In paper based financial messaging system, the messages are sent in defined formats duly signed by the authorized signatories. Paperless initiative and changes in the business practices have changed the authorization process. In electronic messaging, the messages are created, authenticated electronically and sent through electronic medium. Present technology facilitates data transmission with security integrity duly taking care of confidentiality and nonrepudiation of messages. Encryption and digital signature play a vital role in providing such security, integrity, authentication and nonrepudiation for transactions done electronically. Public-key has been around for some time now. A lot of interesting work has been done in different committees (for instance IETF/PKIX and PKCS1) to define Public-key related standards and techniques. This seminars opens the hood and take a look at the engine and at last, get into the details on how publickey encryption and digital signature really work. This seminars is a starting point to get a feel for a vast area named PKI (Public-Key Infrastructure). This includes the mechanics described in this paper as well as an ensemble of software, hardware and processes governed by rules and standards converging to the high level of trust required and expected by the Industry. Public-key is a very appealing and exciting technology, embedding both encryption and digital signature. This constitutes a breakthrough over symmetric-key cryptosystems. It provides 5 layer security ,authentication, authorization, encryption integrity and audit. Beyond the mechanics, there is a need for an infrastructure, a PKI, which includes the tools to effectively manage and use keys and certificates. Finally, beyond the infrastructure, there is a need for preparing, documenting and maintaining the infrastructure. From a technical point of view, implementing a PKI is a matter of days.