Add design considerations section to privacy considerations (fixes #275) (#278)

Author: johannhof

index.html

@@ -62,6 +62,18 @@
           date: "2025-05-28",
           publisher: "W3C"
         },
+        "custom-schemes": {
+          title: "Concerns with custom schemes for identity presentment",
+          href: "https://github.com/w3c-fedid/digital-credentials/blob/main/custom-schemes.md",
+          authors: ["Rick Byers"],
+          date: "2024-05-01",
+          publisher: "W3C"
+        },
+        "presenting-credentials-on-the-web": {
+          title: "Presenting Credentials on the Web",
+          href: "https://docs.google.com/document/d/1Ppaz_EnhzHqPOz5UusRJvbSunh-RXPWgJ3Np_TM2EE0/",
+          authors: ["Simone Onofri"]
+        },
       },
       xref: {
         profile: "web-platform",
@@ -930,6 +942,60 @@ <h2>
         evolving privacy landscape and participate in the corresponding
         evolution of the API.
       </p>
+      <section>
+        <h3>
+          Design Considerations and Alternatives
+        </h3>
+        <p>
+          The Digital Credentials API is designed to mediate requests for
+          digital credentials from websites, being agnostic to the credential
+          format and the information contained in it, as well as the protocol
+          used to exchange it (within the bounds on the protocol registry
+          inclusion criteria). This and other key design choices are derived
+          from the goal of providing a more secure and private credential
+          exchange experience for users than the existing alternatives (e.g.,
+          [[custom-schemes]]), that is still compatible with common exchange
+          protocols for ease of adoption.
+        </p>
+        <p>
+          The API provides the connection interface between [=verifiers=] and
+          [=holders=], i.e. the means by which a [=digital credential/exchange
+          protocol|credential exchange protocol=] is initiated and the user
+          switches to the [=holder=] application to select a credential.
+          Solutions that have been used for this purpose in the past include QR
+          codes and custom URL schemes. As documented in
+          [[[presenting-credentials-on-the-web]]] and [[[custom-schemes]]],
+          those solutions have security, privacy, and accessibility concerns.
+        </p>
+        <p>
+          With adoption of digital credential technology being driven by
+          ecosystem demand and regulatory mandates, the Web platform offers an
+          alternative to the aforementioned less-desirable technologies that is
+          easy to use for developers, is compatible with existing credential
+          exchange protocols and, most importantly, has better user privacy,
+          security, and accessibility properties than these alternatives.
+        </p>
+        <p>
+          The Digital Credentials API offers the user agent the ability to
+          intermediate on behalf of the user (e.g. in the form of a
+          [=credential chooser=]) to contextualize requests and <a href=
+          "#permission-prior-to-wallet-selection">prevent immediate exposure to
+          holder applications</a>. It also enforces certain minimum
+          requirements on supported protocols, such as <a href=
+          "#encrypting-credential-responses">response encryption</a>.
+        </p>
+        <aside class="note">
+          The Digital Credentials API is not intended to inhibit the
+          development of other standardized solutions that enhance user
+          privacy. For example, an API could be standardized that more strictly
+          enforces unlinkability for specific purposes such as age
+          verification. Higher-level, designed-for-purpose APIs often enable
+          <a data-cite="privacy-principles#purpose-limitation">purpose
+          limitation</a>, ease of explanation to the user, and privacy and
+          security protections from <a data-cite=
+          "design-principles#high-level-low-level">user agents</a>.
+        </aside>
+      </section>
       <section data-cite="vc-data-model#spectrum-of-privacy">
         <h3>
           Spectrum of Privacy