What do you mean by Digital Signature ?

A digital signature is an electronic signature that can be used to authenticate the identity of the sender of a message or the signer of a document, and possibly to ensure that the original content of the message or document that has been sent is unchanged. Digital signatures are easily transportable, cannot be imitated by someone else, and can be automatically time-stamped. The ability to ensure that the original signed message arrived means that the sender cannot easily repudiate it later.

A digital signature can be used with any kind of message, whether it is encrypted or not. Digitally signed messages may be anything representable as a bitstring: examples include electronic mail, contracts, or a message sent via some other cryptographic protocol. Digital signatures are commonly used for software distribution, financial transactions, and in other cases where it is important to detect forgery or tampering.

Digital signatures are often used to implement electronic signatures, a broader term that refers to any electronic data that carries the intent of a signature, but not all electronic signatures use digital signatures. In some countries, including the United States, India and members of the European Union, electronic signatures have legal significance.

A digital signature scheme typically consists of three algorithms:

·  A key generation algorithm that selects a private key uniformly at random from a set of possible private keys. The algorithm outputs the private key and a corresponding public key.

·  A signing algorithm that, given a message and a private key, produces a signature.

·  A signature verifying algorithm that, given a message, public key and a signature, either accepts or rejects the message's claim to authenticity.

Some of the well-known digital signature algorithms are:

·  RSA-based signature schemes, such as RSA-PSS

·  DSA and its elliptic curve variant ECDSA

·  ElGamal signature scheme as the predecessor to DSA, and variants Schnorr signature and Pointcheval–Stern signature algorithm

·  Rabin signature algorithm

·  Pairing-based schemes such as BLS

·  Undeniable signatures