diff --git a/spec/integ/crypto/rust-crypto.spec.ts b/spec/integ/crypto/rust-crypto.spec.ts
index c36287668..3f0193b02 100644
--- a/spec/integ/crypto/rust-crypto.spec.ts
+++ b/spec/integ/crypto/rust-crypto.spec.ts
@@ -41,7 +41,7 @@ describe("MatrixClient.initRustCrypto", () => {
         await expect(() => unknownDeviceClient.initRustCrypto()).rejects.toThrow("unknown deviceId");
     });
 
-    it("should create the indexed dbs", async () => {
+    it("should create the indexed db", async () => {
         const matrixClient = createClient({
             baseUrl: "http://test.server",
             userId: "@alice:localhost",
@@ -53,7 +53,25 @@ describe("MatrixClient.initRustCrypto", () => {
 
         await matrixClient.initRustCrypto();
 
-        // should have two dbs now
+        // should have an indexed db now
+        const databaseNames = (await indexedDB.databases()).map((db) => db.name);
+        expect(databaseNames).toEqual(expect.arrayContaining(["matrix-js-sdk::matrix-sdk-crypto"]));
+    });
+
+    it("should create the meta db if given a pickleKey", async () => {
+        const matrixClient = createClient({
+            baseUrl: "http://test.server",
+            userId: "@alice:localhost",
+            deviceId: "aliceDevice",
+            pickleKey: "testKey",
+        });
+
+        // No databases.
+        expect(await indexedDB.databases()).toHaveLength(0);
+
+        await matrixClient.initRustCrypto();
+
+        // should have two indexed dbs now
         const databaseNames = (await indexedDB.databases()).map((db) => db.name);
         expect(databaseNames).toEqual(
             expect.arrayContaining(["matrix-js-sdk::matrix-sdk-crypto", "matrix-js-sdk::matrix-sdk-crypto-meta"]),
@@ -78,6 +96,7 @@ describe("MatrixClient.clearStores", () => {
             baseUrl: "http://test.server",
             userId: "@alice:localhost",
             deviceId: "aliceDevice",
+            pickleKey: "testKey",
         });
 
         await matrixClient.initRustCrypto();
diff --git a/spec/unit/rust-crypto/rust-crypto.spec.ts b/spec/unit/rust-crypto/rust-crypto.spec.ts
index c0afcc524..85f746447 100644
--- a/spec/unit/rust-crypto/rust-crypto.spec.ts
+++ b/spec/unit/rust-crypto/rust-crypto.spec.ts
@@ -54,6 +54,56 @@ const TEST_DEVICE_ID = "TEST_DEVICE";
 
 afterEach(() => {
     fetchMock.reset();
+    jest.restoreAllMocks();
+});
+
+describe("initRustCrypto", () => {
+    function makeTestOlmMachine(): Mocked<OlmMachine> {
+        return {
+            registerRoomKeyUpdatedCallback: jest.fn(),
+            registerUserIdentityUpdatedCallback: jest.fn(),
+            outgoingRequests: jest.fn(),
+        } as unknown as Mocked<OlmMachine>;
+    }
+
+    it("passes through the store params", async () => {
+        const testOlmMachine = makeTestOlmMachine();
+        jest.spyOn(OlmMachine, "initialize").mockResolvedValue(testOlmMachine);
+
+        await initRustCrypto(
+            {} as MatrixClient["http"],
+            TEST_USER,
+            TEST_DEVICE_ID,
+            {} as ServerSideSecretStorage,
+            {} as CryptoCallbacks,
+            "storePrefix",
+            "storePassphrase",
+        );
+
+        expect(OlmMachine.initialize).toHaveBeenCalledWith(
+            expect.anything(),
+            expect.anything(),
+            "storePrefix",
+            "storePassphrase",
+        );
+    });
+
+    it("suppresses the storePassphrase if storePrefix is unset", async () => {
+        const testOlmMachine = makeTestOlmMachine();
+        jest.spyOn(OlmMachine, "initialize").mockResolvedValue(testOlmMachine);
+
+        await initRustCrypto(
+            {} as MatrixClient["http"],
+            TEST_USER,
+            TEST_DEVICE_ID,
+            {} as ServerSideSecretStorage,
+            {} as CryptoCallbacks,
+            null,
+            "storePassphrase",
+        );
+
+        expect(OlmMachine.initialize).toHaveBeenCalledWith(expect.anything(), expect.anything(), undefined, undefined);
+    });
 });
 
 describe("RustCrypto", () => {
@@ -398,10 +448,6 @@ describe("RustCrypto", () => {
             );
         });
 
-        afterEach(() => {
-            jest.restoreAllMocks();
-        });
-
         async function makeEncryptedEvent(): Promise<MatrixEvent> {
             const encryptedEvent = mkEvent({
                 event: true,
@@ -812,5 +858,5 @@ async function makeTestRustCrypto(
     secretStorage: ServerSideSecretStorage = {} as ServerSideSecretStorage,
     cryptoCallbacks: CryptoCallbacks = {} as CryptoCallbacks,
 ): Promise<RustCrypto> {
-    return await initRustCrypto(http, userId, deviceId, secretStorage, cryptoCallbacks, null);
+    return await initRustCrypto(http, userId, deviceId, secretStorage, cryptoCallbacks, null, undefined);
 }
diff --git a/src/client.ts b/src/client.ts
index c6c4b9299..69f21560b 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -343,7 +343,14 @@ export interface ICreateClientOpts {
     deviceToImport?: IExportedDevice;
 
     /**
-     * Key used to pickle olm objects or other sensitive data.
+     * Encryption key used for encrypting sensitive data (such as e2ee keys) in storage.
+     *
+     * This must be set to the same value every time the client is initialised for the same device.
+     *
+     * If unset, either a hardcoded key or no encryption at all is used, depending on the Crypto implementation.
+     *
+     * No particular requirement is placed on the key data (it is fed into an HKDF to generate the actual encryption
+     * keys).
      */
     pickleKey?: string;
 
@@ -1193,7 +1200,18 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
     public store: Store;
     public deviceId: string | null;
     public credentials: { userId: string | null };
+
+    /**
+     * Encryption key used for encrypting sensitive data (such as e2ee keys) in storage.
+     *
+     * As supplied in the constructor via {@link IMatrixClientCreateOpts#pickleKey}.
+     *
+     * If unset, either a hardcoded key or no encryption at all is used, depending on the Crypto implementation.
+     *
+     * @deprecated this should be a private property.
+     */
     public pickleKey?: string;
+
     public scheduler?: MatrixScheduler;
     public clientRunning = false;
     public timelineSupport = false;
@@ -2279,6 +2297,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
             this.secretStorage,
             this.cryptoCallbacks,
             useIndexedDB ? RUST_SDK_STORE_PREFIX : null,
+            this.pickleKey,
         );
         rustCrypto.setSupportedVerificationMethods(this.verificationMethods);
 
diff --git a/src/rust-crypto/index.ts b/src/rust-crypto/index.ts
index 74249bbb9..3288f66e5 100644
--- a/src/rust-crypto/index.ts
+++ b/src/rust-crypto/index.ts
@@ -32,7 +32,10 @@ import { ICryptoCallbacks } from "../crypto";
  * @param secretStorage - Interface to server-side secret storage.
  * @param cryptoCallbacks - Crypto callbacks provided by the application
  * @param storePrefix - the prefix to use on the indexeddbs created by rust-crypto.
- *     If unset, a memory store will be used.
+ *     If `null`, a memory store will be used.
+ * @param storePassphrase - a passphrase to use to encrypt the indexeddbs created by rust-crypto.
+ *     Ignored if `storePrefix` is null. If this is `undefined` (and `storePrefix` is not null), the indexeddbs
+ *     will be unencrypted.
  *
  * @internal
  */
@@ -43,6 +46,7 @@ export async function initRustCrypto(
     secretStorage: ServerSideSecretStorage,
     cryptoCallbacks: ICryptoCallbacks,
     storePrefix: string | null,
+    storePassphrase: string | undefined,
 ): Promise<RustCrypto> {
     // initialise the rust matrix-sdk-crypto-wasm, if it hasn't already been done
     await RustSdkCryptoJs.initAsync();
@@ -59,7 +63,7 @@ export async function initRustCrypto(
         u,
         d,
         storePrefix ?? undefined,
-        (storePrefix && "test pass") ?? undefined,
+        (storePrefix && storePassphrase) ?? undefined,
     );
     const rustCrypto = new RustCrypto(olmMachine, http, userId, deviceId, secretStorage, cryptoCallbacks);
     await olmMachine.registerRoomKeyUpdatedCallback((sessions: RustSdkCryptoJs.RoomKeyInfo[]) =>