Implemented phase 4 for the POC prototype in the Website service.

Services/Website/Sources/Library/Public/Controllers/PasskeyController.swift

@@ -0,0 +1,308 @@
+import Foundation
+import Hummingbird
+import Infrastructure
+import Persistence
+import Utility
+import WebAuthn
+
+/// Serves the passkey ceremonies behind the sign-in screen, as JSON for `js/passkey.js`.
+///
+/// One step both signs in and joins: an unknown email registers a passkey and creates its account together; anything else authenticates with a
+/// passkey registered earlier — a discoverable one when no email is given. A passkey is never added to an existing account: the prototype has
+/// no other proof of who is asking, so that would hand the account over.
+///
+/// - Note: `Context` is the request context the routes are resolved against, and must match the context of the router the routes are added to.
+public struct PasskeyController<Context: RequestContext> {
+
+    // MARK: Properties
+
+    /// The repository accounts and their passkeys are read and created through.
+    private let accounts: AccountRepository
+
+    /// The ceremonies under way.
+    private let ceremonies: PasskeyCeremonies
+
+    /// Whether the cookies are limited to HTTPS.
+    private let isSecure: Bool
+
+    /// The origin the sign-in screen is served at, which passkeys are bound to.
+    private let origin: String
+
+    /// The store sessions are started in.
+    private let sessions: SessionStore
+
+    // MARK: Initializers
+
+    /// Creates a passkey controller.
+    /// - Parameters:
+    ///   - origin: the origin the sign-in screen is served at, e.g. `https://ccn.example`. Browsers bind a passkey to it, and an answer
+    ///     signed for any other origin is turned down — so it is configuration, never read off a request.
+    ///   - accounts: the repository accounts and their passkeys are read and created through.
+    ///   - sessions: the store sessions are started in.
+    ///   - ceremonies: the ceremonies under way; a set of its own by default.
+    public init(
+        origin: String,
+        accounts: AccountRepository,
+        sessions: SessionStore,
+        ceremonies: PasskeyCeremonies = .init()
+    ) {
+        self.accounts = accounts
+        self.ceremonies = ceremonies
+        self.isSecure = origin.hasPrefix("https://")
+        self.origin = origin
+        self.sessions = sessions
+    }
+
+}
+
+// MARK: - RouterController
+
+extension PasskeyController: RouterController {
+
+    // MARK: Properties
+
+    public var routes: RouteCollection<Context> {
+        let routes = RouteCollection(context: Context.self)
+
+        routes.post(
+            .init("\(String.Route.signIn)/passkey/options"),
+            use: options
+        )
+        routes.post(
+            .init("\(String.Route.signIn)/passkey/finish"),
+            use: finish
+        )
+
+        return routes
+    }
+
+}
+
+// MARK: - Helpers
+
+private extension PasskeyController {
+
+    // MARK: Computed
+
+    /// The WebAuthn relying party the ceremonies are run as. Built per request: the manager is not `Sendable`, and holds no state.
+    var manager: WebAuthnManager {
+        .init(configuration: .init(
+            relyingPartyID: URLComponents(string: origin)?.host ?? "localhost",
+            relyingPartyName: "CCN",
+            relyingPartyOrigin: origin
+        ))
+    }
+
+    // MARK: Methods
+
+    /// Handles the browser's signed answer: checks it against the ceremony it belongs to, then starts the session.
+    @Sendable
+    func finish(
+        request: Request,
+        context: Context
+    ) async throws -> Response {
+        guard
+            let token = request.cookies[PasskeyCeremonies.cookieName]?.value,
+            let ceremony = await ceremonies.take(token)
+        else {
+            return Self.refusal(.badRequest, "That sign-in ran out. Try again.")
+        }
+
+        let body = try await request.body.collect(upTo: 64 * 1_024)
+        let user: User
+
+        do {
+            switch ceremony.kind {
+            case .registration(let email):
+                let answer = try JSONDecoder().decode(Answer<RegistrationCredential>.self, from: body)
+                let credential = try await manager.finishRegistration(
+                    challenge: ceremony.challenge,
+                    credentialCreationData: answer.credential,
+                    confirmCredentialIDNotRegisteredYet: { [accounts] in
+                        try await accounts.isUnregistered(credentialID: Self.base64URL($0))
+                    }
+                )
+
+                guard let joined = try await accounts.join(
+                    email: email,
+                    credentialID: Self.base64URL(credential.id),
+                    publicKey: credential.publicKey,
+                    signCount: credential.signCount
+                ) else {
+                    return Self.refusal(.conflict, "That email already has an account.")
+                }
+
+                user = joined
+
+                return try await signedIn(user, returningTo: answer.return)
+            case .authentication:
+                let answer = try JSONDecoder().decode(Answer<AuthenticationCredential>.self, from: body)
+                let credentialID = Self.base64URL(answer.credential.id.asString())
+
+                guard let passkey = try await accounts.passkey(credentialID: credentialID) else {
+                    return Self.refusal(.unauthorized, "That passkey isn't registered with CCN.")
+                }
+
+                let verified = try manager.finishAuthentication(
+                    credential: answer.credential,
+                    expectedChallenge: ceremony.challenge,
+                    credentialPublicKey: passkey.publicKey,
+                    credentialCurrentSignCount: passkey.signCount
+                )
+
+                try await accounts.recordUse(ofPasskey: credentialID, signCount: verified.newSignCount)
+
+                return try await signedIn(passkey.user, returningTo: answer.return)
+            }
+        } catch is DecodingError {
+            return Self.refusal(.badRequest, "That answer couldn't be read.")
+        } catch is WebAuthnError {
+            return Self.refusal(.unauthorized, "That passkey couldn't be verified.")
+        }
+    }
+
+    /// Handles the start of a ceremony: issues the challenge, and tells the script whether to create a passkey or use one.
+    @Sendable
+    func options(
+        request: Request,
+        context: Context
+    ) async throws -> Response {
+        let typed = (try? await request.decode(as: Start.self, context: context))?.email?
+            .trimmingCharacters(in: .whitespaces) ?? ""
+
+        // No email: whichever passkey the browser holds for this site.
+        guard !typed.isEmpty else {
+            return try await begin(.authentication, options: manager.beginAuthentication(), mode: "get")
+        }
+
+        guard let email = NormalizeEmail()(typed) else {
+            return Self.refusal(.unprocessableContent, "That doesn't look like an email address.")
+        }
+
+        guard try await accounts.existingAccount(email: email) != nil else {
+            let options = manager.beginRegistration(user: .init(
+                id: Array(UUID().uuidString.utf8),
+                name: email,
+                displayName: String(email.prefix { $0 != "@" })
+            ))
+
+            return try await begin(.registration(email: email), options: options, challenge: options.challenge, mode: "create")
+        }
+
+        let passkeys = try await accounts.passkeys(email: email)
+
+        guard !passkeys.isEmpty else {
+            return Self.refusal(.conflict, "That account has no passkey. The prototype's demo accounts sign in with the demo sign-in below.")
+        }
+
+        return try await begin(
+            .authentication,
+            options: manager.beginAuthentication(allowCredentials: passkeys.compactMap { passkey in
+                Self.bytes(base64URL: passkey.credentialID).map { .init(type: .publicKey, id: $0) }
+            }),
+            mode: "get"
+        )
+    }
+
+    /// Starts an authentication ceremony and answers with its options.
+    func begin(
+        _ kind: PasskeyCeremonies.Ceremony.Kind,
+        options: PublicKeyCredentialRequestOptions,
+        mode: String
+    ) async throws -> Response {
+        try await begin(kind, options: options, challenge: options.challenge, mode: mode)
+    }
+
+    /// Starts a ceremony and answers with its options, as `{"mode": …, "options": …}`, and the ceremony's cookie.
+    func begin(
+        _ kind: PasskeyCeremonies.Ceremony.Kind,
+        options: some Encodable,
+        challenge: [UInt8],
+        mode: String
+    ) async throws -> Response {
+        let token = await ceremonies.start(kind, challenge: challenge)
+        let options = String(decoding: try JSONEncoder().encode(options), as: UTF8.self)
+        var response = Self.json(.ok, #"{"mode":"\#(mode)","options":\#(options)}"#)
+
+        response.setCookie(PasskeyCeremonies.cookie(token, isSecure: isSecure))
+
+        return response
+    }
+
+    /// Starts the session of a signed-in account, and answers with where to go next.
+    func signedIn(
+        _ user: User,
+        returningTo path: String?
+    ) async throws -> Response {
+        let token = await sessions.start(for: user)
+        let location = String(decoding: try JSONEncoder().encode(String.Route.safeReturn(path)), as: UTF8.self)
+        var response = Self.json(.ok, #"{"location":\#(location)}"#)
+
+        response.setCookie(SessionStore.cookie(token, isSecure: isSecure))
+        response.setCookie(PasskeyCeremonies.cookie("", isSecure: isSecure))
+
+        return response
+    }
+
+    /// A base64 or base64url string as base64url without padding: the one form credential identifiers are kept and compared in.
+    static func base64URL(
+        _ string: String
+    ) -> String {
+        string
+            .replacingOccurrences(of: "+", with: "-")
+            .replacingOccurrences(of: "/", with: "_")
+            .replacingOccurrences(of: "=", with: "")
+    }
+
+    /// The bytes a base64url string encodes, or `nil` when it encodes none.
+    static func bytes(
+        base64URL string: String
+    ) -> [UInt8]? {
+        var base64 = string
+            .replacingOccurrences(of: "-", with: "+")
+            .replacingOccurrences(of: "_", with: "/")
+
+        base64 += String(repeating: "=", count: (4 - base64.count % 4) % 4)
+
+        return Data(base64Encoded: base64).map(Array.init)
+    }
+
+    /// A JSON response, never stored.
+    static func json(
+        _ status: HTTPResponse.Status,
+        _ body: String
+    ) -> Response {
+        .init(
+            status: status,
+            headers: [
+                .contentType: "application/json; charset=utf-8",
+                .cacheControl: "no-store",
+            ],
+            body: .init(byteBuffer: .init(string: body))
+        )
+    }
+
+    /// A refusal, as `{"error": …}`, for the script to show.
+    static func refusal(
+        _ status: HTTPResponse.Status,
+        _ message: String
+    ) -> Response {
+        let message = (try? JSONEncoder().encode(message)).map { String(decoding: $0, as: UTF8.self) } ?? "\"\""
+
+        return json(status, #"{"error":\#(message)}"#)
+    }
+
+}
+
+// MARK: - Structs
+
+/// What the script posts to start a ceremony.
+private struct Start: Decodable {
+    let email: String?
+}
+
+/// What the script posts to finish a ceremony: the browser's credential, and where to go afterwards.
+private struct Answer<Credential: Decodable>: Decodable {
+    let credential: Credential
+    let `return`: String?
+}