A document with a digital signature. Such a certificate is used to authenticate identity. Digital certificates are often signed by a trusted third party known as a certificate authority. Such a company provides a private key and a public key. The latter is used to carry out the decryption of the signature associated with the certificate; the former is used to carry out the encryption. To enable digital certificates to be successful the certificate authority has to be of such a status that they can guarantee that the private keys that are used are securely stored. Virtually the only certificates in use on the Internet are those which conform to the X509 standard. Digital certificates can be categorized as identifying certificates, authorizing certificates, transactional certificates, and time stamping certificates.