The signatures in the real world represent and reveal the senders identity. They also bind the signer with the contents of message. Requirement of any signature system is two folded, first it represent a unique mark by creator of the document in a distinguished manner, secondly the recipient as well as third party may verify the ‘genuineness’ of the document. The authentication and verification of signed document in digital medium is far more complex then hand written signatures. Adoption of digital signature system addresses several issues like privacy, confidentiality and integrity of the document apart from authentication and verification of the document. For digital message system, a solution must be found to these problems simultaneously addressing the legal requirement as laid down in IT Act 2000. In this paper, we present the general model of cryptographic system and will show how public key cryptographic system serves the requirement of digital signature in the light of IT Act 2000.