A digital certificate is akin to an electronic "credit card" that establishes a client’s credentials and authenticity when establishing a communication session and is issued by a certification authority (CA). It contains various information used for encrypting messages and digital signatures. In addition, the certificate contains the digital signature of the certificate-issuing authority so that it can be verified as being real. Some digital certificates conform to a standard, such X.509. Digital certificates can be kept in registries so that authenticating users can look up other users' public keys. See also Certificate Authority (CA).