC algorithm for DNSSEC as described in using MTL and ML-DSA . The cryptographic operations for applying MTL to ML-DSA in the DNSSEC use case are specified within this document. The reader does not need to be familiar with the more general specification to implement the ML-DSA-MTL signature scheme for DNSSEC. MTL is designed to reduce the size, memory, and computational impact of PQC signature algorithms. For DNSSEC, MTL reduces the size and memory requirements for PQC signatures as the signature data in the zone database or cache is primarily comprised of Merkle proofs and only occasionally of signed MTLs . It also reduces the computational requirements as only the full signature incurs the signing and/or verification overhead of the underlying algorithm. ML-DSA was formally published as a standard by NIST in August 2024 . This document selected ML-DSA as one application of MTL to DNSSEC because lattice-based techniques are well understood and offer a conservative choice for long-term security relative to newer NIST candidate post-quantum signature schemes.

Definitions The provided terms in alphabetical order describe concepts utilized in this document.

Authentication Path

The set of sibling hash values from a leaf hash value to a rung.

Index

An integer value I that identifies a leaf node in a node set. Indexes start at 0 and are consecutively assigned.

Index Pair

A pair of integer values (L,R) that together identify a node in a node set based on the lowest and highest indexes of the consecutively-indexed leaf nodes that the node authenticates. For a leaf node, the index pair is (I,I) which is used interchangeably with the leaf index I.

Internal Node

A node in a node set whose value is the hash of two child nodes.

Ladder/Merkle Tree Ladder (MTL)

A collection of one or more rungs that can be used to verify an authentication path.

Leaf Node

A node in a node set whose value is the hash of a single message.

Message

A set of bytes that are intended to be signed and later verified.

Node Set

An evolving set of hash nodes, each of which is part of a union of tree structures either as a leaf node or an internal node. A node set is acyclic, i.e., every node is either a leaf node or the ancestor of two or more leaf nodes, and no node is an ancestor of itself. Every node set has one or more root nodes.

Root Node

A node in a node set that has no ancestors.

Rung

A node from a node set that can be used to authenticate one or more leaf nodes within that node set. A rung may be a root node.

Series Identifier (SID)

A unique value associated with an instance of a MTL node set.

Binary Rung Strategy In the MTL operations in this document, the ladder is selected according to what is called the binary rung strategy. In this strategy, the index pairs for the rungs are based on the binary representation of the number of messages in the message series. More specifically, the first rung is the apex of the largest perfect binary tree that can be formed from the leaf nodes corresponding to the messages, starting from the left; the second rung is the apex of the largest perfect binary tree that can be formed from the remaining leaf nodes; and so on. The sizes of the trees decrease from left to right. The following figure shows a node set with 14 leaf nodes based on this strategy. The internal node hash function is denoted H and the leaf node hash function is not shown. The rungs are marked with asterisks (*). Merkle tree authentication paths in binary rung strategy are constructed from the perfect binary tree that a given rung covers. For example, the authentication path for leaf 9 would include the hash of leaf 8 and (10,11) which is sufficient to re-create rung (8,11) which covers leaf 9. The following table gives examples of ladders for values of N up to 19 to showcase how the rungs evolve as new messages are appended to a given node set. Different rung strategies may result in rungs and authentication paths that differ from the above. This document does not limit to any specific rung strategy, although for interoperability in constructing/evaluating authentication paths the current draft assumes each rung is a perfect binary tree. Additional rung strategies may be defined in updates to this draft or in future drafts.

Related Work The binary rung strategy appears under different names in other cryptographic constructions based on Merkle trees. Champine defines a binary numeral tree with similar structure (the successive perfect binary subtrees are called eigentrees). Other similar Merkle tree-based constructions include Crosby and Wallach's history trees , Todd's Merkle mountain ranges , and Reyzin and Yakoubov's cryptographic accumulator .

Conventions Used in This Document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. The single pipe character, "|", is used to denote concatenation as is done in . All numeric DNSKEY elements and RRSIG elements specified in this document are unsigned integers in network byte order (big endian order).

DNSKEY Resource Records The DNSKEY format MUST be the same as in . The Algorithm field of the DNSKEY RR MUST be set to the value allocated for ML-DSA-44-MTL-SHAKE-128 (see ).

RRSIG Resource Records The value of the signature field in the RRSIG RR consists of a variable-length value starting with one-octet MTL-Type and followed by MTL Authentication Data: The MTL-Type octet is provided for extensibility. In this specification, the type MUST be set to full (0x01). The value 0x00 MUST NOT be used and is reserved for future use. Other specifications may define additional types. The MTL Authentication Data includes the signature information necessary to evaluate the MTL response and is described in .

MTL Implementation Details This section describes the data structures for the full MTL-Type in , how to instantiate the hashing algorithms in , and summarizes how this covers individual DNS RRsets in . Taken together, it allows a signer or verifier to sign and verify MTL DNSSEC signatures in a consistent, interoperable manner. and include high-level steps a signer or verifier could take to implement these concepts. The octets required in this section are based on the values for ML-DSA-44 as specified in and could be different if other underlying signature algorithms were supported.

Full Signatures This section describes the format of the full MTL-Type Authentication Data referred to in , also referred to as full signatures in this document. A full signature consists of two base components: Where:

• condensed_signature is described in
• signed_ladder is described in

Condensed Signatures A condensed MTL signature contains a Merkle proof (authentication path) and consists of: Where:

• flags, string providing future extensibility; it MUST be 0x0000 in this document
• SID, series identifier of the node set
• randomizer, randomizer value associated with leaf_index
• leaf_index, the leaf index of the message being authenticated, a non-negative integer between 0 and 2^64-1
• rung_left, the left index of the target rung, a non-negative integer between 0 and 2^64-1
• rung_right, the right index of the target rung, a non-negative integer between 0 and 2^64-1
• sibling_hash_count, the number of sibling hash values in the authentication path, a non-negative integer between 0 and 2^16-1
• sibling_hashes, zero or more sibling hash values

The SID is RECOMMENDED to be generated from an approved Random Bit Generator and the randomizer MUST be generated from an approved Random Bit Generator.

Signed Ladder A signed ladder data structure consists of: Where:

• flags, string providing future extensibility; the initial value for this field MUST be 0x0000
• SID, same value as defined in
• rung_count, the number of rungs in the ladder, a positive integer between 1 and 2^16-1
• rungs, one or more rung data structures
• sig_len, the length in octets of the underlying signature on the ladder, a positive integer between 1 and 2^32-1
• sig, the underlying signature on the ladder (i.e., flags, SID, rung_count, and rungs)

The format of the sig MUST be the same as in , however the message signed is the ladder and not the data described in 3.1.8.1 of . Each Rung Data consists of: Where:

• left_index, the left index of the rung, a non-negative integer between 0 and 2^64-1
• right_index, the right index of the rung, a non-negative integer between left_index and 2^64-1
• hash, the rung hash value

MTL Hash Function Instantiation This document specifies the use of cSHAKE128 as the hash function for use in MTL as defined in NIST SP 800-185 . For MTL the hash algorithm is utilized to generate the Merkle tree(s) that comprise a node set where the inputs to the hash function depend on whether the node is a leaf node or internal node. For a leaf node, LEAF = (SID | Randomizer | leaf_index | len(ctx_msg) | ctx_msg | msg) where:

• SID, a 32-octet value as described in
• leaf_index, an 8-octet value denoting the leaf node's index
• Randomizer, a 16-octet value as described in
• len(ctx_msg), a 1-octet value of the length of the included message-specific context string; MUST be 0x00 in this draft
• ctx_msg, variable length byte array
• msg, variable length byte array containing the message corresponding to the leaf node, i.e., RRSIG_RDATA | RR(1) | RR(2)...

The inclusion of ctx_msg is provided for future extensibility and consistency with other recent specifications that support per-message context strings. For an internal node, INTERNAL = (SID | left_index | right_index | hash_left | hash_right) where:

• SID, a 32-octet value
• left_index, an 8-octet value denoting the internal node's left index
• right_index, an 8-octet value denoting the internal node's right index
• hash_left, a 16-octet value denoting the left child hash
• hash_right, a 16-octet value denoting the right child hash

This is then applied to cSHAKE as follows to produce a cSHAKE128 output:

• H_leaf = cSHAKE(LEAF, 128, "", HASH_CUSTOMIZATION_STRING_LEAF)
• H_int = cSHAKE(INTERNAL, 128, "", HASH_CUSTOMIZATION_STRING_INT)

where HASH_CUSTOMIZATION_STRING_LEAF is the user customization string input allowed by cSHAKE and MUST be an octet string represented by "{'M','T','L','L','E','A','F'}" and HASH_CUSTOMIZATION_STRING_INT MUST be an octet string represented by "{'M','T','L','I','N','T'}".

MTL Signature Coverage MTL signatures protect the same DNS RRset responses as non-MTL DNSSEC responses because the leaf nodes correspond to the same RRSIG_RDATA | RR(1) | RR(2)... observed in 3.1.8.1 of . The primary difference is how this is covered by the full MTL-Type signature related data as described next. A full MTL-Type signature signs a ladder rather than individual DNS RRsets, i.e., signature = sign(FLAGS | SID | RUNG_COUNT | RUNG_DATA). Meanwhile, the msg input to the leaf node hash operation contains the data. This enables one signature on a signed ladder to authenticate every RRset included in that ladder rather than signing each RRset individually. A condensed signature does not provide a signature using sign(...) but provides enough information to prove inclusion in a Merkle tree node set covered by a signed ladder. The association with a full signature that has sign(...) data covers the condensed signature.

Algorithm Numbers for DS, DNSKEY, and RRSIG Resource Records The algorithm number associated with the use of ML-DSA-44-MTL-SHAKE-128 in DS, DNSKEY, and RRSIG resource records is TBD. This registration is fully defined in the IANA Considerations section.

Implementation Considerations

Batch Signing Signing RRsets in batches, i.e., as multiple additions to a node set, rather than as individual additions or as messages can leverage MTL to reduce the number of signature and verification operations performed with the underlying signature algorithm. This results in reducing the average computational overhead per message signed/verified. This practice can also reduce the load on a hardware security module. Batches also benefit the verifier by reducing the number of full signatures required for validation because multiple RRSIGs can be verified by the signed ladder covering a batch. The appropriate batch size will depend on the properties of the zone and the requirements of the zone operator. Batch size needs to be considered carefully to ensure that new signatures are available in a timely manner while still gaining the benefits of batch signing .

Online Signing MTL can accommodate online signing, also known as dynamic signing, where responses are generated dynamically at query time. How exactly this is achieved is dependent on the operator's requirements. One example, but not limiting, is to generate a new MTL node set for each query response. Implementors should be aware that online signing may limit the amortization benefits of MTL to only data within the same query.

IANA Considerations This document updates the IANA registry for DNSSEC "Domain Name System Security (DNSSEC) Algorithm Numbers" located at https://www.iana.org/assignments/dns-sec-alg-numbers/dns-sec-alg-numbers.xhtml. The following entries are requested to be added to the registry subject to the Number update:

• There has been no determination of standardization of the use of these algorithms with Transaction Security.

Implementation Status NOTE: Please remove this section and the reference to RFC 7942 prior to publication as an RFC. This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in RFC 7942. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist. According to RFC 7942, "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit". For testing purposes, ML-DSA-44-MTL-SHAKE-128 has been implemented in the following DNS open-source applications:

• LDNS for key generation, zone signing, and zone verification with MTL:
• NSD authoritative name server with MTL:
• Unbound recursive resolver with MTL:

These implementations depend on the reference implementation of MTL which is available in C. The MTL library can be found at .

Security Considerations The security considerations of and are inherited in the usage of ML-DSA-MTL in DNSSEC. ML-DSA-44-MTL-SHAKE-128 is intended to operate at around the 128-bit security level against classical attacks and the 64-bit level against quantum attacks, consistent with NIST's security level I and comparable to non-PQC DNSSEC algorithms such as ECDSAP256SHA256 (algorithm 13). Future documents may describe ML-DSA-65 or ML-DSA-87 if stronger security levels are needed. A private key used for DNSSEC MUST NOT be used for other purposes. Otherwise, cross-protocol or cross-application attacks are possible. Implementers MUST NOT use the same SID for multiple MTL instantiations, e.g., the MTL instantiation for a KSK node set and the MTL instantiation for a ZSK node set MUST use different SIDs to provide cryptographic separability. Post-quantum algorithms in DNSSEC may introduce larger keys, signatures, and/or signing/verifying effort which may lead to differences in resource capacity compared to existing DNS operational profiles. Accounting for such factors will help mitigate potential resource exhaustion attacks. The following highlights such factors as it relates to ML-DSA in MTL:

• ML-DSA signatures will require more space to store in both authoritative and caching resolvers; while condensed signatures help address this challenge at least one full ML-DSA signature will be needed.
• Additional TCP traffic compared to non-PQC DNSSEC algorithms is expected. Full MTL-Type responses in this draft will always exceed DNS over UDP.
• Clients concerned about re-tries, e.g., a UDP query being truncated and then re-attempted over TCP, are RECOMMENDED to perform all queries over TCP when encountering a zone signed with ML-DSA-44-MTL-SHAKE-128.
• Signing and verifying times in ML-DSA are competitive with existing non-PQC DNSSEC algorithms. Even so, clients should test their system's capacity to handle full signature responses signed with ML-DSA.

Acknowledgements This I-D has drawn from helpful examples of document structure and specification text from various DNSSEC algorithm RFCs. The authors express their gratitude to the authors of those RFCs for their contributions.

References Normative References National Institute of Standards and Technology (NIST) Informative References National Institute of Standards and Technology (NIST) Verisign Verisign Verisign Verisign Virginia Tech Virginia Tech

Change Log

• 00: Initial draft of the document.
• 01: Removed EDNS(0) option and specified full MTL-Type responses are the only response type for every RRset. Future drafts may define other types that leverage the same MTL data structures and wire format.

Signer Steps A signer, e.g., an authoritative server, performs the following set of operations to sign messages in MTL. The first step is performed once per key pair:

1. Generate a public / private key pair for ML-DSA-44, e.g., for use as a KSK or ZSK.

The second step is performed once per node set to be signed:

2. Generate a series identifier (SID) for the node set and initialize a node set for the series. The counter of the node set is set to 0 in this step.

The third and fourth steps are performed once per message to be signed in a node set:

3. Sample a randomizer and compute the leaf hash for the message.
4. Append the leaf hash to the node set. The counter of the node set is incremented in this step.

The fifth and sixth steps are performed whenever the signer wants to produce a new signed ladder. The signer could do so after each new message is added, or after a new batch of new messages is added.

5. Compute the current ladder for the node set using one or more rungs that collectively authenticate all the nodes in the node set.
6. Sign the ladder using the ML-DSA-44 private key generated in (1).

The seventh step is performed whenever the signer wants to provide a full signature to a requester, e.g., upon receiving a DNS query.

7. Provide the full signature as described in .

The eighth step is performed whenever the signer wants to compute a new authentication path for a message relative to the current ladder. The signer could do so after each new message is added, after a batch of new messages is added, and/or later, as needed, to update the authentication paths for older messages so that they are relative to the current ladder.

8. Compute an authentication path for the message at a specified message index relative to the current ladder.

Verifier Steps A verifier, e.g., a DNSSEC-aware resolver, performs the following to verify signatures in MTL:

1. Obtain the signer's public key, e.g., from appropriate DNSKEY records.

The following steps are performed as needed for each full MTL-Type response to be authenticated:

2. Validate the included signed ladder signature against the public key in (1).
3. From the condensed signature, obtain the authentication path (i.e., sibling hash values) and the SID.
4. Compute a leaf hash from the message as described in .
5. Verify the authentication path and leaf hash relative to the compatible rung found in (3).

A verifier may opt to cache the signed ladder of (2) separately as future responses may re-use the same signed ladder which does not need to be re-validated again. When caching, a verifier should take care to ensure the signed ladder is associated with the signer's name in case another signer uses the same SID.