sign = async (identity, data) => {
    const signingKey = await keystore.getKey(identity.id)

    if (!signingKey) {
      throw new Error('Private signing key not found from KeyStore')
    }

    return await signMessage(signingKey, data)
  }
Signs data using an identity.
@param {module:Identities~Identity} identity The identity to use for
signing.
@param {string} data The data to sign.
@return {string} The signed data.
@throws Private signing key not found from KeyStore when no signing key can
be retrieved.
@memberof module:Identities~Identities
@instance
@private