What is digital signature? How does it work? How does it differ from a digital certificate?

Subject E-Commerce
NU Year Set: 3.(c) Marks: 2+3+3=8 Year: 2013

Digital Signature is a process that guarantees that the contents of a message have not been altered in transit.

When you, the server, digitally sign a document, you add a one-way hash (encryption) of the message content using your public and private key pair.

How does a Digital Signature Work?

The digital signature can be considered as a numerical value that is represented as a sequence of characters. The creation of a digital signature is a complex mathematical process that can only be created by a computer.

Consider a scenario where Alice has to digitally sign a file or an email and send it to Bob.

  • Alice selects the file to be digitally signed or clicks on 'sign' in her email application
  • The hash value of the file content or the message is calculated by Alice's computer
  • This hash value is encrypted with Alice's Signing Key (which is a Private Key) to create the Digital Signature.
  • Now, the original file or email message along with its Digital Signature are sent to Bob.
  • After Bob receives the signed message, the associated application (such as email application) identifies that the message has been signed. Bob's computer then proceeds to:

    • Decrypt the Digital Signature using Alice's Public Key
    • Calculate the hash of the original message
    • Compare the (a) hash it has computed from the received message with the (b) decrypted hash received with Alice's message.
  • Any difference in the hash values would reveal tampering of the message.


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