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Digital Signature: * Digital signature, first proposed in 1976 by Whitfield Diffie of Stanford University, is a digital code (encrypted message digest) that can be attached to an electronically transmitted message that uniquely identifies the sender. “Like a written signature, the purpose of a digital signature is to guarantee that the individual sending the message really is who he or she claims to be. It is linked to the data in such a manner that if the data is changed, the digital signature is invalidated. “When making a digital signature, cryptographic hash functions are generally used to construct the message digest. * A digital signature servers three important purposes: Q Verifies data integrity. Q Provides authentication of the sender. Q Provides non-repudiation * A digital signature is a mathematical scheme that is used to authenticate the sender of an electronic document . * A digital signature is nothing but an attachment to any piece of electronic information, which represents the content of the document and the identity of the owner of that document uniquely Motivation ® To provide Authenticity, Integrity and Non-repudiation to electronic documents. © To use the Internet as the safe and secure medium for e-Commerce and e-Governance Basic requirements © Private Key The private key is one which is accessible only to the signer. It is used to generate the digital signature which is then attached to the message. © Public Key The public key is made available to all those who receive the signed messages from the sender. It is used for verification of the received message How does it work? => [ Message : fans UserB A's Private Key = UserA = Hash Signed Digest Public Key Digital Signatures (2) In practice someone cannot alter the message without modifying the digest ca Digest operation very hard to invert Encrypt digest with sender's private key Ka, Kq*: private and public keys of A Alice's computer m Bob's computer m Hash m , function, | H y ——___— Hash Alice's Alice's function, private key, ® public key, Compare OK H Ky } Ki T T H(m) Ky (H(m)) H(m) 
