In cryptography and computer security, a root certificate is either an unsigned public key certificate or a self-signed certificate that identifies the Root Certificate Authority (CA). A root certificate is part of a public key infrastructure scheme. The most common commercial variety is based on the ITU-T X.509 standard, which normally includes a digital signature from a certificate authority (CA).
A certificate authority can issue multiple certificates in the form of a tree structure. A root certificate is the top-most certificate of the tree, the private key of which is used to "sign" other certificates. All certificates immediately below the root certificate inherit the trustworthiness of the root certificate - a signature by a root certificate is somewhat analogous to "notarizing" an identity in the physical world. Certificates further down the tree also depend on the trustworthiness of the intermediates (often known as "subordinate certification authorities").
Many software applications assume these root certificates are trustworthy on the user's behalf. For example, a web browser uses them to verify identities within SSL/TLS secure connections. However, this implies that the user trusts their browser's publisher, the certificate authorities it trusts, and any intermediates the certificate authority may have issued a certificate-issuing-certificate, to faithfully verify the identity and intentions of all parties that own the certificates. This (transitive) trust in a root certificate is the usual case and is integral to the X.509 certificate chain model.
The root certificate is usually made trustworthy by some mechanism other than a certificate, such as by secure physical distribution. For example, some of the most well-known root certificates are distributed in the Internet browsers by their manufacturers.
See also 
- DNS-based Authentication of Named Entities, a working group developing protocols that allow certificates to be bound to DNS names using Domain Name System Security Extensions (DNSSEC)