Public key cryptography solves two basic problems. The first problem is a matter of security of encrypted data. By decoupling the keys used for encryption and decryption, it avoids the problems associated with a pre-shared key. It also introduces the possibility of using its mechanism for identity.
Unfortunately, public key cryptography doesn't solve the problem of identification just by itself. It still requires one fundamental principle. That principle is trust.
To quote the Wiktionary, trust is "confidence in or reliance on some person or quality". In case of public key cryptography, this relates to trust that a certain public key is really tied to the private key belonging to a certain person. The quality in this case if the guarantee that this private key is uniquely owned by a single person, and that this person's computer hasn't been compromised.
The trust model is one of the most difficult areas within PKI in general. This part of the book will deal with two possible trust models - peer-to-peer trust model and Central authority trust model.