220.127.116.11 How are certificates used?
Certificates are typically used to generate confidence in the legitimacy of a public key. Certificates are essentially digital signatures that protect public keys from forgery, false representation, or alteration. The verification of a signature therefore can include checking the validity of the certificate for the public key involved. Such verification steps can be performed with greater or lesser rigor depending on the context.
The most secure use of authentication involves associating one or more certificates with every signed message. The receiver of the message would verify the certificate using the certifying authority's public key and, now confident of the public key of the sender, verify the message's signature. There may be two or more certificates enclosed with the message, forming a hierarchical certificate chain, wherein one certificate testifies to the authenticity of the previous certificate. At the end of a certificate hierarchy is a top-level certifying authority, which is trusted without a certificate from any other certifying authority. The public key of the top-level certifying authority must be independently known, for example, by being widely published. It is interesting to note that there are alternative trust models being pursued by a variety of researchers that avoid this hierarchical approach.
The more familiar the sender is to the receiver of the message, or more precisely, the more trust the receiver places in the claim that the public key really is that of the sender, the less need there is to enclose and verify certificates. If Alice sends messages to Bob every day, Alice can enclose a certificate chain on the first day that Bob verifies. Bob thereafter stores Alice's public key and no more certificates or certificate certifications are necessary. A sender whose company is known to the receiver may need to enclose only one certificate (issued by the company), whereas a sender whose company is unknown to the receiver may need to enclose two or more certificates. A good rule of thumb is to enclose just enough of a certificate chain so the issuer of the highest level certificate in the chain is well known to the receiver. If there are multiple recipients then enough certificates should be included to cover what each recipient might need.