Green: unseal shared collection keys with the account keypair

decryptCollection now takes the full key material {masterKey,
publicKey, secretKey} and dispatches on keyDecryptionNonce: present
means an owned collection (secretbox under the master key), absent
means a shared collection (sealed box to our public key). Client
already held the keypair for unsealing the auth token, so it just
passes it through.

src/model/decrypt.ts

@@ -1,10 +1,16 @@
-import { decryptBlob, decryptBox, fromBase64 } from "../crypto/index.js";
+import {
+    decryptBlob,
+    decryptBox,
+    decryptSealed,
+    fromBase64,
+} from "../crypto/index.js";
 import type {
     Collection,
     CollectionType,
     EnteFile,
     FileMetadata,
     FileType,
+    KeyMaterial,
     RawCollection,
     RawEnteFile,
     RawMagicMetadata,
@@ -30,14 +36,25 @@ const parseFileType = (n: number): FileType => FILE_TYPE_MAP[n] ?? "unknown";
 
 export const decryptCollection = (
     raw: RawCollection,
-    masterKey: Uint8Array,
+    keys: KeyMaterial,
     currentUserID?: number,
 ): Collection => {
-    const key = decryptBox(
-        fromBase64(raw.encryptedKey),
-        fromBase64(raw.keyDecryptionNonce),
-        masterKey,
-    );
+    // Owned collections carry their key as a secretbox under our master
+    // key, with the nonce in keyDecryptionNonce. Collections shared with
+    // us carry it as an anonymous sealed box to our public key and have
+    // no keyDecryptionNonce at all (sealed boxes embed an ephemeral
+    // public key instead).
+    const key = raw.keyDecryptionNonce
+        ? decryptBox(
+              fromBase64(raw.encryptedKey),
+              fromBase64(raw.keyDecryptionNonce),
+              keys.masterKey,
+          )
+        : decryptSealed(
+              fromBase64(raw.encryptedKey),
+              keys.publicKey,
+              keys.secretKey,
+          );
 
     let name = "";
     if (raw.encryptedName && raw.nameDecryptionNonce) {