Monopoly

• a single organization is the CA for everyone

• No such universally trusted organization
• Requires everyone to authenticate physically with the same CA
• Recovery from compromise is difficult (Due to a single embedded public key of CA on all products)
• Once established, the CA can abuse its position
• Requires perfect security at the CA
• Getting a certificate from a remote CA is vulnerable to attacks and/or is expensive

Oligarchy

• many root CAs exist trusted by verifiers

• N security-sensitive sites instead of one
• Compromise of any may put the entire system at a risk
• Users can be tricked into trusting fake CAs (depending on implementation)
• Users cannot tell if trust anchors in use are good or bad

Anarchy

• users decide on whom to trust and how to authenticate their public keys

• Not scalable as the public database grows significantly (works well for informal non-sensitive applications, e.g., PGP)

Top-Down With Name Constraints

• the root is known to everyone
• root CA delegates other CAs each of which is responsible for its own name space
• searching a path to a name is easy – from the root downward

Bottom-Up With Name Constraints

• the model is very similar to how DNS service is organized and works
• each organization has its CA responsible for names in its organization
• each parent-child in the hierarchy certifies each other
• uses cross-certificates among remote nodes