Digital signature is one of the best method in the security of message in digital data communication. Technically the digital signature is inherited form the public key cryptography. The public key digital signature provide data integrity and authenticity. Digital signature also able to provide non repudiation which means that the identity of sender is well known so s/he didn’t dot chance to claim that the information is not send from his/her device or ID, this feature enable privacy much more enhance.
Digital signature can perform same purpose as handwritten hard copy Signature but handwritten signature can be easily to counterfeit, but digital signature is advance to handwritten signature it is nearly impossible to copy same signature more than one plus it is attested to the information along to identity of signer.
In some sense, In older days digital signature is used as a medium of encryption and decryption. The message is simply signed with private key and decrypt with public key if it matched then the signed document is genuine but it has some demerits the document size is more the communication process is slow and many more now in modern digital signature the concept of hash function and message digest are arrived.
An improvement on the older schema the addition of hash function is applied. Hash function is one way function that takes million bits of input and produce a fixed length of output, lets say in 64 bit then if a bit change in original information then the entire different result is produce in output which secure the originality of the information
From The original data and the strong hash function the message digest is generated . message digest is the process of generation of fixed length of data items.(Any bit change in data cause entire change in digest) the digest and private key are used to create signature and transmitted the signature along the information. As the hash function there is no chance of taking someone’s signature form any document and applied for another. The slight change may cause failure in verification process of digital signature.
As long as hash function is used, there is no way to take someone’s signature from one document and attach it to another, or to alter a signed message in any way. A single change in a signed document will cause the digital signature verification process to fail.