How to use the @tanker/crypto.tcrypto.getEncryptionKeyPairFromPrivateKey function in @tanker/crypto

async _decryptPrivateProvisionalKeys(recipientUserPublicKey: Uint8Array, encryptedPrivateProvisionalKeys: Uint8Array): Promise {
    const userKeyPair = await this._localUserManager.findUserKey(recipientUserPublicKey);

    const provisionalUserPrivateKeys = tcrypto.sealDecrypt(encryptedPrivateProvisionalKeys, userKeyPair);
    const appEncryptionKeyPair = tcrypto.getEncryptionKeyPairFromPrivateKey(new Uint8Array(provisionalUserPrivateKeys.subarray(0, tcrypto.ENCRYPTION_PUBLIC_KEY_SIZE)));

    const tankerEncryptionKeyPair = tcrypto.getEncryptionKeyPairFromPrivateKey(new Uint8Array(provisionalUserPrivateKeys.subarray(tcrypto.ENCRYPTION_PUBLIC_KEY_SIZE)));

    return { appEncryptionKeyPair, tankerEncryptionKeyPair };
  }

async _decryptPrivateProvisionalKeys(recipientUserPublicKey: Uint8Array, encryptedPrivateProvisionalKeys: Uint8Array): Promise {
    const userKeyPair = await this._localUserManager.findUserKey(recipientUserPublicKey);

    const provisionalUserPrivateKeys = tcrypto.sealDecrypt(encryptedPrivateProvisionalKeys, userKeyPair);
    const appEncryptionKeyPair = tcrypto.getEncryptionKeyPairFromPrivateKey(new Uint8Array(provisionalUserPrivateKeys.subarray(0, tcrypto.ENCRYPTION_PUBLIC_KEY_SIZE)));

    const tankerEncryptionKeyPair = tcrypto.getEncryptionKeyPairFromPrivateKey(new Uint8Array(provisionalUserPrivateKeys.subarray(tcrypto.ENCRYPTION_PUBLIC_KEY_SIZE)));

    return { appEncryptionKeyPair, tankerEncryptionKeyPair };
  }

async _claimProvisionalIdentity(provisionalIdentity: SecretProvisionalIdentity, tankerKeys: TankerProvisionalKeys): Promise {
    await this._localUserManager.updateLocalUser();

    const appProvisionalUserPrivateSignatureKey = utils.fromBase64(provisionalIdentity.private_signature_key);
    const appProvisionalUserPrivateEncryptionKey = utils.fromBase64(provisionalIdentity.private_encryption_key);

    const provisionalUserKeys = {
      ...tankerKeys,
      appEncryptionKeyPair: tcrypto.getEncryptionKeyPairFromPrivateKey(appProvisionalUserPrivateEncryptionKey),
      appSignatureKeyPair: tcrypto.getSignatureKeyPairFromPrivateKey(appProvisionalUserPrivateSignatureKey),
    };

    const { userId, deviceId, currentUserKey } = this._localUserManager.localUser;
    const { payload, nature } = makeProvisionalIdentityClaim(userId, deviceId, currentUserKey.publicKey, provisionalUserKeys);

    await this._client.send('push block', this._localUserManager.localUser.makeBlock(payload, nature), true);
  }
}

_localUserKeysFromPrivateKey = (encryptedPrivateKey: Uint8Array, encryptionKeyPair: tcrypto.SodiumKeyPair, existingLocalUserKeys: ?LocalUserKeys): LocalUserKeys => {
    const privateKey = tcrypto.sealDecrypt(encryptedPrivateKey, encryptionKeyPair);
    const keyPair = tcrypto.getEncryptionKeyPairFromPrivateKey(privateKey);
    const b64PublicKey = utils.toBase64(keyPair.publicKey);
    const res = {};
    res[b64PublicKey] = keyPair;

    if (existingLocalUserKeys) {
      return {
        userKeys: { ...existingLocalUserKeys.userKeys, ...res },
        currentUserKey: existingLocalUserKeys.currentUserKey,
      };
    }
    return {
      userKeys: res,
      currentUserKey: keyPair,
    };
  }

export const decryptUserKeyForGhostDevice = (ghostDevice: GhostDevice, encryptedUserKey: EncryptedUserKeyForGhostDevice) => {
  const ghostDeviceEncryptionKeyPair = tcrypto.getEncryptionKeyPairFromPrivateKey(ghostDevice.privateEncryptionKey);

  const decryptedUserPrivateKey = tcrypto.sealDecrypt(
    encryptedUserKey.encryptedPrivateUserKey,
    ghostDeviceEncryptionKeyPair
  );

  return tcrypto.getEncryptionKeyPairFromPrivateKey(decryptedUserPrivateKey);
};