diff --git a/src/client/initMatrix.ts b/src/client/initMatrix.ts
index f49d53e46..b563af1f8 100644
--- a/src/client/initMatrix.ts
+++ b/src/client/initMatrix.ts
@@ -87,7 +87,7 @@ export const initClient = async (session: Session): Promise<MatrixClient> => {
     // the user (KeyUploadBanner) instead of retrying silently forever.
     fetchFn: watchKeyUploads((input, init) => globalThis.fetch(input, init)),
     timelineSupport: true,
-    cryptoCallbacks: cryptoCallbacks as any,
+    cryptoCallbacks,
     // SAS (emoji) + QR-code verification (show/scan/reciprocate).
     verificationMethods: ['m.sas.v1', 'm.qr_code.show.v1', 'm.qr_code.scan.v1', 'm.reciprocate.v1'],
     oauthClientId: oidcIdentity?.oidc.clientId,
diff --git a/src/client/secretStorageKeys.js b/src/client/secretStorageKeys.js
deleted file mode 100644
index 78c0cb4cb..000000000
--- a/src/client/secretStorageKeys.js
+++ /dev/null
@@ -1,37 +0,0 @@
-const secretStorageKeys = new Map();
-
-export function storePrivateKey(keyId, privateKey) {
-  if (privateKey instanceof Uint8Array === false) {
-    throw new Error('Unable to store, privateKey is invalid.');
-  }
-  secretStorageKeys.set(keyId, privateKey);
-}
-
-function hasPrivateKey(keyId) {
-  return secretStorageKeys.get(keyId) instanceof Uint8Array;
-}
-
-function getPrivateKey(keyId) {
-  return secretStorageKeys.get(keyId);
-}
-
-export function clearSecretStorageKeys() {
-  secretStorageKeys.clear();
-}
-
-async function getSecretStorageKey({ keys }) {
-  const keyIds = Object.keys(keys);
-  const keyId = keyIds.find(hasPrivateKey);
-  if (!keyId) return undefined;
-  const privateKey = getPrivateKey(keyId);
-  return [keyId, privateKey];
-}
-
-function cacheSecretStorageKey(keyId, keyInfo, privateKey) {
-  secretStorageKeys.set(keyId, privateKey);
-}
-
-export const cryptoCallbacks = {
-  getSecretStorageKey,
-  cacheSecretStorageKey,
-};
diff --git a/src/client/secretStorageKeys.test.ts b/src/client/secretStorageKeys.test.ts
new file mode 100644
index 000000000..6d7193c4f
--- /dev/null
+++ b/src/client/secretStorageKeys.test.ts
@@ -0,0 +1,26 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { clearSecretStorageKeys, cryptoCallbacks, storePrivateKey } from './secretStorageKeys';
+
+const ask = (...ids: string[]) =>
+  cryptoCallbacks.getSecretStorageKey!(
+    { keys: Object.fromEntries(ids.map((id) => [id, {} as never])) },
+    'm.cross_signing.master',
+  );
+
+test('[#210] secret storage key cache: null when unknown, key when stored, copies input', async () => {
+  clearSecretStorageKeys();
+  assert.equal(await ask('k1'), null);
+
+  const key = new Uint8Array([1, 2, 3]);
+  storePrivateKey('k1', key);
+  key[0] = 9; // the cache keeps its own copy
+  assert.deepEqual(await ask('k0', 'k1'), ['k1', new Uint8Array([1, 2, 3])]);
+
+  cryptoCallbacks.cacheSecretStorageKey!('k2', {} as never, new Uint8Array([7]));
+  assert.deepEqual(await ask('k2'), ['k2', new Uint8Array([7])]);
+
+  clearSecretStorageKeys();
+  assert.equal(await ask('k1', 'k2'), null);
+  assert.throws(() => storePrivateKey('k3', 'nope' as never), /invalid/);
+});
diff --git a/src/client/secretStorageKeys.ts b/src/client/secretStorageKeys.ts
new file mode 100644
index 000000000..8d805d6f7
--- /dev/null
+++ b/src/client/secretStorageKeys.ts
@@ -0,0 +1,27 @@
+import type { CryptoCallbacks } from 'matrix-js-sdk/lib/crypto-api';
+
+const secretStorageKeys = new Map<string, Uint8Array<ArrayBuffer>>();
+
+export function storePrivateKey(keyId: string, privateKey: Uint8Array) {
+  if (privateKey instanceof Uint8Array === false) {
+    throw new Error('Unable to store, privateKey is invalid.');
+  }
+  // Our own copy, on a plain ArrayBuffer (what the SDK's callback type wants).
+  secretStorageKeys.set(keyId, new Uint8Array(privateKey));
+}
+
+export function clearSecretStorageKeys() {
+  secretStorageKeys.clear();
+}
+
+export const cryptoCallbacks: CryptoCallbacks = {
+  async getSecretStorageKey({ keys }) {
+    const keyId = Object.keys(keys).find((id) => secretStorageKeys.get(id) instanceof Uint8Array);
+    const privateKey = keyId ? secretStorageKeys.get(keyId) : undefined;
+    // The SDK expects null (not undefined) when no key is cached.
+    return keyId && privateKey ? [keyId, privateKey] : null;
+  },
+  cacheSecretStorageKey(keyId, keyInfo, privateKey) {
+    secretStorageKeys.set(keyId, privateKey);
+  },
+};