One-way Hash Algorithm

In cryptographic application such as digital signature, we usually operate on the message with one-way hash algorithm before we apply the sender’s private key on it.

Why? It is because one-way hash algorithm can convert the message of whatever length  into a fixed length code (usually of 128 or 196 bits). In this way, we can easily apply asymmetric encryption on the shortened code instead of the original message.

This is because the original message can be infinitely long and the operation of asymmetric encryption on it can take a long time.

As one-way hash algorithm serves as the digital fingerprint of the original message it has to process the following properties:

  • It has to be speedy in operation.
  • It produces Hash of fixed length irrespective of the original message length.
  • Once the hash produced, no one can reverse the process and reconstruct the original message from it (as the name one-way implies)
  • Even if there is a little change in the original message, the resultant hash has to be changed in most of its bit (this is called diffusion in the cryptographic technology).

The last property prevents someone from constructing a modified message that produces the same hash.