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<body>
<section id='abstract'>
<p>
A [=controller document=] is a set of data that specifies one or more
relationships between a [=controller=] and a set of data, such as a set of
public cryptographic keys.
</p>
</section>
<section id='sotd'>
</section>
<section class="informative">
<h2>Introduction</h2>
<p>
A [=controller document=] is a set of data that specifies one or more
relationships between a [=controller=] and a set of data, such as a set of
public cryptographic keys. The [=controller document=] SHOULD
contain [=verification relationships=] that explicitly permit the use of
certain [=verification methods=] for specific purposes.
</p>
<p>
It is expected that other specifications using this specification
will profile the features that it defines,
requiring and/or recommending the use of some and
prohibiting and/or deprecating the use of others.
</p>
<section id="conformance">
<p>
A <dfn>conforming controller document</dfn> is any concrete expression of the
data model that follows the relevant normative requirements in Sections
[[[#data-model]]] and [[[#contexts-and-vocabularies]]].
</p>
<p>
A <dfn>conforming verification method</dfn> is any concrete expression of the
data model that follows the relevant normative requirements in Sections
[[[#verification-methods]]] and
[[[#contexts-and-vocabularies]]].
</p>
<p>
A <dfn class="lint-ignore">conforming document</dfn> is either a
[=conforming controller document=], or a
[=conforming verification method=].
</p>
<p>
A <dfn class="lint-ignore">conforming processor</dfn> is any algorithm realized
as software and/or hardware that generates and/or consumes a
[=conforming document=] according to the relevant normative statements in
Section [[[#algorithms]]]. Conforming processors MUST produce errors
when non-conforming documents are consumed.
</p>
</section>
<section class="informative">
<h3>Terminology</h3>
<p>
This section defines the terms used in this specification. A link to the relevant
definition is included whenever one of these terms appears in this specification.
</p>
<dl class="termlist definitions" data-sort="ascending">
<dt><dfn>public key</dfn></dt>
<dd>
Cryptographic material that can be used to verify digital proofs created with a
corresponding [=private key=].
</dd>
<dt><dfn>private key</dfn></dt>
<dd>
Cryptographic material that can be used to generate digital proofs.
</dd>
<dt><dfn data-lt="authenticated|authenticate">authentication</dfn></dt>
<dd>
A process by which an entity can prove to a verifier that it has a specific
attribute or controls a specific secret.
</dd>
<dt><dfn class="export" data-lt="cryptosuite">cryptographic suite</dfn></dt>
<dd>
A specification defining how to use specific cryptographic primitives
to achieve a particular security goal. These documents are often used
to specify [=verification methods=], digital signature types,
their identifiers, and other related properties.<!-- See Section -->
<!-- [[[#cryptographic-suites]]] for further detail. -->
</dd>
<dt><dfn class="export" data-lt="controller(s)|Controllers">controller</dfn></dt>
<dd>
An entity that has the capability to make changes to a
[=controller document=].
</dd>
<dt><dfn class="export">controller document</dfn></dt>
<dd>
A set of data that specifies one or more relationships between a
[=controller=] and a set of data, such as a set of public cryptographic keys.
</dd>
<dt><dfn data-lt="subjects">subject</dfn></dt>
<dd>
The entity identified by the `id` property in a [=controller document=].
Anything can be a subject: person, group, organization, physical thing, digital
thing, logical thing, etc.
</dd>
<dt><dfn class="export">verification method</dfn></dt>
<dd>
<p>
A set of parameters that can be used together with a process to independently
verify a proof. For example, a cryptographic public key can be used as a
verification method with respect to a digital signature; in such use, it
verifies that the signer possessed the associated cryptographic private key.
</p>
<p>
"Verification" and "proof" in this definition are intended to apply broadly. For
example, a cryptographic public key might be used during Diffie-Hellman key
exchange to negotiate a shared symmetric key for encryption. This guarantees the
integrity of the key agreement process. It is thus another type of verification
method, even though descriptions of the process might not use the words
"verification" or "proof."
</p>
</dd>
<dt><dfn class="export">verification relationship</dfn></dt>
<dd>
<p>
An expression of the relationship between a [=subject=] and a
[=verification method=]. One example of a verification relationship is
[[[#authentication]]].
</p>
</dd>
</dl>
</section>
</section>
<section>
<h3>Data Model</h3>
<p>
A [=controller document=] is a set of data that specifies one or more
relationships between a [=controller=] and a set of data, such as a set of
public cryptographic keys. The [=controller document=] SHOULD
contain [=verification relationships=] that explicitly permit the use of
certain [=verification methods=] for specific purposes.
</p>
<div class="issue">
Add examples of common Controller documents.
</div>
<p class="note"
title="Property names used in map of different types">
The property names `id`, `type`, and `controller` can be present in map of
different types with possible differences in constraints.
</p>
<section>
<h2>Controller Documents</h2>
<p>
The following sections define the properties in a [=controller document=],
including whether these properties are required or optional. These properties
describe relationships between the [=subject=] and the value of the
property.
</p>
<p>
The following tables contain informative references for the core properties
defined by this specification, with expected values, and whether or not they are
required. The property names in the tables are linked to the normative
definitions and more detailed descriptions of each property.
</p>
<table class="simple">
<thead>
<tr>
<th>Property</th>
<th>Required?</th>
<th>Value constraints</th>
</tr>
</thead>
<tbody>
<tr>
<td>`id`</td>
<td>yes</td>
<td>
A <a data-cite="INFRA#string">string</a> that conforms to the URL syntax
defined in Section [[[#subjects]]].
</td>
</tr>
<tr>
<td>`alsoKnownAs`</td>
<td>no</td>
<td>
A <a data-cite="INFRA#ordered-set">set</a> of
<a data-cite="INFRA#string">strings</a> that conform to the rules of
[[RFC3986]] for URIs as defined in Section [[[#also-known-as]]].
</td>
</tr>
<tr>
<td>`controller`</td>
<td>no</td>
<td>
A <a data-cite="INFRA#string">string</a> or a
<a data-cite="INFRA#ordered-set">set</a> of
<a data-cite="INFRA#string">strings</a>, each of which conforms to the URL syntax
defined in the [[[URL]]] as defined in Section [[[#controllers]]].
</td>
</tr>
<tr>
<td>`verificationMethod`</td>
<td>no</td>
<td>
A <a data-cite="INFRA#ordered-set">set</a> of
[=verification method=] <a data-cite="INFRA#ordered-map">maps</a>
that conform to the rules in Section [[[#verification-methods]]].
</td>
</tr>
<tr>
<td>`authentication`</td>
<td>no</td>
<td>
A <a data-cite="INFRA#ordered-set">set</a> of
[=verification method=] <a data-cite="INFRA#ordered-map">maps</a>
that conform to the rules in Section [[[#authentication]]]; or a
a <a data-cite="INFRA#ordered-set">set</a> of
<a data-cite="INFRA#string">strings</a>, each of which conforms to the URL syntax
defined in the [[[URL]]].
</td>
</tr>
<tr>
<td>`assertionMethod`</td>
<td>no</td>
<td>
A <a data-cite="INFRA#ordered-set">set</a> of
[=verification method=] <a data-cite="INFRA#ordered-map">maps</a>
that conform to the rules in Section [[[#assertion]]]; or a
a <a data-cite="INFRA#ordered-set">set</a> of
<a data-cite="INFRA#string">strings</a>, each of which conforms to the URL syntax
defined in the [[[URL]]].
</td>
</tr>
<tr>
<td>`keyAgreement`</td>
<td>no</td>
<td>
A <a data-cite="INFRA#ordered-set">set</a> of
[=verification method=] <a data-cite="INFRA#ordered-map">maps</a>
that conform to the rules in Section [[[#key-agreement]]]; or a
a <a data-cite="INFRA#ordered-set">set</a> of
<a data-cite="INFRA#string">strings</a>, each of which conforms to the URL syntax
defined in the [[[URL]]].
</td>
</tr>
<tr>
<td>`capabilityInvocation`</td>
<td>no</td>
<td>
A <a data-cite="INFRA#ordered-set">set</a> of
[=verification method=] <a data-cite="INFRA#ordered-map">maps</a>
that conform to the rules in Section [[[#capability-invocation]]]; or a
a <a data-cite="INFRA#ordered-set">set</a> of
<a data-cite="INFRA#string">strings</a>, each of which conforms to the URL syntax
defined in the [[[URL]]].
</td>
</tr>
<tr>
<td>`capabilityDelegation`</td>
<td>no</td>
<td>
A <a data-cite="INFRA#ordered-set">set</a> of
[=verification method=] <a data-cite="INFRA#ordered-map">maps</a>
that conform to the rules in Section [[[#capability-delegation]]]; or a
a <a data-cite="INFRA#ordered-set">set</a> of
<a data-cite="INFRA#string">strings</a>, each of which conforms to the URL syntax
defined in the [[[URL]]].
</td>
</tr>
</tbody>
</table>
<section>
<h3>Subjects</h3>
<p>
The identifier for a particular [=subject=] is expressed using the
`id` property in the [=controller document=].
</p>
<dl>
<dt>id</dt>
<dd>
The value of `id` MUST be a <a data-cite="INFRA#string">string</a>
that conforms to the rules in the [[[URL]]].
</dd>
</dl>
<p>
A [=controller document=] MUST contain an `id` value in the
root <a data-cite="INFRA#ordered-map">map</a>.
</p>
<pre class="example nohighlight">
{
"id": "https://controller.example/123"
}
</pre>
<p>
The `id` property only denotes the identifier of the [=subject=] when it is
present in the <em>topmost</em>
<a data-cite="INFRA#ordered-map">map</a> of the [=controller document=].
</p>
</section>
<section>
<h3>Controllers</h3>
<p>
A [=controller=] is an entity that is authorized to make changes to a
[=controller document=].
</p>
<dl>
<dt>controller</dt>
<dd>
The `controller` property is OPTIONAL. If present, its value MUST
be a <a data-cite="INFRA#string">string</a> or a <a
data-cite="INFRA#ordered-set">set</a> of <a
data-cite="INFRA#string">strings</a>, each of which conforms to the rules in
the [[[URL]]]. The corresponding [=controller document=](s) SHOULD
contain [=verification relationships=] that explicitly permit the use of
certain [=verification methods=] for specific purposes. If the `controller`
property is not present, the value expressed by the `id` property MUST be
treated as if it were also set as the value of the `controller` property.
</dd>
</dl>
<p>
When a `controller` property is present in a [=controller document=], its
value expresses one or more identifiers. Any [=verification methods=] contained
in the [=controller documents=] for those identifiers SHOULD
be accepted as authoritative, such that proofs that satisfy those
[=verification methods=] are considered equivalent to proofs provided
by the [=subject=].
</p>
<pre class="example nohighlight"
title="Controller document with a controller property">
{
"@context": "https://www.w3.org/ns/controller/v1",
"id": "https://controller1.example/123",
"controller": "https://controllerB.example/abc",
}
</pre>
<p class="note" title="Authorization vs authentication">
Note that authorization provided by the value of `controller` is
separate from authentication as described in Section [[[#authentication]]].
This is particularly important for key recovery in the cases of cryptographic key
loss, where the [=subject=] no longer has access to their keys, or cryptographic
key compromise, where the [=controller=]'s trusted third parties need to
override malicious activity by an attacker. See [[[#security-considerations]]]
for information related to threat models and attack vectors.
</p>
</section>
<section>
<h3>Also Known As</h3>
<p>
A [=subject=] can have multiple identifiers that are used for different purposes
or at different times. The assertion that two or more identifiers (or other types
of URI) refer to the same [=subject=] can be made using the
`alsoKnownAs` property.
</p>
<dl>
<dt>alsoKnownAs</dt>
<dd>
The `alsoKnownAs` property is OPTIONAL. If present, its value MUST
be a <a data-cite="INFRA#ordered-set">set</a> where each item in the
set is a URI conforming to [[RFC3986]].
</dd>
<dd>
This relationship is a statement that the subject of this identifier is
also identified by one or more other identifiers.
</dd>
</dl>
<div class="note" title="Equivalence and alsoKnownAs">
<p>
Applications might choose to consider two identifiers related by `alsoKnownAs`
to be equivalent <em>if</em> the `alsoKnownAs` relationship expressed in the
controller document of one [=subject=] is also expressed in the reverse direction
(i.e., reciprocated) in the controller document of the other [=subject=]. It is
best practice <em>not</em> to consider them
equivalent in the absence of this reciprocating relationship. In other words,
the presence of an `alsoKnownAs` assertion does not prove that this assertion
is true. Therefore, it is strongly advised that a requesting party obtain
independent verification of an `alsoKnownAs` assertion.
</p>
<p>
Given that the [=subject=] might use different identifiers for different
purposes, such as enhanced privacy protection, an expectation of strong
equivalence between the two identifiers, or taking action to
merge the information from the two corresponding [=controller documents=], is
not necessarily appropriate, <em>even with</em> a reciprocal relationship.
</p>
</div>
</section>
</section>
<section>
<h2>Verification Methods</h2>
<p>
A [=controller document=] can express [=verification methods=], such as
cryptographic [=public keys=], which can be used to [=authenticate=] or
authorize interactions with the [=controller=] or associated parties. For
example, a cryptographic [=public key=] can be used as a <a>verification
method</a> with respect to a digital signature; in such use, it verifies that
the signer could use the associated cryptographic private key. <a>Verification
methods</a> might take many parameters. An example of this is a set of five
cryptographic keys from which any three are required to contribute to a
cryptographic threshold signature.
</p>
<dl>
<dt><dfn class="lint-ignore">verificationMethod</dfn></dt>
<dd>
<p>
The `verificationMethod` property is OPTIONAL. If present, the value
MUST be a <a data-cite="INFRA#ordered-set">set</a> of <a>verification
methods</a>, where each [=verification method=] is expressed using a <a
data-cite="INFRA#ordered-map">map</a>. The [=verification method=] <a
data-cite="INFRA#ordered-map">map</a> MUST include the `id`,
`type`, `controller`, and specific verification material
properties that are determined by the value of `type` and are defined
in [[[#verification-material]]]. A [=verification method=] MAY
include additional properties.
<!-- VC-DATA-INTEGRITY-SPECIFIC:
[=Verification methods=] SHOULD be registered
in the Data Integrity Specification Registries [TBD - DIS-REGISTRIES].
-->
</p>
<dl>
<dt>id</dt>
<dd>
<p>
The value of the `id` property for a [=verification method=]
MUST be a <a data-cite="INFRA#string">string</a> that
conforms to the [[URL]] syntax.
</p>
</dd>
<dt>type</dt>
<dd>
The value of the `type` property MUST be a <a
data-cite="INFRA#string">string</a> that references exactly one <a>verification
method</a> type.
<!-- VC-DATA-INTEGRITY-SPECIFIC:
In order to maximize global interoperability, the
[=verification method=] type SHOULD be registered in the Data Integrity Specification
Registries [TBD -- DIS-REGISTRIES].
-->
<!-- VC-JOSE-COSE-SPECIFIC:
To maximize interoperability, the
[=verification method=] type SHOULD be `JsonWebKey`.
-->
</dd>
<dt><span id="defn-controller">controller</span></dt>
<dd>
The value of the `controller` property MUST be a <a
data-cite="INFRA#string">string</a> that conforms to the [[URL]] syntax.
</dd>
<!-- VC-DATA-INTEGRITY-SPECIFIC:
<dt id="defn-vm-expires">expires</dt>
<dd>
The `expires` property is OPTIONAL. It is set, in advance, by the
[=controller=] of a [=verification method=] to signal when that method
can no longer be used for verification purposes. If provided, it MUST be an
[[XMLSCHEMA11-2]] `dateTimeStamp` string specifying when the
[=verification method=] SHOULD cease to be used. Once the value is set, it is
not expected to be updated, and systems depending on the value are expected to
not verify any proofs associated with the [=verification method=] at or after
the time of expiration.
</dd>
-->
<dt><dfn class="lint-ignore">revoked</dfn></dt>
<dd>
The `revoked` property is OPTIONAL.
<!-- VC-DATA-INTEGRITY-SPECIFIC:
It is set by the [=controller=] of a
[=verification method=] to signal when that method is to no longer to be used
for verification purposes, such as after a security compromise of the
[=verification method=].
-->
If present, it MUST be an [[XMLSCHEMA11-2]]
`dateTimeStamp` string specifying when the [=verification method=]
SHOULD cease to be used. Once the value is set, it is not expected to be
updated, and systems depending on the value are expected to not verify any
proofs associated with the [=verification method=] at or after the time of
revocation.
</dd>
</dl>
</dd>
</dl>
<pre class="example nohighlight"
title="Example verification method structure">
{
"@context": [
"https://www.w3.org/ns/controller/v1",
"https://www.w3.org/ns/credentials/v2",
"https://w3id.org/security/jwk/v1",
"https://w3id.org/security/data-integrity/v2"
]
"id": "https://controller.example/123456789abcdefghi",
<span class="comment">...</span>
"verificationMethod": [{
"id": <span class="comment">...</span>,
"type": <span class="comment">...</span>,
"controller": <span class="comment">...</span>,
"publicKeyJwk": <span class="comment">...</span>
}, {
"id": <span class="comment">...</span>,
"type": <span class="comment">...</span>,
"controller": <span class="comment">...</span>,
"publicKeyMultibase": <span class="comment">...</span>
}]
}
</pre>
<p class="note"
title="Verification method controller(s) and controller(s)">
The semantics of the `controller` property are the same when the
subject of the relationship is the [=controller document=] as when the subject of
the relationship is a [=verification method=], such as a cryptographic public
key. Since a key can't control itself, and the key controller cannot be inferred
from the [=controller document=], it is necessary to explicitly express the identity
of the controller of the key. The difference is that the value of
`controller` for a [=verification method=] is <em>not</em>
necessarily a [=controller=]. [=Controllers=] are expressed
using the `controller` property at the highest level of the
[=controller document=].
</p>
<section>
<h3>Verification Material</h3>
<p>
Verification material is any information that is used by a process that applies
a [=verification method=]. The `type` of a [=verification method=] is
expected to be used to determine its compatibility with such processes. Examples
of verification methods include `JsonWebKey` and `Multikey`.
A [=cryptographic suite=] specification is responsible for specifying the
[=verification method=] `type` and its associated verification material
format. For examples, see
<a href="https://www.w3.org/TR/vc-jose-cose/">Securing Verifiable Credentials using JOSE and COSE</a>,
<a href="https://www.w3.org/TR/vc-di-ecdsa/">the Data Integrity ECDSA
Cryptosuites</a> and <a href="https://w3c-ccg.github.io/lds-ed25519-2020/">
the Data Integrity EdDSA Cryptosuites</a>.
<!-- VC-DATA-INTEGRITY-SPECIFIC:
For a list of <a>verification
method</a> types, please see the [[?SECURITY-VOCABULARY]].
-->
</p>
<p>
To increase the likelihood of interoperable implementations, this specification
limits the number of formats for expressing verification material in a
[=controller document=]. The fewer formats that implementers have to
implement, the more likely it will be that they will support all of them. This
approach attempts to strike a delicate balance between easing implementation
and providing support for formats that have historically had broad deployment.
</p>
<p>
A [=verification method=] MUST NOT contain multiple verification material
properties for the same material. For example, expressing key material in a
[=verification method=] using both `publicKeyJwk` and
`publicKeyMultibase` at the same time is prohibited.
</p>
<p>
Implementations MAY convert keys between formats as desired for operational purposes or
to interface with cryptographic libraries. As an internal implementation detail, such
conversion MUST NOT affect the external representation of key material.
</p>
<p>
An example of a [=controller document=] containing <a>verification
methods</a> using both properties above is shown below.
</p>
<pre id="example-various-verification-method-types"
class="example nohighlight"
title="Verification methods using publicKeyJwk and publicKeyMultibase">
{
"@context": [
"https://www.w3.org/ns/controller/v1",
"https://www.w3.org/ns/credentials/v2",
"https://w3id.org/security/jwk/v1",
"https://w3id.org/security/multikey/v1"
]
"id": "https://controller.example/123456789abcdefghi",
<span class="comment">...</span>
"verificationMethod": [{
"id": "https://controller.example/123#_Qq0UL2Fq651Q0Fjd6TvnYE-faHiOpRlPVQcY_-tA4A",
"type": "JsonWebKey", <span class="comment">// external (property value)</span>
"controller": "https://controller.example/123",
"publicKeyJwk": {
"crv": "Ed25519", <span class="comment">// external (property name)</span>
"x": "VCpo2LMLhn6iWku8MKvSLg2ZAoC-nlOyPVQaO3FxVeQ", <span class="comment">// external (property name)</span>
"kty": "OKP", <span class="comment">// external (property name)</span>
"kid": "_Qq0UL2Fq651Q0Fjd6TvnYE-faHiOpRlPVQcY_-tA4A" <span class="comment">// external (property name)</span>
}
}, {
"id": "https://controller.example/123456789abcdefghi#keys-1",
"type": "Multikey", <span class="comment">// external (property value)</span>
"controller": "did:example:pqrstuvwxyz0987654321",
"publicKeyMultibase": "z6MkmM42vxfqZQsv4ehtTjFFxQ4sQKS2w6WR7emozFAn5cxu"
}],
<span class="comment">...</span>
}
</pre>
</section>
<section>
<h3>Multikey</h3>
<p>
The Multikey data model is a specific type of [=verification method=] that
encodes key types into a single binary stream that is then encoded as a
Multibase value as described in Section [[[#multibase-0]]].
</p>
<p>
When specifying a `Multikey`, the object takes the following form:
</p>
<dl>
<dt>type</dt>
<dd>
The value of the `type` property MUST contain the string `Multikey`.
</dd>
<dt><dfn class="lint-ignore">publicKeyMultibase</dfn></dt>
<dd>
The `publicKeyMultibase` property is OPTIONAL. If present, its value MUST be a
Multibase encoded value as described in Section [[[#multibase-0]]].
</dd>
<dt><dfn class="lint-ignore">secretKeyMultibase</dfn></dt>
<dd>
The `secretKeyMultibase` property is OPTIONAL. If present, its value MUST be a
Multibase encoded value as described in Section [[[#multibase-0]]].
</dd>
</dl>
<p>
The example below expresses an Ed25519 public key using the format defined
above:
</p>
<pre class="example nohighlight"
title="Multikey encoding of a Ed25519 public key">
{
"@context": ["https://w3id.org/security/multikey/v1"],
"id": "https://controller.example/123456789abcdefghi#keys-1",
"type": "Multikey",
"controller": "https://controller.example/123456789abcdefghi",
"publicKeyMultibase": "z6MkmM42vxfqZQsv4ehtTjFFxQ4sQKS2w6WR7emozFAn5cxu"
}
</pre>
<p>
The public key values are expressed using the rules in the table below:
</p>
<table class="simple">
<thead>
<th>Key type</th>