sentences:  For non-repudiation a hash function can be applied successively to additional pieces of data in order to record the chronology of data's existence.A hash chain is a successive application of a cryptographic hash function formula_1 to a string formula_2.For example,
  
  formula_3
  gives a hash chain of length 4, often denoted formula_4
  
  Lamport suggested the use of hash chains as a password protection scheme in an insecure environment.