•Hash value of a message when encrypted with the private key of a person is his digital signature on that e-Document –Digital Signature of a person therefore varies from document to document thus ensuring authenticity of each word of that document. –As the public key of the signer is known, anybody can verify the message and the digital signature
Each individual generates his own key pair [Public key known to everyone & Private key only to the owner] Private Key – Used for making digital signature
Public Key – Used to verify the digital signature
Digital signatures are one of the most important inventions of modern cryptography. The problem is how can a user sign a message such that everybody (or the intended addressee only) can verify the digital signature and the signature is good enough also for legal purposes. Assume that all users use a public-key cryptosystem. Signing a message ‘Apa Kabar ?’ by a user A so that any user can verify the signature; dA(Apa Kabar) Signing a message w by a user A so that only user B can verify the signature; eB(dA(Apa Kabar)) Sending a message w and a signed message digest of w obtained by using a hash function standard h: (w, dA(h(Apa Kabar)))
Example : Alice succeeds after 20 years to factor the integer, Bob used, as modulus, to sign documents, using RSA, 20 years ago. Even if the key is already expired, she can write Bob's will, leaving fortune to Alice, and date it 20 years ago. Moral : It may pay of to factor a single integers using many years of many computer power.