Red: shared collections arrive as sealed boxes with no keyDecryptionNonce

Collections shared with the account are not encrypted with the master
key: the sharer only knows the recipient's public key, so the server
delivers encryptedKey as crypto_box_seal to that key and omits
keyDecryptionNonce entirely. decryptCollection assumed the owned-only
wire format and crashed on fromBase64(undefined) for any account with
an incoming shared album, taking down listCollections and every
command built on it (backup, backup-metadata, ...).

The previous "shared" fixture was unfaithful (secretbox + nonce with a
foreign ownerID, a shape the server never sends), which is why the
suite stayed green. These tests model the real wire format and change
decryptCollection to take the full key material {masterKey, publicKey,
secretKey} so it can unseal shared collection keys.

test/client/usage.test.ts

@@ -91,6 +91,7 @@ interface ServerState {
     thumbHeader: Uint8Array;
     thumbCiphertext: Uint8Array;
     collectionKey: Uint8Array;
+    sharedCollectionKey: Uint8Array;
 }
 
 let server: ServerState;
@@ -211,6 +212,35 @@ const buildServer = async (): Promise<ServerState> => {
         updationTime: 1700000000000000,
     };
 
+    // A collection another user shared WITH us. The sharer does not have
+    // our master key, only our public key, so the real server delivers the
+    // collection key as an anonymous sealed box (crypto_box_seal) to our
+    // public key and the response carries NO keyDecryptionNonce. Every
+    // account with an incoming shared album has one of these in its
+    // /collections/v2 response, so the mock must include one too.
+    const sharedCollectionKey = sodium.crypto_secretbox_keygen();
+    const sealedSharedKey = sodium.crypto_box_seal(
+        sharedCollectionKey,
+        kp.publicKey,
+    );
+    const sharedNameNonce = sodium.randombytes_buf(
+        sodium.crypto_secretbox_NONCEBYTES,
+    );
+    const encSharedName = sodium.crypto_secretbox_easy(
+        new TextEncoder().encode("Friend's Wedding"),
+        sharedNameNonce,
+        sharedCollectionKey,
+    );
+    const rawSharedCollection = {
+        id: 2,
+        owner: { id: 99 },
+        encryptedKey: toBase64(sealedSharedKey),
+        encryptedName: toBase64(encSharedName),
+        nameDecryptionNonce: toBase64(sharedNameNonce),
+        type: "album",
+        updationTime: 1700000000000000,
+    };
+
     const rawFile = {
         id: 100,
         collectionID: 1,
@@ -230,6 +260,8 @@ const buildServer = async (): Promise<ServerState> => {
     // can return them. This is ugly plumbing; in a real program you
     // never see any of it.
     (globalThis as Record<string, unknown>).__mockRawCollection = rawCollection;
+    (globalThis as Record<string, unknown>).__mockRawSharedCollection =
+        rawSharedCollection;
     (globalThis as Record<string, unknown>).__mockRawFile = rawFile;
 
     return {
@@ -253,6 +285,7 @@ const buildServer = async (): Promise<ServerState> => {
         thumbHeader: thumbPush.header,
         thumbCiphertext,
         collectionKey,
+        sharedCollectionKey,
     };
 };
 
@@ -303,9 +336,13 @@ const buildMockFetch = (s: ServerState) => {
             });
         }
         if (path === "/collections/v2") {
-            const raw = (globalThis as Record<string, unknown>)
-                .__mockRawCollection;
-            return json({ collections: [raw] });
+            const g = globalThis as Record<string, unknown>;
+            return json({
+                collections: [
+                    g.__mockRawCollection,
+                    g.__mockRawSharedCollection,
+                ],
+            });
         }
         if (path === "/collections/v2/diff") {
             const raw = (globalThis as Record<string, unknown>).__mockRawFile;
@@ -412,6 +449,10 @@ describe("quak Client usage guide", () => {
      * encryption key. `listCollections()` fetches them from the server,
      * decrypts the keys and names, and returns typed objects.
      *
+     * This works transparently for both kinds of collection: ones you
+     * own (key encrypted with your master key) and ones shared with you
+     * (key sealed to your public key, flagged with `isShared: true`).
+     *
      * ```ts
      * const collections = await client.listCollections();
      * for (const c of collections) {
@@ -419,7 +460,7 @@ describe("quak Client usage guide", () => {
      * }
      * ```
      */
-    it("3. list and decrypt collections", async () => {
+    it("3. list and decrypt collections, owned and shared", async () => {
         const client = await Client.login({
             email: TEST_EMAIL,
             password: TEST_PASSWORD,
@@ -428,12 +469,21 @@ describe("quak Client usage guide", () => {
 
         const collections = await client.listCollections();
 
-        expect(collections.length).toBe(1);
-        expect(collections[0]!.name).toBe("Vacation");
-        expect(collections[0]!.type).toBe("album");
-        expect(collections[0]!.id).toBe(1);
+        expect(collections.length).toBe(2);
+
+        const owned = collections.find((c) => c.id === 1)!;
+        expect(owned.name).toBe("Vacation");
+        expect(owned.type).toBe("album");
+        expect(owned.isShared).toBe(false);
         // The decrypted collection key is available for advanced use.
-        expect(collections[0]!.key.length).toBe(32);
+        expect(owned.key.length).toBe(32);
+
+        const shared = collections.find((c) => c.id === 2)!;
+        expect(shared.name).toBe("Friend's Wedding");
+        expect(shared.isShared).toBe(true);
+        // The key was unsealed with our keypair; the decrypted name above
+        // already proves it round-trips, but check it exactly too.
+        expect(shared.key).toEqual(server.sharedCollectionKey);
     });
 
     /**