The demand for securing communications is increasing dramatically day by day. Along with this demand, more
techniques have been proposed to achieve the maximum security. The most commonly used technique is the
encryption with two kinds of cryptography. First one is called private key or symmetric cryptography; it can be
applied really fast but the key is hard to be managed since there is just one key each user. Despite this issue, it
still plays a role in the most encryption systems. Other kind of cryptography is proposed to overcome this issue
under the name of public key or asymmetric cryptography where each user is provided with a pair of keys:
public and private keys. Since the public key is not a secret, then the key management problem was solved but
equivalent problem was raised which is the authentication or name management problem. Public key infrastructure
(PKI) was developed to solve this problem and support the public key cryptography. Authentication is
the process of using all PKIs. We have introduced several PKIs with different architectures and processes and
briefly discussed a comprehensive survey of these PKIs.