to give his public key to the users SO that no user can accept a public key forged as that of the receiver. The problem behind public key certification is that one certificate may have the public key in one format and another certificate may have the public key in another format. To overcome this ITU has devised the X.509 protocol that describes the Fundamentals of EMS, NMS and OSS/BSS certificate in a structural way. When we use public key universally many DNS servers are required to answer the queries, such that the servers are arranged in a hierarchical relationship that is called public key infrastructure (PKI). 31.4 Message Security The following services are offered under security: Privacy: It is a security service in which the transmitted message makes sense only to the intended receiver. Privacy can be achieved either by using symmetric key cryptography or public key cryptography. Message authentication: It is a security service in which the sender of the mes- sage is verified to be authentic and having the required privileges for sending a message to and requesting services from a specific receiver. Once the sender is authenticated, then the sender is authorized to perform a set of operations based on security rules. Message integrity: It is a security service in which the data reaches the receiver as they were sent without any accidental or malicious changes that may occur during transmission. Nonrepudiation: It is a security service in which the receiver must be able to prove that the message received by the receiver came from a specific sender. Before starting a communication, the sender identity is verified using the authenti- cation service. User authentication can be done by both symmetric key cryptogra- phy and public key cryptography. User authentication with symmetric-key cryptography: Here the identity of one entity (person/process/client/server) is verified for another entity. For the entire process of system access, the identity of the user is verified only once. In one approach the sender se