diff --git a/packages/shared/src/crypto/index.ts b/packages/shared/src/crypto/index.ts
index 3d6d61c..4d267de 100644
--- a/packages/shared/src/crypto/index.ts
+++ b/packages/shared/src/crypto/index.ts
@@ -3,3 +3,4 @@ export * from './box';
 export * from './keys';
 export * from './recoveryCode';
 export * from './sessionKeys';
+export * from './userKey';
diff --git a/packages/shared/src/crypto/userKey.test.ts b/packages/shared/src/crypto/userKey.test.ts
new file mode 100644
index 0000000..ffa9ea5
--- /dev/null
+++ b/packages/shared/src/crypto/userKey.test.ts
@@ -0,0 +1,51 @@
+import { describe, expect, it, beforeAll } from 'vitest';
+
+import { setCryptoBackend } from './backend';
+import { makeWasmTestBackend } from './testBackend';
+import { generateUserKeyPair, sealUserKey, openUserKey, KDF_PRESET } from './userKey';
+
+beforeAll(async () => { setCryptoBackend(await makeWasmTestBackend()); });
+
+describe('userKey', () => {
+  it('generates a 32-byte X25519 keypair', async () => {
+    const kp = await generateUserKeyPair();
+    expect(kp.publicKey).toHaveLength(32);
+    expect(kp.privateKey).toHaveLength(32);
+  });
+
+  it('seals and opens a private key with the same PIN', async () => {
+    const kp = await generateUserKeyPair();
+    const sealed = await sealUserKey({ privateKey: kp.privateKey, pin: '123456' });
+    const opened = await openUserKey({
+      sealed: sealed.sealedPrivateKey,
+      pin: '123456',
+      salt: sealed.salt,
+      kdfParams: sealed.kdfParams,
+    });
+    expect(Array.from(opened)).toEqual(Array.from(kp.privateKey));
+  });
+
+  it('throws when opening with the wrong PIN', async () => {
+    const kp = await generateUserKeyPair();
+    const sealed = await sealUserKey({ privateKey: kp.privateKey, pin: '123456' });
+    await expect(
+      openUserKey({ sealed: sealed.sealedPrivateKey, pin: '654321', salt: sealed.salt, kdfParams: sealed.kdfParams }),
+    ).rejects.toThrow();
+  });
+
+  it('throws when opening with the wrong salt', async () => {
+    const kp = await generateUserKeyPair();
+    const sealed = await sealUserKey({ privateKey: kp.privateKey, pin: '123456' });
+    const wrongSalt = new Uint8Array(sealed.salt.length); wrongSalt.fill(7);
+    await expect(
+      openUserKey({ sealed: sealed.sealedPrivateKey, pin: '123456', salt: wrongSalt, kdfParams: sealed.kdfParams }),
+    ).rejects.toThrow();
+  });
+
+  it('emits the documented KDF preset', async () => {
+    const kp = await generateUserKeyPair();
+    const sealed = await sealUserKey({ privateKey: kp.privateKey, pin: '123456' });
+    expect(sealed.kdfParams.algo).toBe('argon2id');
+    expect(sealed.kdfParams.preset).toBe(KDF_PRESET);
+  });
+});
diff --git a/packages/shared/src/crypto/userKey.ts b/packages/shared/src/crypto/userKey.ts
new file mode 100644
index 0000000..67b3fa7
--- /dev/null
+++ b/packages/shared/src/crypto/userKey.ts
@@ -0,0 +1,82 @@
+import sodium from 'libsodium-wrappers-sumo';
+
+import { getCryptoBackend } from './backend';
+import type { KeyPair } from './backend';
+
+export const KDF_PRESET = 'moderate' as const;
+const SALT_LEN = 16;
+
+export interface KdfParams {
+  algo: 'argon2id';
+  preset: typeof KDF_PRESET;
+  opslimit: number;
+  memlimit: number;
+}
+
+export interface SealedUserKey {
+  sealedPrivateKey: Uint8Array; // nonce(24) || ciphertext
+  salt: Uint8Array;             // 16 bytes
+  kdfParams: KdfParams;
+}
+
+export async function generateUserKeyPair(): Promise<KeyPair> {
+  return getCryptoBackend().generateKeyPair();
+}
+
+async function deriveKek(pin: string, salt: Uint8Array, params: KdfParams): Promise<Uint8Array> {
+  await sodium.ready;
+  return sodium.crypto_pwhash(
+    sodium.crypto_secretbox_KEYBYTES,
+    pin,
+    salt,
+    params.opslimit,
+    params.memlimit,
+    sodium.crypto_pwhash_ALG_ARGON2ID13,
+  );
+}
+
+function defaultKdfParams(): KdfParams {
+  return {
+    algo: 'argon2id',
+    preset: KDF_PRESET,
+    opslimit: sodium.crypto_pwhash_OPSLIMIT_MODERATE,
+    memlimit: sodium.crypto_pwhash_MEMLIMIT_MODERATE,
+  };
+}
+
+export async function sealUserKey(opts: {
+  privateKey: Uint8Array;
+  pin: string;
+  salt?: Uint8Array;
+  kdfParams?: KdfParams;
+}): Promise<SealedUserKey> {
+  await sodium.ready;
+  const backend = getCryptoBackend();
+  const salt = opts.salt ?? backend.randomBytes(SALT_LEN);
+  const kdfParams = opts.kdfParams ?? defaultKdfParams();
+  const kek = await deriveKek(opts.pin, salt, kdfParams);
+  try {
+    const nonce = backend.randomBytes(backend.secretboxNonceLength);
+    const cipher = backend.secretbox(opts.privateKey, nonce, kek);
+    const sealedPrivateKey = new Uint8Array(nonce.length + cipher.length);
+    sealedPrivateKey.set(nonce, 0);
+    sealedPrivateKey.set(cipher, nonce.length);
+    return { sealedPrivateKey, salt, kdfParams };
+  } finally { sodium.memzero(kek); }
+}
+
+export async function openUserKey(opts: {
+  sealed: Uint8Array;
+  pin: string;
+  salt: Uint8Array;
+  kdfParams: KdfParams;
+}): Promise<Uint8Array> {
+  await sodium.ready;
+  const backend = getCryptoBackend();
+  const nonceLen = backend.secretboxNonceLength;
+  if (opts.sealed.length <= nonceLen) throw new Error('sealed user key blob too short');
+  const nonce = opts.sealed.slice(0, nonceLen);
+  const cipher = opts.sealed.slice(nonceLen);
+  const kek = await deriveKek(opts.pin, opts.salt, opts.kdfParams);
+  try { return backend.secretboxOpen(cipher, nonce, kek); } finally { sodium.memzero(kek); }
+}