What do you understand by Digital Signatures? Explain Symmetric-key signature and public-key signature methods with suitable example?

Digital Signature:- Digital Signature is a mechanism to authenticate the text communicated by one party to other party. In actual physical documents are authorized by signing, but in electronic transmission documents are authorized by digital signature methods.

Following are the three problems that are avoided by the digital signatures:-

  • If a text is sent by the sender, later on sender should not able to deny about the transmission.
  • Receiver should not able to generate false transmission on behalf of sender.
  • Receiver should able to verify that message is coming from right sender.

Following are two methods to implement digital signature mechanism:-

Symmetric-Key Signatures: Symmetric-Key Signature method is based on the concept of Big Brother. In this method it is assumed that all parties faith on one entity that is Big brother. He knows encryption keys of all parties but never shares keys with others.

In this method, sender (say A) sends Message P along with timestamp t, (the time at which it was sent by A). The message is sent to Big brother not to B in fact. The Big brother kept a receipt of it along with the timestamp t then sends this message to B. Big brother also sends a receipt Kb(B,t,P) to B along with the message. In case some controversy arises later on, sender may show the receipt issued by the Big brother, receipt issued by the big brother is treated as authenticated.  

The main problem with this method is that appropriate and reliable big brother is not available at all. Big brothers may be Govt. Servers basically but All can not faith on one entity completely.

Public-Key Signatures: Public-Key Signature method is based on the concept of public and private keys. In this method each party has two types of keys. One key is public and one key is private. Public keys are known to all whereas private key is known to respective party only.

Two assumptions are made in this method:-
a)     Encryption of decrypted text will give original plain text.  i.e. Ke(Kd(P)) = P.
b)     Decryption of encryption text will give original plain text. i.e. Kd(Ke(P)) = P.

Public Key  = Ea                                                               Public Key  = Eb
Private Key = Da                                                               Private Key = Db


Sender A will encrypt the message P with B’s Public Key and A’s Private Key, thus message sent will be  Da Eb(P). At Receiver side, B will encrypt the message with B’s Private Key and then will encrypt with public key of A. This process will give the original message.   


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