]> CIRA

jesse.carter@cira.ca

CIRA

jacques.latour@cira.ca

NorthernBlock

mathieu@northernblock.io

Digital Governance Council

tim.bouma@dgc-cgn.org

next generation unicorn sparkling distributed ledger This document outlines a method for improving the authenticity, discoverability, and portability of Decentralized Identifiers (DIDs) by utilizing the current DNS infrastructure and its technologies. This method offers a straightforward procedure for a verifier to cryptographically cross-validate a DID using data stored in the DNS, separate from the DID document. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Source for this draft and an issue tracker can be found at .

Introduction In the ever-evolving digital world, the need for secure and verifiable identities is paramount. DIDs have emerged as a promising solution, providing a globally unique, persistent identifier that does not require a centralized registration authority. However, like any technology, DIDs face challenges in terms of authenticity, discoverability, and portability. This is where the Domain Name System (DNS), a well-established and globally distributed internet directory service, comes into play. By leveraging the existing DNS infrastructure, we can enhance the verification process of DIDs. Specifically, we can use Transport Layer Security Authentication (TLSA) and Uniform Resource Identifier (URI) DNS records to add an additional layer of verification and authenticity to DIDs. TLSA records in DNS allow us to associate a certificate or public key with the domain name where the record is found, thus providing a form of certificate pinning. URI records, on the other hand, provide a way to publish mappings from hostnames to URIs, such as DIDs. By storing crucial information about a DID, such as the DID itself and its Public Key Infrastructure (PKI) in these DNS records, we can provide a verifier with a simple yet effective method to cross-validate and authenticate a DID. This not only ensures the authenticity of the DID document but also allows for interaction with material signed by the DID without access to the DID document itself. In essence, the integration of DIDs with DNS, specifically through the use of TLSA and URI records, provides a robust solution to some of the challenges faced by DIDs, paving the way for a more secure and trustworthy digital identity landscape.

Conventions and Definitions 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.

Securing a DID using the DNS Much like presenting two pieces of ID to provide a higher level of assurance when proving your identity or age, replicating important information about a DID into a different domain (like the DNS) enables a similar form of cross validation. This enhances the initial trust establishment between the user and the DID document, as the key information can be compared and verified across two segregated sets of infrastructure. This also acts as a form of ownership verification in a similar way to 2FA, as the implementer must have control over both the DNS zone and the DID document to properly duplicate the relevant information. | DID | | | +-------+ | | +-------+ | | +-------+ | | +-------+ | | | PKI |<---+-----+-->| PKI | | | +-------+ | | +-------+ | | | | | +----------------+ +----------------+ ]]> The diagram above illustrates how a web server storing the DID document, and the DNS server storing the URI and TLSA records shares and links the key information about the DID accross to independant sets of infrastructure.

Specifically for did:web With did:web, there’s an inherent link between the DNS needed to resolve the associated DID document and the domain where the relevant supporting DNS records are located. This means that the domain specified by the did:web identifier (for example, did:web:example.ca) is also the location where you can find the supporting DNS records.

Other DID methods In the case of other DID methods, the association between a DID and a DNS domain is still possible although less obvious than with the aformentioned did:web. The W3C DID Core spec supports multiple ways of creating the association between a DID to a domain. This is most intuitively accomplished using one of two different fields. alsoKnownAs: The assertion that two or more DIDs (or other types of URI, such as a domain name) refer to the same DID subject can be made using the property. Services: Alternatively, are used in DID documents to express ways of communicating with the DID subject or associated entities. In this case we are referring specifically to the "LinkedDomains" service type.

DIDs with URI records However, this association stemming only from the DID is unidirectional. By leveraging URI records as outlined in , we can create a bidirectional relationship, allowing a domain to publish their associated DIDs in the DNS. Ex: _did.example-issuer.ca IN URI 1 0 “did:example:XXXXXXX” This relationship enhances security, as an entity would require control over both the DID and the domain’s DNS server to create this bidirectional association, reducing the likelihood of malicious impersonation. The ability for an organization to publish a list of their DIDs on the DNS is also beneficial as it establishes a link between the DNS, which is ubiquitously supported, and the distributed ledger (or other places) where the DID document resides on which may not have the same degree of access or support, enhancing discoverability.

URI record scoping

• The records MUST be scoped by setting the global underscore name of the URI RRset to _did (0x5F 0x64 0x69 0x64).

Issuer Handles An issuer may have multiple sub entities issuing credentials on their behalf, such as the different faculties in a university issuing diplomas. Each of these entities may have one or more DIDs of their own. For this reason, the introduction of an issuer handle, represented as a subdomain in the resource record name, provides a simple way to facilitate the distinction of DIDs, their public keys, and credentials they issue in their relationship to an issuer or root authority. Ex: _did.diplomas.example-issuer.ca IN URI 1 0 “did:example:XXXXXXX” Ex: _did.certificates.example-issuer.ca IN URI 1 0 “did:example:XXXXXXX”

PKI with TLSA records The DID to DNS mapping illustrated in section 4 provides a way of showing the association between a DID and a domain, but no way of verifying that relationship. By hosting the public keys of that DID in its related domain’s zone, we can provide a cryptographic linkage to bolster this relationship while also providing access to the DID’s public keys outside of the infrastructure where the DID document itself resides, facilitating interoperability. If a verifier is presented with a credential issued or signed by a DID using a method they do not support, they would have the option to perform the cryptographic verification of the credential's signature using the public key stored in the DNS. TLSA records provide a simple way of hosting cryptographic information in the DNS.

TLSA Record Scoping, Selector Field When public keys related to DIDs are published in the DNS as TLSA records:

• The records MUST be scoped by setting the global underscore name of the TLSA RRset to _did (0x5F 0x64 0x69 0x64).
• The Selector Field of the TLSA record must be set to 1, SubjectPublicKeyInfo: DER-encoded binary structure as defined in .

Issuer Handles As mentioned in section 4.2, an issuer may have multiple sub entities issuing credentials on their behalf, likely with their own set or sets of keypairs. Because these keypairs will need to be represented in the DNS as TLSA records, the use of an issuer handle as outlined in section 4.2 will facilitate the distinction of the different public keys in their relation to the issuer. Ex: _did.diplomas.example-issuer.ca IN TLSA 3 1 0 “4e18ac22c00fb9...b96270a7b2” Ex: _did.certificates.example-issuer.ca IN TLSA 3 1 0 “4e18ac22c00fb9...b96270a7b3”

Instances of Multiple DIDs It is also likely an issuer may be using or wish to associate multiple DIDs with a single domain or subdomain. In this case it is possible to expand the name of the RRset using both the related DID method and identifier to more clearly associate the public key and its corresponding DID. In this circumstance, we propose using another 2 additional sub names, the first following the _did global identifier denoting the method, and the second denoting the DID's id. Ex: _did.example.123abc.example-issuer.ca IN TLSA 3 1 0 “4e18ac22c00fb9...b96270a7b2” Ex: _did.example2.456abc.example-issuer.ca IN TLSA 3 1 0 “4e18ac22c00fb9...b96270a7b3”

Instances of Multiple Key Pairs Depending on the needs of the issuer, it is possible they may use multiple keypairs associated with a single DID to sign and issue credentials. In this case, a TLSA record will be created per and then be bundled into the corresponding TLSA RRset. A resolver can then parse the returned records for the corresponding verificationMethod they wish to interact with or verify. Ex: _did.example-issuer.ca IN TLSA 3 1 0 "4e18ac22c00fb9...b96270a7b4" Ex: _did.example-issuer.ca IN TLSA 3 1 0 "4e18ac22c00fb9...b96270a7b5"

Benefits of Public Keys in the DNS Hosting the public keys in TLSA records provides a stronger mechanism for the verifier to verify the issuer with, as they are able to perform a cryptographic challenge against the DID using the corresponding TLSA records, or against the domain using the corresponding in the DID document. The accessibility of the public keys is also beneficial, as the verifier does not need to resolve the DID document using a did method they do not support to access the key material. This limits the burden of having to interoperate with a multitude of different did methods and for credential verification, facilitating interoperability and adoption.

Role of DNSSEC for Assurance and Revocation It is hihgly recommended that all the participants in this digital identity ecosystem enable DNSSEC signing for the DNS instances they operate. See . DNSSEC provides cryptographic assurance that the DNS records returned in response to a query are authentic and have not been tampered with. This assurance within the context of the _did URI and _did TLSA records provides another mechanism to ensure the integrity of the DID and its public keys outside of infrastructure it resides on directly from the domain of its owner. Within this use-case, DNSSEC also provides revocation checks for both DIDs and public keys. In particular, a DNS query for a specific _did URI record or _did TLSA record can return an NXDOMAIN response if the DID or public key has been revoked. This approach can simplify the process of verifying the current validity of DIDs and public keys by reducing the need for complex revocation mechanisms or implementation specific technologies.

Digital Signature and Proof Value of the DID Document Digital signatures ensure the integrity of the DID Document, and by extent the public keys, authentication protocols, and service endpoints necessary for initiating trustworthy interactions with the identified entity. The use of digital signatures in this context provides a robust mechanism for verifying that the DID Document has not been tampered with and indeed originates from the correct entity. In accordance with W3C specifications, we propose including a data integrity proof such as those outlined in and , with the mandatory inclusions of the "created" and "expiry" fields. The inclusion of which acts as a lifespan for the document, similar to the TTL for a DNS record. Depending on the use case and security requirement, a longer or shorter expiry period would be used as necessary. javascript "proof": { "type": "DataIntegrityProof", "cryptosuite": "ecdsa-jfc-2019", "created": "2023-10-11T15:27:27Z", "expires": "2099-10-11T15:27:27Z", "verificationMethod": "did:web:trustregistry.ca#key-1", } The data integrety proof SHOULD be signed using a verificationMethod that has an associated TLSA record to allow for the verification of the data integrity proof using data contained outside of the DID document. This provides an added layer of authenticity, as the information contained in the DID document would need to be supported accross 2 different domains.

Verification Process Using the new DNS records and proof object in the DID document, we enable a more secure and higher assurance verification process for the DID. It is important to note that while not strictly necessary, DNSSEC verification should be performed each time a DNS record is resolved to ensure authenticity.

1. Initial presentation: The user is presented with a DID document, ex. did:web:example.ca.
2. Verification of the DID: The user verifies the DID is represented as a URI record in the associated domain.
   1. In the case of did:web, the domain to be queried is indicated by the last segment of the did. ex. did:web:example.ca -> _did.example.ca
   2. In the case of other did methods, the domain to be queried is indicated by the value held in the "alsoKnownAs" or "service" fields.
      1. ex. javascript {"alsoKnownAs": "example.ca"} -> _did.example.ca
      2. ex. javascript {"services": [{ "id":"did:example:123abc#linked-domain", "type": "LinkedDomains", "serviceEndpoint": "https://example.ca" -> _did.example.ca }] }
3. Verification of the PKI: With the association between the DID and the domain verified, the user would then proceed to verify the key material between the DID and the domain.
   1. The user would query for a TLSA record. Depending on the record/s returned, the user would verify either the hash of the verificationMethod or verificationMethod itself matches what was returned by the TLSA record content.
      1. Note: This may require some conversion, as TLSA records store key material as hex encoded DER format, and this representation is not supported by . However, there are many well supported cryptography libraries in a variety of languages that facilitate the conversion process.
4. Verification of the DID document's integrity: After verifying that the did's key material matches what is represented in the TLSA records of the associated domain, the user would then verify the "proof" object to ensure the integrity of the DID document.
   1. This can be accomplished by using either the directly from the did document, or using the key material stored in the TLSA record. Using the TLSA record would provide a higher level of assurance as this confirms the key material is being accurately represented accross 2 different domains, both at the DID document level and the DNS level.
   2. As mentioned above, if using the TLSA record, some conversion will be necessary to convert the DER format public key to whatever is required by the proof's cryptosuite.

Verification Failure If at any given step verification fails, the DID document should be deemed INSECURE. Whether it is due to the DID and DNS being out of sync with recent updates, or the DID document or DNS zone themselves have been compromised, it is highly advised that the user stop interacting with the given DID until verification succeeds and cross-verification is restored.

Control Requirements This section defines a simple framework to define a set of technical controls that can be implemented and mapped into levels of assurance for did:web identifiers. To assist in decision-making and implementation, The controls are ordered in increasing level of security assurance and are grouped into levels of assurance from LOW- to HIGH+

• Issuing Authority is the entity accountable for the did:web identifier.
• Issuing Service is the entity responsible for operating the did:web identifier insfrastructure.

In many cases the Issuing Authority may delegate elements of providing a high assurance did:web identitifier to an Issuing Service that may be a commercial provider. In the simplest case, the Issuing Authority can be regarded as the same as the Issuing Service. Note that Controls 9, 10, and 11 CANNOT BE DELEGATED to an Issuing Service 11 technical controls are defined. These controls would be implemented in order of precedence for an increasing level of security assurance. (e.g., Control No. N would need to be implemented before implementing Control No. N+1)

Control No.	Control Name	Description
1	DID Resource Control	The Issuing Service MUST control the resource that generates the DID document. (i.e., website)
2	DID Document Management	The Issuing Service MUST have the ability to do CRUD operations on the DID document.
3	DID Document Data Integrity	The Issuing Service MUST ensure the data integrity of the DID document by cryptographic means, typically a digital signature or other means. The use of approved or established cryptographic algorithmsis HIGHLY RECOMMENDED
4	DID Document Key Control	The Issuing Service MUST control the keys required to sign the DID document.
5	DID Document Key Generation	With proper delegation from the Issuing Authority, the DID Document signing key MAY be generated by the Issuing Service. Otherwise, the signing key must be generated by the Issuing Authority.
6	Domain Zone Control	The Issuing Service MUST have control of the domain zone (or subdomain zone).If direct control of the domain is not feasible, the use of an accredited DNS provider is HIGHLY RECOMMENDED
7	Domain Zone Mapping	There MUST be domain zone records that map the necessary URI, TLSA, CERT and/or TXT records to the specified did:web identifier.
8	Domain Zone Signing	The domain zone records MUST be signed according to DNSSEC. (RRSIG)
9	Domain Zone Signing Key Control	The Issuing Authority MUST have control over the domain zone keys used for signing and delegation. (KSK and ZSK)
10	Domain Zone Signing Key Generation	The signing keys MUST be generated under the control of the Issuing Authority.
11	Hardware Security Module	A FIPS 140-2 compliant hardware security module must be under the control of the Issuing Authority.

Levels of Assurance Many trust frameworks specify levels of assurance to assist in determing which controls must be implemented. The following table is not a definitive mapping to trust framework levels of assurance. It is intended to assist in determing mappings by grouping the controls within a range from LOW- to HIGH+ relating to the appropriate risk level. Note that controls are additive in nature. (i.e.,, controls of the preceding level must be fulfilled).

Level of Assurance	Controls	Description
LOW-	Control 1	SHOULD only be used for low risk transactions where attribution to originator is desireable.
LOW	Control 2	SHOULD only be used for lower risk transactions where establishing the accountablity of the originator is desirable.
MEDIUM	Controls 3, 4 and 5	MAY be used for medium risk commercial transactions, such as correspondence, proposals, etc.
MEDIUM+	Controls 6 and 7	MAY be used for higher risk transcations, such as signing and verifying invoices, contracts, or official/legal docmentation
HIGH	Controls 8, 9 and 10	MUST be high risk transactions, such as government transactions for signing and verifying licenses, certifications or identification
HIGH+	Control 11	MUST be used for extremely high risk transactions where there may be systemic or national security implications

Security Considerations TODO Security

IANA Considerations Per , IANA is requested to add the following entries to the "Underscored and Globally Scoped DNS Node Names" registry:

References Normative References n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Encrypted communication on the Internet often uses Transport Layer Security (TLS), which depends on third parties to certify the keys used. This document improves on that situation by enabling the administrators of domain names to specify the keys used in that domain's TLS servers. This requires matching improvements in TLS client software, but no change in TLS server software. [STANDARDS-TRACK] This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] This document describes the DNS Security Extensions (commonly called "DNSSEC") that are specified in RFCs 4033, 4034, and 4035, as well as a handful of others. One purpose is to introduce all of the RFCs in one place so that the reader can understand the many aspects of DNSSEC. This document does not update any of those RFCs. A second purpose is to state that using DNSSEC for origin authentication of DNS data is the best current practice. A third purpose is to provide a single reference for other documents that want to refer to DNSSEC. This document states clearly that when a DNS resolver receives a response with a response code of NXDOMAIN, it means that the domain name which is thus denied AND ALL THE NAMES UNDER IT do not exist. This document clarifies RFC 1034 and modifies a portion of RFC 2308: it updates both of them. Formally, any DNS Resource Record (RR) may occur under any domain name. However, some services use an operational convention for defining specific interpretations of an RRset by locating the records in a DNS branch under the parent domain to which the RRset actually applies. The top of this subordinate branch is defined by a naming convention that uses a reserved node name, which begins with the underscore character (e.g., "_name"). The underscored naming construct defines a semantic scope for DNS record types that are associated with the parent domain above the underscored branch. This specification explores the nature of this DNS usage and defines the "Underscored and Globally Scoped DNS Node Names" registry with IANA. The purpose of this registry is to avoid collisions resulting from the use of the same underscored name for different services. Informative References

W3C Considerations

1. We propose the inclusion of an optional data integrity proof for the DID document, as outlined in and .
2. We propose the inclusion of an optional TTL ("timeToLive") field in the DID document to indicate the amount of time a resolver should cache the document.

Acknowledgments TODO acknowledge.