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<!DOCTYPE html>
<html>
<head>
<title>Verifiable Credential Rendering Methods</title>
<meta http-equiv="Content-Type" content="text/html;charset=utf-8" />
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shortName: "vc-render-method",
group: "credentials",
// if you wish the publication date to be other than today, set this
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</head>
<body>
<section id="abstract">
<p>
This specification describes mechanisms that can be used to represent a
Verifiable Credential through a visual, auditory, or haptic medium. It covers
rendering a Verifiable Credential to a physical document, digital image,
screen reader, or braille output.
</p>
</section>
<section id="sotd">
<p>
This is an experimental specification and is undergoing regular revisions. It is
not fit for production deployment.
</p>
</section>
<section>
<h2>Introduction</h2>
<p>
Rendering methods can be used when the <a>issuer</a> has a specific way that
they want to express a <a>verifiable credential</a> to an observer through
a visual, auditory, or haptic mechanism. For example, an <a>issuer</a> of an
employee badge credential might want to include rich imagery of their corporate
logo and specific placement of employee information in specific areas of the
badge. They might also want to provide an audio read out of the important
aspects of the badge for individuals that have accessibility needs related
to their eyesight.
</p>
<section id="terminology">
<h3>Terminology</h3>
<div data-include="https://w3c.github.io/vc-data-model/terms.html"></div>
</section>
<section id="conformance">
<p>
A <dfn>conforming render method</dfn> is any concrete expression of the data
model that complies with the normative statements in this specification.
Specifically, all relevant normative statements in Sections
<a href="#data-model"></a> and <a href="#algorithms"></a>
of this document MUST be enforced.
</p>
<p>
A <dfn class="lint-ignore">conforming processor</dfn> is any algorithm realized
as software and/or hardware that generates or consumes a
<a>conforming render method</a>. Conforming processors MUST produce errors when
non-conforming documents are consumed.
</p>
<p>
This document also contains examples that contain JSON and JSON-LD content. Some
of these examples contain characters that are invalid JSON, such as inline
comments (`//`) and the use of ellipsis (`...`) to denote
information that adds little value to the example. Implementers are cautioned to
remove this content if they desire to use the information as valid JSON or
JSON-LD.
</p>
</section>
</section>
<section>
<h2>Data Model</h2>
<p>
The following sections outline the data model that is used by this specification
for rendering methods
</p>
<section>
<h3>The `renderMethod` Property</h3>
<p>
The `renderMethod` property is a
<a href="https://www.w3.org/TR/vc-data-model-2.0/#reserved-extension-points">
reserved extension point</a> in the Verifiable Credentials Data Model
specification [[VC-DATA-MODEL-2.0]]. An <a>issuer</a> can utilize this
property in a <a>verifiable credential</a> to express one or more preferred
render methods.
</p>
<dl>
<dt><var>renderMethod</var></dt>
<dd>
The value of the `renderMethod` property MUST specify one or
more rendering methods that can be used by software to express the
<a>verifiable credential</a> using a visual, auditory, or haptic mechanism. Each
`renderMethod` value MUST specify its `type`, for example,
`SvgRenderingTemplate`. The precise contents of each rendering
hint is determined by the specific `renderMethod` `type`
definition.
</dd>
</dl>
</section>
<section>
<h4>SvgRenderingTemplate</h4>
<p>
When an <a>issuer</a> desires to specify SVG rendering instructions for a
<a>verifiable credential</a>, they MAY add a `render` property that uses the
data model described below.
</p>
<table class="simple">
<thead>
<tr>
<th style="white-space: nowrap">Property</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td>id</td>
<td>
A URL that dereferences to an SVG image [[SVG]] with an associated
`image/svg+xml` media type.
</td>
</tr>
<tr>
<td>type</td>
<td>
The <code>type</code> property MUST be <code>SvgRenderingTemplate</code>.
</td>
</tr>
<tr>
<td>name</td>
<td>
A name that can be used by a display to express the type of rendering that
is associated with the render method. This property might be used in a
graphical interface that enables an individual to select between multiple
presentation modes.
</td>
</tr>
<tr>
<td>css3MediaQuery</td>
<td>
A CSS Level 3 Media Query that can be used to specify desired characteristics
in the display device. If there are multiple render methods, the best match
to the CSS Level 3 Media Query will be used by default. It is expected that
an individual could override this preference through a manual process.
</td>
</tr>
<tr>
<td>digestMultibase</td>
<td>
An optional multibase-encoded multihash of the SVG image. The multibase value
MUST be `z` and the multihash value MUST be SHA-2 with 256-bits of output
(`0x12`).
</td>
</tr>
</tbody>
</table>
<p>
The data model shown above is expressed in a <a>verifiable credential</a>
in the example below.
</p>
<pre class="example nohighlight"
title="Usage of the render property by an issuer">
{
"@context": [
"https://www.w3.org/ns/credentials/v2",
"https://www.w3.org/ns/credentials/examples/v2",
"https://w3id.org/vc/render-method/v1"
],
"id": "http://example.edu/credentials/3732",
"type": ["VerifiableCredential", "UniversityDegreeCredential"],
"issuer": "https://example.edu/issuers/14",
"validFrom": "2010-01-01T19:23:24Z",
"credentialSubject": {
"id": "did:example:ebfeb1f712ebc6f1c276e12ec21",
"degree": {
"type": "BachelorDegree",
"name": "Bachelor of Science and Arts"
}
},
<span class="highlight">"renderMethod": [{
"id": "https://example.edu/credentials/BachelorDegree.svg",
"type": "SvgRenderingTemplate",
"name": "Portrait Mode",
"css3MediaQuery": "@media (orientation: portrait)",
"digestMultibase": "zQmAPdhyxzznFCwYxAp2dRerWC85Wg6wFl9G270iEu5h6JqW"
}]
</span>
}
</pre>
<p>
In the example above, the <a>issuer</a> has provided an SVG rendering template
for a bachelor's degree that will be filled in with the information in the
<a>verifiable credential</a>. The rendering is preferred when the device or
paper is in a portrait (display is taller than it is wider) orientation.
</p>
</section>
<section>
<h4>OpenAttestationEmbeddedRenderer</h4>
<p>
OpenAttestationEmbeddedRenderer is used by an issuer to render a verifiable
credential. The verifiable credential is rendered in HTML within an embedded
<code><iframe></code> through a Template Renderer website referenced in
the document. This arrangement allows for interactive selective disclosure
using OpenAttestationMerkleProofSignature2018.
</p>
<p>
The Template Renderer is a web application embedded in an iframe. It renders
verifiable credentials based on selected templates and must listen for
specific messages from the Host application to facilitate the rendering
process.
</p>
<p>
There currently exists a number of OpenAttestationEmbeddedRenderer issuers
and decentralized renderer implementations.
</p>
<p>
When an <a>issuer</a> desires to specify an embedded rendering instructions
for a <a>verifiable credential</a>, they MAY add a `renderMethod` property
that uses the data model described below.
</p>
<table class="simple">
<thead>
<tr>
<th>Property</th>
<th>Description</th>
</tr>
</thead>
<tbody>
<tr>
<td>id</td>
<td>
A URL that locates a website that implements the
OpenAttestationEmbeddedRenderer Action API.
</td>
</tr>
<tr>
<td>type</td>
<td>The `type` property MUST be `OpenAttestationEmbeddedRenderer`.</td>
</tr>
<tr>
<td>renderName</td>
<td>
Name of the template used by the website specified by `id` to render
the document. A different template can be used for the decentralized
renderer to present a different HTML view of the verifiable
credential.
</td>
</tr>
</tbody>
</table>
<p>
The data model shown above is expressed in a <a>verifiable credential</a> in
the example below.
</p>
<pre
class="example nohighlight"
title="Usage of the renderMethod property by an issuer"
>
{
"@context": [
"https://www.w3.org/ns/credentials/v2",
"https://schemata.openattestation.com/com/openattestation/4.0/alpha-context.json"
],
"type": ["VerifiableCredential", "OpenAttestationCredential"],
"validFrom": "2021-03-08T12:00:00+08:00",
"name": "Republic of Singapore Driving Licence",
"issuer": {
"id": "did:ethr:0xB26B4941941C51a4885E5B7D3A1B861E54405f90",
"type": "OpenAttestationIssuer",
"name": "Government Technology Agency of Singapore (GovTech)",
"identityProof": { "identityProofType": "DNS-DID", "identifier": "example.openattestation.com" }
},
"credentialSubject": {
"id": "urn:uuid:a013fb9d-bb03-4056-b696-05575eceaf42",
"type": ["DriversLicense"],
"name": "John Doe",
"address": "123 Choa Chu Kang Road, Singapore 123456",
"licenses": [
{
"class": "3",
"description": "Motor cars with unladen weight <= 3000kg",
"effectiveDate": "2013-05-16T00:00:00+08:00"
},
{
"class": "3A",
"description": "Motor cars with unladen weight <= 3000kg",
"effectiveDate": "2013-05-16T00:00:00+08:00"
}
]
},
<span class="highlight">"renderMethod": [{
"id": "https://demo-renderer.opencerts.io",
"type": "OpenAttestationEmbeddedRenderer",
"rendererName": "GOVTECH_DEMO"
}]</span>
}
</pre>
<p>
The <a>verifiable credential</a> specifies a decentralized renderer at
`https://demo-renderer.opencerts.io`, using the template named
`GOVTECH_DEMO`.
</p>
<p>
The decentralized renderer can support different templates that can provide
different views of the verifiable credentials. Below are two renderings of
the same <a>verifiable credential</a> using different templates.
</p>
<figure id="oa-embedded-certificate">
<img
style="width: 100%"
src="images/oa-embedded-certificate.png"
alt="Certificate Template"
/>
<figcaption>Certificate Template</figcaption>
</figure>
<figure id="oa-embedded-transcript">
<img
style="width: 100%"
src="images/oa-embedded-transcript.png"
alt="Transcript Template"
/>
<figcaption>Transcript Template</figcaption>
</figure>
<section>
<h5>Action API</h5>
<p>
The Host is the application that displays the document with the help of the
Template Renderer. The Template Renderer MUST be a web application embedded
within an iframe specified by `renderMethod`.`id`. It MUST communicate with the
Host application using postMessage API to perform actions.
</p>
<p>
All actions follow the same structure. They are composed of `type` and
`payload`:
</p>
<ol class="algorithm">
<li>
`type` indicates the kind of action being executed, for instance,
`RENDER_DOCUMENT` means rendering a document. The type of an action is
mandatory.
</li>
<li>
`payload` indicates optional data associated to the type, for instance,
the content of the document to render.
</li>
</ol>
<p>
An open source reference implementation is available on
<a
href="https://github.com/Open-Attestation/decentralized-renderer-react-components"
>GitHub</a
>.
</p>
<p>
The interaction between the Host and the Template Renderer is illustrated in
the following diagram.
</p>
<figure id="oa-embedded-sequence-diagram">
<img
style="width: 100%"
src="images/oa-embedded-sequence-diagram.png"
alt="Sequence Diagram"
/>
<figcaption>Sequence Diagram</figcaption>
</figure>
<section>
<h6>Host-to-Frame Actions</h6>
<p>
When the iframe is first displayed, the host sends commands to the iframe
to render the document. The 4 types of actions supported are described
below.
</p>
<table class="simple">
<thead>
<tr>
<th>type</th>
<th>payload</th>
<th>action</th>
</tr>
</thead>
<tbody>
<tr>
<td>`GET_TEMPLATES`</td>
<td><pre>{ type: "GET_TEMPLATES" }</pre></td>
<td>
Obtain a list of templates supported by the renderer for the given
document. The list of templates is returned from UPDATE_TEMPLATES
call from the iframe.
</td>
</tr>
<tr>
<td>`SELECT_TEMPLATE`</td>
<td>
<pre>
{
type: "SELECT_TEMPLATE",
payload: "CUSTOM_TEMPLATE"
}
</pre>
</td>
<td>
Select the template to be used for rendering. It should be from the
list returned by GET_TEMPLATES. If not found, a default template is
used.
</td>
</tr>
<tr>
<td>`RENDER_DOCUMENT`</td>
<td>
<pre>
{
type: "RENDER_DOCUMENT",
payload: {
document: {
credentialSubject: ...,
renderMethod: ...
}
}
}
</pre>
</td>
<td>
Render the verifiable credential inside the IFRAME using the
selected template. Document is JSON object form of the verifiable
credential.
</td>
</tr>
<tr>
<td>`PRINT`</td>
<td><pre>{ type: "PRINT" }</pre></td>
<td>
MUST show the print dialog so the contents of the IFRAME can be
printed.
</td>
</tr>
</tbody>
</table>
</section>
<section>
<h6>Frame-to-Host Actions</h6>
<p>
These are used by the iframe to update the host to make adjustments on
formatting, or selective redaction.
</p>
<table class="simple">
<thead>
<tr>
<th>type</th>
<th>payload</th>
<th>action</th>
</tr>
</thead>
<tbody>
<tr>
<td>`OBFUSCATE`</td>
<td>
<pre>
{
type: "OBFUSCATE",
payload: "a[0].b.c",
}
</pre
>
</td>
<td>
This is used for selective redaction. The Host is informed of the
path of the field that has been obfuscated so that the host can
create an updated version of the document with the selected field
obfuscated.
</td>
</tr>
<tr>
<td>`UPDATE_HEIGHT`</td>
<td>
<pre>
{
type: "UPDATE_HEIGHT",
payload: 150,
}
</pre>
</td>
<td>
Notify the Host of the height of the embedded iframe in pixels so
the Host can adjust the size on the browser.
</td>
</tr>
<tr>
<td>`UPDATE_TEMPLATES`</td>
<td>
<pre>
{
type: "UPDATE_TEMPLATES",
payload: [
{
id: "template1",
label: "template1",
},
{
id: "template2",
label: "template2",
}
]
}
</pre>
</td>
<td>
Notify the Host of the list of template names that are usable from
RENDER_METHOD, or GET_TEMPLATES calls.
</td>
</tr>
</tbody>
</table>
</section>
</section>
</section>
</section>
<section>
<h2>Algorithms</h2>
<p>
The following sections outline the algorithms that is used by this specification
for rendering methods.
</p>
<section>
<h5>Render (SvgRenderingTemplate)</h5>
<p>
The following algorithm is used to transform the SVG image template into the
final SVG image that is displayed. The inputs to the algorithm are the
<a>verifiable credential</a> (`verifiableCredential`) and the SVG image
source code (`svgImage`). The output is a SVG image.
</p>
<ol class="algorithm">
<li>
Generate a map, `replacementMap`, by finding all strings in `svgImage` that
start with `{{` (double open braces) and end with `}}`
(double close braces). For each string, `templateKey`, that is found:
<ol class="algorithm">
<li>
Generate another string, `templateValue`, by evaluating the value of
`templateKey` (without the opening or closing braces) using the JSON
Pointer [[RFC6901]] algorithm with the `verifiableCredential` as input to the
algorithm. If the evaluation is `null`, set `templateValue` to the empty string.
</li>
<li>
Set a key in `replacementMap` by using `templateKey` and associate it with
`templateValue`.
</li>
</ol>
</li>
<li>
For every key in `replacementMap`, replace each corresponding
string in `svgImage` that matches the key with the associated value in the
`replacementMap`.
</li>
</ol>
</section>
</section>
<section>
<h2>Security Considerations</h2>
<p class="issue" title="Summary of Security Considerations">
The list of security considerations listed below need to be converted into
sections:
</p>
<ul>
<li>
If a cryptographic hash for a display template isn't provided, you can't know
if it has been tampered with since issuance. There are good reasons an issuer
might allow for this, such as being able to update the look/feel of a
long-lived credential over time.
</li>
<li>
If a cryptographic hash for a display template is provided, implementations
that don't check it run the risk of display attacks, which might sound benign,
but they aren't; Verifiers might use verifiable displays to show what the
issuer intended, and if those displays don't check the cryptographic hash,
they might show the wrong info to an individual checking the credential on
the verifiable display.
</li>
<li>
Fetching resources from external locations can be used as a denial-of-service
attack vector where the resource is never served, or served at a very slow pace.
</li>
</ul>
</section>
<section>
<h2>Privacy Considerations</h2>
<p class="issue" title="Summary of Privacy Considerations">
The list of privacy considerations listed below need to be converted into
sections:
</p>
<ul>
<li>
An issuer could use a rendering method `id` to track individuals. These
values should be aggressively cached client-side, or looked up using a
mixnet or proxy service.
</li>
</ul>
</section>
<section>
<h2>Internationalization Considerations</h2>
<p class="issue" title="Summary of Internationalization Considerations">
The list of internationalization considerations listed below need to be
converted into sections:
</p>
<ul>
<li>
Are we allowed to use media queries to detect language and provide different
displays based on language?
</li>
</ul>
</section>
</body>
</html>