A message digest algorithm, or one way has function, takes an arbitrary sized string (the message) and generates a fixed length string (the digest or hash). A Digest has the following characteristics:

• It should be computationally infeasible to find another message that will generate the same digest:
• The digest does not reveal anything about the message


A digital signature consists of two pieces information: a string of bits that is computed from the data that is being ‘signed’, along with the private key of the individual or organization wishing the signature. The signature can be used to verify that the data comes from this individual or organization. Like a handwritten signature, a digital signature has many useful properties:

• Its authenticity can be verified, using a computation based on corresponding public key;
• It cannot be forged ( assuming the private key is kept secret);
• It is a function of the data signed and cannot be claimed to be the signature for any other data;
• The signed data cannot be changed, otherwise the signature will no longer verify the data as being authentic.

Some digital signature algorithms use message digest algorithms for parts of their computations; others, for efficiency, compute the digest of a message and digitally sign the digest rather than signing the message itself.