signIdentity = async (data, { id } = {}) => {
    if (!id) {
      throw new Error('id is required')
    }

    const key = await keystore.getKey(id)
    if (!key) {
      throw new Error(`Signing key for '${id}' not found`)
    }

    return signMessage(key, data)
  }
Signs an identity using the identity's id.
@memberof module:IdentityProviders.IdentityProvider-PublicKey
@param {*} data The identity data to sign.
@param {Object} params One or more parameters for configuring Database.
@param {string} [params.id] The identity's id.
@return {string} A signature.
@instance