When no central identity provider or governance agreement is present, participants assert their own identities and each individual decides who they trust and who they do not. Each participant is a peer with equal standing and each can communicate with anyone else in the network.
Examples: The most familiar peer-to-peer network is probably e-mail. An internet host can join the e-mail network with little more effort than updating its DNS entry and installing some software. Once a host has joined the network, individual e-mail addresses are easily created with no requirement for approval by any central authority. This flexibility and ease of account creation helped spur the growth of the internet, but also allows spam marketers to create false emails.
The best known secure peer-to-peer identity networks on the Internet have been implemented using public key cryptography, which allows participants to trust messages sent over insecure channels like email. Products like PGP and it’s open source counterpart gpg are the most common implementations of public key messaging tools.
When To Use: No central identity provider is available but network participants can exchange credentials.
Advantages: No dependence on a central identity provider. No formal agreement needed to join the network. Participants can assert any identity that they want. Secure peer-to-peer technologies can provide a high degree of confidence once identities have been exchanged. Peer-to-peer models are very flexible, and can support a wide range of trust policies.
Disadvantages: No governing agreement or requirement to implement any policies. Secure deployment requires a high degree of technical sophistication and active management. Individually verifying each participant can be labor intensive. Tracking identities that have been revoked can be complex and error prone.
Ability to Scale: If security requirements are low, peer-to-peer networks can grow very large because new members can join easily. Higher levels of security can be complex to deploy and operate, and can impose a practical limit on the size of the network.
The full papers is downloadable [Field-Guide-Internet-TrustID] Here is a link to introduction of the paper and a at the bottom of that post is a link to all the other models with descriptions. Below are links to all the different models.