func (m *accountManager) GrantStaff(
	ctx scope.Context, accountID snowflake.Snowflake, kmsCred security.KMSCredential) error {

	m.b.Lock()
	defer m.b.Unlock()

	account, ok := m.b.accounts[accountID]
	if !ok {
		return proto.ErrAccountNotFound
	}
	memAcc := account.(*memAccount)

	kms := kmsCred.KMS()
	key := memAcc.sec.SystemKey.Clone()
	if err := kms.DecryptKey(&key); err != nil {
		return err
	}

	nonce, err := kms.GenerateNonce(key.KeyType.BlockSize())
	if err != nil {
		return err
	}

	capability, err := security.GrantSharedSecretCapability(&key, nonce, kmsCred.KMSType(), kmsCred)
	if err != nil {
		return err
	}

	memAcc.staffCapability = capability
	return nil
}

func (b *AccountManagerBinding) GrantStaff(
	ctx scope.Context, accountID snowflake.Snowflake, kmsCred security.KMSCredential) error {

	// Look up the target account's (system) encrypted client key. This is
	// not part of the transaction, because we want to interact with KMS
	// before we proceed. That should be fine, since this is an infrequently
	// used action.
	var row struct {
		EncryptedClientKey []byte `db:"encrypted_system_key"`
		Nonce              []byte `db:"nonce"`
	}
	err := b.DbMap.SelectOne(
		&row, "SELECT encrypted_system_key, nonce FROM account WHERE id = $1", accountID.String())
	if err != nil {
		if err == sql.ErrNoRows {
			return proto.ErrAccountNotFound
		}
		return err
	}

	// Use kmsCred to obtain kms and decrypt the client's key.
	kms := kmsCred.KMS()
	clientKey := &security.ManagedKey{
		KeyType:      proto.ClientKeyType,
		Ciphertext:   row.EncryptedClientKey,
		ContextKey:   "nonce",
		ContextValue: base64.URLEncoding.EncodeToString(row.Nonce),
	}
	if err := kms.DecryptKey(clientKey); err != nil {
		return err
	}

	// Grant staff capability. This involves marshalling kmsCred to JSON and
	// encrypting it with the client key.
	nonce, err := kms.GenerateNonce(clientKey.KeyType.BlockSize())
	if err != nil {
		return err
	}

	capability, err := security.GrantSharedSecretCapability(clientKey, nonce, kmsCred.KMSType(), kmsCred)
	if err != nil {
		return err
	}

	// Store capability and update account table.
	t, err := b.DbMap.Begin()
	if err != nil {
		return err
	}

	rollback := func() {
		if err := t.Rollback(); err != nil {
			backend.Logger(ctx).Printf("rollback error: %s", err)
		}
	}

	dbCap := &Capability{
		ID:                   capability.CapabilityID(),
		NonceBytes:           capability.Nonce(),
		EncryptedPrivateData: capability.EncryptedPayload(),
		PublicData:           capability.PublicPayload(),
	}
	if err := t.Insert(dbCap); err != nil {
		rollback()
		return err
	}

	result, err := t.Exec(
		"UPDATE account SET staff_capability_id = $2 WHERE id = $1",
		accountID.String(), capability.CapabilityID())
	if err != nil {
		rollback()
		return err
	}
	n, err := result.RowsAffected()
	if err != nil {
		rollback()
		return err
	}
	if n != 1 {
		rollback()
		return proto.ErrAccountNotFound
	}

	if err := t.Commit(); err != nil {
		return err
	}

	return nil
}