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<!DOCTYPE html>
<html>
<head>
<title>
Decentralized Identifiers (DIDs) v0.13
</title>
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<body>
<section id='abstract'>
<p>
<a>Decentralized identifiers</a> (DIDs) are a new type of identifier for
verifiable, decentralized digital identity. These new identifiers
are designed to enable the controller of a <a>DID</a> to prove control over
it and to be implemented independently of any centralized registry,
identity provider, or certificate authority. <a>DIDs</a> are URLs that relate
a <a>DID subject</a> to means for trustable interactions with that subject.
<a>DIDs</a> resolve to <a>DID documents</a> — simple documents that describe how
to use that specific <a>DID</a>. Each <a>DID document</a> may express cryptographic
material, verification methods, and/or <a>service endpoints</a>. These provide
a set of mechanisms which enable a <a>DID controller</a> to prove control of the
<a>DID</a>. <a>Service endpoints</a> enable trusted interactions with the <a>DID
subject</a>.
</p>
<p>
This document specifies a common data model, format, and operations that
all <a>DIDs</a> support.
</p>
</section>
<section id='sotd'>
<p>
Comments regarding this document are welcome. Please file issues
directly on <a href="https://github.com/w3c-ccg/did-spec/issues/">GitHub</a>, or send them
to <a href="mailto:public-credentials@w3.org">public-credentials@w3.org</a> (
<a href="mailto:public-credentials-request@w3.org?subject=subscribe">subscribe</a>,
<a href="https://lists.w3.org/Archives/Public/public-credentials/">archives</a>).
</p>
<p>
Portions of the work on this specification have been funded by the
United States Department of Homeland Security's Science and Technology
Directorate under contracts HSHQDC-16-R00012-H-SB2016-1-002 and
HSHQDC-17-C-00019. The content of this specification does not
necessarily reflect the position or the policy of the U.S. Government
and no official endorsement should be inferred.
</p>
<p>
Work on this specification has also been supported by the Rebooting the
Web of Trust community facilitated by Christopher Allen, Shannon
Appelcline, Kiara Robles, Brian Weller, Betty Dhamers, Kaliya Young,
Kim Hamilton Duffy, Manu Sporny, Drummond Reed, Joe Andrieu, and
Heather Vescent.
</p>
</section>
<section class="informative">
<h1>
Introduction
</h1>
<p>
Conventional <a href="https://en.wikipedia.org/wiki/Identity_management">identity
management</a> systems are based on centralized authorities such as
corporate <a href="https://en.wikipedia.org/wiki/Directory_service">directory
services</a>, <a href="https://en.wikipedia.org/wiki/Certificate_authority">certificate
authorities</a>, or <a href="https://en.wikipedia.org/wiki/Domain_name_registry">domain name
registries</a>. From the standpoint of cryptographic trust
verification, each of these centralized authorities serves as its own
<a href="https://en.wikipedia.org/wiki/Trust_anchor">root of
trust</a>. To make identity management work across these systems
requires implementing <a href="https://en.wikipedia.org/wiki/Federated_identity">federated identity
management</a>.
</p>
<p>
The emergence of <a>distributed ledger technology</a> (DLT), sometimes
referred to as <a>blockchain technology</a>, provides the opportunity for
fully <a>decentralized identity management</a>. In a decentralized
identity system, entities (in the sense of discrete identifiable units
such as — but not limited to — people, organizations, and
things) are free to use any shared root of trust.
Globally distributed ledgers, decentralized P2P networks, or other systems
with similar capabilities, provide the means for managing a root of
trust without introducing a centralized authority or a single point of
failure. In combination, <a>DLTs</a> and <a>decentralized identity management</a>
systems enable any entity to create and manage their own identifiers on any
number of distributed, independent roots of trust.
</p>
<p>
Entities are identified by <a>decentralized identifiers</a> (DIDs), and
may authenticate via proofs (e.g., digital signatures,
privacy-preserving biometric protocols, etc.). <a>DIDs</a> point to <a>DID
documents</a>. A <a>DID document</a> contains a set of <a>service endpoints</a>
for interacting with the entity the <a>DID</a> identifies (aka the <a>DID
subject</a>). Following the dictums of
<a href="https://en.wikipedia.org/wiki/Privacy_by_design">Privacy by
Design</a>, any entity may have as many <a>DIDs</a> as necessary (and
corresponding <a>DID documents</a> and <a>service endpoints</a>), to respect the
entity’s desired separation of identities, personas, and contexts
(in the everyday sense of these words).
</p>
<p>
<a>DID methods</a> are the mechanism by which a <a>DID</a> and its associated <a>DID
document</a> are created, read, updated, and deactivated on a specific
<a>distributed ledger</a> or network. <a>DID methods</a> are defined using separate
<a>DID method</a> specifications.
</p>
<p>
This design eliminates dependence on centralized registries for
identifiers as well as centralized certificate authorities for key
management — the standard pattern in hierarchical
<a href="https://en.wikipedia.org/wiki/Public_key_infrastructure">PKI
(public key infrastructure</a>). In cases where the <a>DID registry</a>
is a <a>distributed ledger</a>, each entity may serve as its own root authority
— an architecture referred to as
<a href="https://github.com/WebOfTrustInfo/rebooting-the-web-of-trust/blob/master/final-documents/dpki.pdf">DPKI
(decentralized PKI)</a>.
</p>
<p>
Note that <a>DID methods</a> may also be developed for identifiers
registered in federated or centralized identity management systems.
For their part, all types of identifier systems may add support for
<a>DIDs</a>. This creates an interoperability bridge between the worlds of
centralized, federated, and <a>decentralized identifiers</a>.
</p>
<p>
The first purpose of this specification is to define the generic
<a>DID scheme</a> and a generic set of operations on <a>DID documents</a> that
can be implemented for any <a>DID registry</a>. The second
purpose of this specification is to
define the conformance requirements for a <a>DID method</a>
specification — a separate specification that defines a specific <a>DID
scheme</a> and specific set of <a>DID document</a> operations for a specific
<a>DID registry</a>.
</p>
<p class="note">
Conceptually, the relationship of this specification and a <a>DID
method</a> specification is similar to the relationship of the IETF
generic <a>URI</a> specification ([[RFC3986]]) and a specific <a>URI</a> scheme
([[IANA-URI-SCHEMES]] (such as the http: and https: schemes
specified in [[RFC7230]]). It is also similar to the relationship
of the IETF generic URN specification ([[RFC8141]]) and a specific URN
namespace definition (such as the <a>UUID</a> URN namespace defined in
[[RFC4122]]). The difference is that a <a>DID method</a> specification, in
addition to defining a specific <a>DID scheme</a>, also specifies the
methods for resolving and deactivating <a>DIDs</a> on, and writing <a>DID
documents</a> to, the appropriate <a>DID registry</a>.
</p>
<p>
The hierarchical design of a generic <a>DID</a> specification with specific <a>DID method</a>
specifications introduces some of the same concepts as the <a>URI</a> specification:
</p>
<ul>
<li>
<a>DIDs</a> from different <a>DID methods</a> may not be interoperable, just as
<a>URIs</a> from different URI schemes may not be interoperable.
</li>
<li>
Entities may need multiple <a>DIDs</a> to support different relationships, as the other
party may only support certain <a>DID methods</a>, just as some browsers may only
support certain URI schemes.
</li>
<li>
Entities may need multiple <a>DIDs</a> to support the different cryptographic schemes
of different <a>DID methods</a>, as not all parties will support the same cryptographic
schemes, just as not all browsers support the same URI schemes.
</li>
<li>
Managing multiple <a>DIDs</a>, and tracking which <a>DID</a> belongs to which relationship,
under which cryptographic scheme, introduces similar logistical challenges as
managing multiple web addresses and tracking which address belongs to which
website, or tracking which email address belongs to which relationship.
</li>
</ul>
<p>
For a list of <a>DID methods</a> and their corresponding specifications,
see the DID Method Registry [[DID-METHOD-REGISTRY]].
</p>
<section class="informative">
<h2>
A Simple Example
</h2>
<p>
A <a>DID</a> is a simple text string that consists of three parts: 1) the URL scheme
identifier (<code>did</code>), 2) the identifier for the
<a>DID method</a>, and 3) the DID method-specific identifier.
</p>
<pre class="example nohighlight" title="A simple example of a decentralized identifier (DID)">
did:example:123456789abcdefghi
</pre>
<p>
The <a>DID</a> above resolves to a <a>DID document</a>. A <a>DID document</a>
contains information associated with the <a>DID</a> such as ways to cryptographically
authenticate the entity in control of the <a>DID</a>, as well as services that can be
used to interact with the entity.
</p>
<pre class="example nohighlight" title="Minimal self-managed DID document">
{
"@context": "https://www.w3.org/2019/did/v1",
"id": "did:example:123456789abcdefghi",
"authentication": [{
<span class="comment">// used to authenticate as did:...fghi</span>
"id": "did:example:123456789abcdefghi#keys-1",
"type": "RsaVerificationKey2018",
"controller": "did:example:123456789abcdefghi",
"publicKeyPem": "-----BEGIN PUBLIC KEY...END PUBLIC KEY-----\r\n"
}],
"service": [{
<span class="comment">// used to retrieve Verifiable Credentials associated with the DID</span>
"id":"did:example:123456789abcdefghi#vcs",
"type": "VerifiableCredentialService",
"serviceEndpoint": "https://example.com/vc/"
}]
}
</pre>
</section>
<section class="informative">
<h2>
Design Goals
</h2>
<p>
<a>Decentralized Identifiers</a> are a component of larger systems, such as
the Verifiable Credentials ecosystem [[?VC-DATA-MODEL]], which have driven
the design goals for this specification. This section summarizes the
primary design goals for this specification.
</p>
<table class="simple">
<thead>
<tr>
<th>
Goal
</th>
<th>
Description
</th>
</tr>
</thead>
<tbody>
<tr>
<td>
Decentralization
</td>
<td>
Eliminate the requirement for
centralized authorities or single points of failure in
identifier management, including the registration of globally
unique identifiers, public verification keys, <a>service
endpoints</a>, and other metadata.
</td>
</tr>
<tr>
<td>
Control
</td>
<td>
Give entities, both human and
non-human, the power to directly control their digital
identifiers without the need to rely on external authorities.
</td>
</tr>
<tr>
<td>
Privacy
</td>
<td>
Enable entities to control the privacy
of their information, including minimal, selective, and
progressive disclosure of attributes or other data.
</td>
</tr>
<tr>
<td>
Security
</td>
<td>
Enable sufficient security for relying
parties to depend on <a>DID documents</a> for their required level of
assurance.
</td>
</tr>
<tr>
<td>
Proof-based
</td>
<td>
Enable the <a>DID subject</a> to provide
cryptographic proof when interacting with other entities.
</td>
</tr>
<tr>
<td>
Discoverability
</td>
<td>
Make it possible for entities to
discover <a>DIDs</a> for other entities to learn more about or
interact with those entities.
</td>
</tr>
<tr>
<td>
Interoperability
</td>
<td>
Use interoperable standards so <a>DID</a>
infrastructure can make use of existing tools and software
libraries designed for interoperability.
</td>
</tr>
<tr>
<td>
Portability
</td>
<td>
Be system and network-independent and
enable entities to use their digital identifiers with any
system that supports <a>DIDs</a> and <a>DID methods</a>.
</td>
</tr>
<tr>
<td>
Simplicity
</td>
<td>
Favor a reduced set of simple features in order to make the technology
easier to understand, implement, and deploy.
</td>
</tr>
<tr>
<td>
Extensibility
</td>
<td>
When possible, enable extensibility
provided it does not greatly hinder interoperability,
portability, or simplicity.
</td>
</tr>
</tbody>
</table>
</section>
<section>
<h3>Interoperability</h3>
<p>Interoperability of implementations for <a>DIDs</a> and <a>DID documents</a> will be tested by evaluating an implementation's ability to create and parse <a>DIDs</a> and <a>DID documents</a> that conform to the specification. Interoperability for <a>DID methods</a> will be determined by evaluating each <a>DID method</a>'s specification to determine, at a minimum, </p>
<ol>
<li> the <a>DID method</a> name is unique and not used by an existing, incompatible <a>DID method</a>, </li>
<li> the required operations are supported,</li>
<li> operations requiring descriptions are described,</li>
<li> the specification is specific, detailed, and complete enough for independent implementation, and</li>
<li> the specification contains sections describing security and privacy considerations.</li>
</ol>
<p>Interoperability for producers and consumers of <a>DIDs</a> and <a>DID documents</a> is provided by ensuring the <a>DIDs</a> and <a>DID documents</a> conform. Interoperability for method specifications is provided by the details in each method specification. It is understood that, just like a web browser is not required to implement all known URI schemes, conformant software that works with <a>DIDs</a> is not required to implement all known <a>DID methods</a>. However, all implementations of a given <a>DID method</a> must be interoperable for that method.</p>
</section>
</section>
</section>
<section class="informative">
<h1>
Terminology
</h1>
<div data-include="terms.html" data-oninclude="restrictReferences">
</div>
</section>
<section class="informative">
<h1>
Data Model
</h1>
<p>
This section outlines the <a>decentralized identifier</a> data model concepts,
in particular how keys, services, and the <a>DID subject</a> are related to the
<a>DID document</a>.
</p>
<p>
For information about how the data model can be extended, see
<a href="#extensibility"></a>.
</p>
<section>
<h2>
Identifier
</h2>
<p>
Identifiers are used in the data model to identify specific instances of people,
organizations, devices, keys, services, and things in general. Identifiers are
typically URLs, or more generally, <a>URIs</a>. Non-URI based identifiers are not
advised as, while the data model supports them, they tend to not be easy to
use with other Internet-based identifiers.
</p>
</section>
<section>
<h2>
DID Document
</h2>
<p>
A <a>DID document</a> is the resource that is associated with a <a>decentralized
identifier</a> (DID). The <a>DID document</a> typically expresses verification methods (such
as public keys) and services that can be used to interact with a
<a>DID controller</a>.
</p>
<p>
A <a>DID document</a> is serialized according to a particular syntax (see
<a href="#did-document-syntax"></a>). The <a>DID</a> itself is
contained in the <code>id</code> property.
</p>
<p>
The properties that can be present in a <a>DID document</a> are detailed
further in <a href="#did-documents"></a>.
</p>
<p>
The properties present in a <a>DID document</a> can be updated according to
the applicable <a href="#did-methods"></a>.
</p>
</section>
<section>
<h2>
Cryptographic Keys and Other Verification Methods
</h2>
<p>
A <a>DID document</a> can express cryptographic keys and other verification methods
that can be used to authenticate or authorize interactions with the <a>DID subject</a>
or associated parties. The information expressed often includes globally
unambiguous identifiers and public key material which can be used to verify
digital signatures. Other information can be expressed such as status
information for the key, such as if it has been suspended or revoked, or other
attributes that enable one to determine whether it is a hardware-backed
cryptographic key.
</p>
<p>
With respect to cryptographic key material, <a href="#public-keys"></a> can be
included in a <a>DID document</a> using, for example, the <code>publicKey</code>
or <code>authentication</code> properties depending on what they are to be used
for. Each public key has an identifier (<code>id</code>) of its own, a
<code>type</code>, and a <code>controller</code>, as well as other properties
which depend on what type of key it is.
</p>
</section>
<section>
<h2>
Services
</h2>
<p>
<a>Service endpoints</a> are used in <a>DID documents</a> to express ways in which one can
communicate with the <a>DID subject</a> or associated entities. Services listed
in the <a>DID document</a> can contain information about privacy preserving messaging
services, or more public information such as social media accounts, personal
websites, and email addresses. The metadata associated with services are often
service specific. For example, the metadata associated with an encrypted
messaging service can express how to initiate the encrypted link before
messaging begins.
</p>
<p>
Pointers to services are expressed using the <code>service</code> property.
Each service has its own <code>id</code> and <code>type</code>, as well as a
<code>serviceEndpoint</code> with a <a>URI</a> or further properties describing the
service.
</p>
<p>
For more information see <a href="#service-endpoints"></a>.
</p>
</section>
</section>
<section class="normative">
<h1>
Decentralized Identifiers (DIDs)
</h1>
<p>
The concept of a globally unique <a>decentralized identifier</a> is not
new; <a>Universally
Unique Identifiers</a> (UUIDs) were first developed in the 1980s and
later became a standard feature of the Open Software Foundation’s
<a href="https://en.wikipedia.org/wiki/Distributed_Computing_Environment">Distributed
Computing Environment</a>. <a>UUIDs</a> achieve global uniqueness without a
centralized registry service by using an algorithm that generates
128-bit values with sufficient entropy that the chance of collision are
infinitesimally small. <a>UUIDs</a> are formally specified in [[RFC4122]] as a
specific type of Unified Resource Name (URN).
</p>
<p>
A <a>DID</a> is similar to a <a>UUID</a> except: (a) like a URL, it can be resolved
or dereferenced to a standard resource describing the subject (a <a>DID
document</a> — see Section <a href="#did-documents"></a>), and (b) unlike
a URL, the <a>DID document</a> typically contains cryptographic material that
enables authentication of the <a>DID subject</a>.
</p>
<section>
<h2>
Generic DID Syntax
</h2>
<p>
The generic <a>DID scheme</a> is a URI scheme conformant with
[[!RFC3986]]. The <a>DID scheme</a> specializes only the scheme and
authority components of a <a>DID URL</a> — the <code>path-abempty</code>,
<code>query</code>, and <code>fragment</code> components are
identical to the ABNF rules defined
in [[!RFC3986]].
</p>
<p class="note">
The term <a>DID</a> refers only to the <a>URI</a>
conforming to the <code>did</code> rule in the ABNF below. A
<a>DID</a> always identifies the <a>DID subject</a>. The term <a>DID URL</a>,
defined by the <code>did-url</code> rule,
refers to a URL that begins with a <a>DID</a> followed by one or more
additional components. A <a>DID URL</a> always identifies the resource to
be located.
</p>
<p>
The following is the ABNF definition using the syntax in [[!RFC5234]]
which defines <code>ALPHA</code> and <code>DIGIT</code>. All other
rule names not defined in this ABNF are defined in [[RFC3986]].
</p>
<pre class="nohighlight">
did = "did:" method-name ":" method-specific-id
method-name = 1*method-char
method-char = %x61-7A / DIGIT
method-specific-id = *idchar *( ":" *idchar )
idchar = ALPHA / DIGIT / "." / "-" / "_"
did-url = did *( ";" param ) path-abempty [ "?" query ]
[ "#" fragment ]
param = param-name [ "=" param-value ]
param-name = 1*param-char
param-value = *param-char
param-char = ALPHA / DIGIT / "." / "-" / "_" / ":" /
pct-encoded
</pre>
<p class="issue" data-number="34">
The grammar currently allows an empty <code>method-specific-id</code>,
e.g., <code>did:example:</code> would be a valid <a>DID</a> that could identify
the <a>DID method</a> itself.
</p>
</section>
<section>
<h2>
Method-Specific Syntax
</h2>
<p>
A <a>DID method</a> specification MUST further restrict the generic <a>DID</a>
syntax by defining its own <code>method-name</code> and its own
<code>method-specific-id</code> syntax. See Section <a href="#did-methods"></a>.
</p>
</section>
<section>
<h2>
Generic DID Parameter Names
</h2>
<p>
<a>DID URL</a> syntax supports a simple, generalized format for parameters based on the
matrix parameter syntax ([[MATRIX-URIS]]).
The ABNF above does not specify any parameter names (the <code>param-name</code>
rule).
</p>
<p>
Some generic DID parameter names (e.g., for service selection) are completely
independent of any specific <a>DID method</a> and MUST always function the same way
for all <a>DIDs</a>.
Others (e.g., for versioning) MAY be supported by
certain <a>DID methods</a>, but MUST operate uniformly across those <a>DID methods</a>
that do support them.
</p>
<p>
Parameter names that are completely method-specific are covered in
<a href="#method-specific-did-parameter-names"></a>.
</p>
<p>
The following table defines a set of generic DID parameter names:
</p>
<table class="simple">
<thead>
<tr>
<th>Generic DID Parameter Name</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td>
<code>hl</code>
</td>
<td>
A resource hash of the <a>DID document</a> to add integrity
protection, as specified in [[HASHLINK]].
</td>
</tr>
<tr>
<td>
<code>service</code>
</td>
<td>
Identifies a service from the <a>DID document</a> by service ID.
</td>
</tr>
<tr>
<td>
<code>version-id</code>
</td>
<td>
Identifies a specific version of a <a>DID document</a> to be resolved (the version ID
could be sequential, or a <a>UUID</a>, or method-specific). Note: This parameter may
not be supported by all <a>DID methods</a>.
</td>
</tr>
<tr>
<td>
<code>version-time</code>
</td>
<td>
Identifies a certain version timestamp of a <a>DID document</a> to be resolved
(i.e., the <a>DID document</a> that was valid for a <a>DID</a> at a certain time).
Note: This parameter may not be supported by all <a>DID methods</a>.
</td>
</tr>
</tbody>
</table>
<p>
The exact processing rules for these parameters are specified in [[DID-RESOLUTION]].
</p>
<p class="note">
Note that there may be additional parameters or options that are not part
of the <a>DID URL</a> but instead passed to a DID resolver "out of band", i.e.,
using a resolution protocol or some other mechanism. Such options could
for example control caching or the desired format of a resolution result.
This is similar to HTTP, where caching or result format are expressed as
HTTP headers rather than as part of an HTTP URL. The important distinction
is that DID parameters that are part of the <a>DID URL</a> specify what resource
is being identified, whereas DID resolver options that are not part of the
<a>DID URL</a> control how that resource is dereferenced.
</p>
</section>
<section>
<h2>
Method-Specific DID Parameter Names
</h2>
<p>
A <a>DID method</a> specification MAY specify additional method-specific parameter
names. A method-specific parameter name MUST be prefixed by the method name
as defined by the <code>method-name</code> rule.
</p>
<p>
For example, if the method <code>did:foo:</code> defines the parameter bar,
the parameter name must be <code>foo:bar</code>. An example <a>DID URL</a> using
this method and this method-specific parameter would be:
</p>
<p>
<code>did:foo:21tDAKCERh95uGgKbJNHYp;foo:bar=high</code>
</p>
<p class="issue" data-number="35">
Consider using kebab-case style instead of colon separator,
e.g., <code>foo-bar</code> instead of <code>foo:bar</code>.
</p>
<p>
A method-specific parameter name defined by one <a>DID method</a> MAY
be used by other <a>DID methods</a>. For example:
</p>
<p>
<code>did:example:21tDAKCERh95uGgKbJNHYp;foo:bar=low</code>
</p>
<p>
Method-specific parameter names MAY be combined with generic parameter
names in any order.
</p>
<p>
<code>did:example:21tDAKCERh95uGgKbJNHYp;service=agent;foo:bar=high</code>
</p>
<p>
Both <a>DID method</a> namespaces and method-specific parameter
namespaces MAY include colons, so they may be partitioned hierarchically
as defined by a <a>DID method</a> specification. Here is an example <a>DID
URL</a> that illustrates both:
</p>
<p>
<code>did:foo:baz:21tDAKCERh95uGgKbJNHYp;foo:baz:hex=b612</code>
</p>
</section>
<p class="issue" data-number="36">
Review what exactly we want to say about method-specific parameters
defined by one method but used in a <a>DID URL</a> with a different method.
Also discuss hierarchical method namespaces in DID parameter names.
</p>
<section>
<h2>
Path
</h2>
<p>
A generic <a>DID path</a> is identical to a URI path and MUST
conform to the <code>path-abempty</code> ABNF rule in [[!RFC3986]]. A
<a>DID path</a> SHOULD be used to address resources available via a
<a>service endpoint</a>. See Section <a href="#service-endpoints"></a>.
</p>
<p>
A specific <a>DID scheme</a> MAY specify ABNF rules for <a>DID paths</a> that are
more restrictive than the generic rules in this section.
</p>
<pre class="example nohighlight">
did:example:123456/path
</pre>
</section>
<section>
<h2>
Query
</h2>
<p>
A generic <a>DID query</a> is identical to a URI query and MUST
conform to the <code>query</code> ABNF rule in [[!RFC3986]]. A
<a>DID query</a> SHOULD be used to address resources available via a
<a>service endpoint</a>. See Section <a href="#service-endpoints"></a>.
</p>
<p>
A specific <a>DID scheme</a> MAY specify ABNF rules for <a>DID queries</a> that are
more restrictive than the generic rules in this section.
</p>
<pre class="example nohighlight">
did:example:123456?query=true
</pre>
</section>
<section>
<h2>
Fragment
</h2>
<p>
A generic <a>DID fragment</a> is identical to a URI
fragment and MUST conform to the <code>fragment</code> ABNF rule in
[[RFC3986]]. Implementers are strongly discouraged from using a <a>DID fragment</a>
for anything other than a method-independent
reference into the <a>DID document</a> to identify a component of a <a>DID
document</a> (e.g. a unique <a>public key description</a> or <a>service endpoint</a>). To resolve
this reference, the complete <a>DID URL</a> including the <a>DID fragment</a> MUST be used as
input to the <a>DID URL</a> dereferencing algorithm (see [[DID-RESOLUTION]]) for the
target component in the <a>DID document</a> object.
</p>
<p>
A specific <a>DID scheme</a> MAY specify ABNF rules for <a>DID fragments</a> that
are more restrictive than the generic rules in this section.
</p>
<p>
It is desirable that we enable tree-based processing of <a>DIDs</a> that include
<a>DID fragments</a> (which resolve directly within the <a>DID document</a>) to locate
metadata contained directly in the <a>DID document</a> or the service resource
given by the target URL without needing to rely on graph-based processing.
</p>
<p>
Implementations SHOULD NOT prevent the use of <a>JSON Pointer</a> ([[!RFC6901]]).
</p>
<pre class="example nohighlight">
did:example:123456#oidc
</pre>
</section>
<section>
<h2>
Normalization
</h2>
<p>
For the broadest interoperability, <a>DID</a> normalization should be as
simple and universal as possible. Therefore:
</p>
<ol start="1">
<li>
The did: scheme name MUST be lowercase.
</li>
<li>
The method name MUST be lowercase.
</li>
<li>
Case sensitivity and normalization of the value of the
<code>method-specific-id</code> rule in Section <a href="#generic-did-syntax">
</a> MUST be defined by the governing <a>DID method</a> specification.
</li>
</ol>
</section>
<section>
<h2>
Persistence
</h2>
<p>
A <a>DID</a> is expected to be persistent and immutable, i.e., bound exclusively
and permanently to its one and only subject. Even after a <a>DID</a> has been
deactivated, it is intended that it never be repurposed.
</p>
<p>
Ideally a <a>DID</a> would be a completely
abstract decentralized identifier (like a <a>UUID</a>) that could be bound
to multiple underlying <a>DID registries</a> over time,
thus maintaining its persistence independent of any particular system.
However registering the same identifier on multiple
<a>DID registries</a> introduces extremely hard entityship and
<a href="https://en.wikipedia.org/wiki/List_of_DNS_record_types%23SOA">start-of-authority</a>
(SOA) problems. It also greatly increases implementation complexity
for developers.