The centralized model is based on authorizing institutes, so called Certificate Authorities (CA) to verify the identity of the user or organization and certify it in a digital certificate. Since there is no single, worldwide CA guaranteeing the identity of everyone, the identity of a party can be considered valid if its certificate was signed by a trusted CA. A trusted CA is a CA that has been decided to be trustworthy, there is no general algorithm or method to determine which CAs can be trusted. A 'trusted CA list' includes the certificates of all the CAs deemed trustworthy.