law: Information Technology Act, 2000
  3. Authentication of electronic records. – 
  (1) Subject to the provisions of this section,
  
  any subscriber may authenticate an electronic record by affixing his digital
  
  signature.
  (2) The authentication of the electronic record
  
  shall be effected by the use of asymmetric crypto system and hash function
  
  which envelop and transform the initial electronic record into another
  
  electronic record.
  Explanation.- For the purposes of
  
  this sub-section, "hash function" means an algorithm mapping or
  
  translation of one sequence of bits into another, generally smaller, set known
  
  as "hash result" such that an electronic record yields the same hash
  
  result every time the algorithm is executed with the same electronic record as
  
  its input making it computationally infeasible-
  (a) to derive or reconstruct the original
  
  electronic record from the hash result produced by the algorithm;
  (b) that two electronic records can produce
  
  the same hash result using algorithm.
  (3) Any person by the use of a public key of
  
  the subscriber can verify the electronic record.
  (4) The private key and the public key are
  
  unique to the subscriber and constitute a functioning key pair.