Hash Functions.txt

#Brief write up on Cryptographic Hash Functions

1. Hash functions in cryptography are basically mathematical functions.
2. Inputs given to these functions are converted into some value based upon the
function given. These are called hash values or digest.
3. The length of hash values or digests is generally fixed at some value between
128 at 512 bits usually.
4. The length of the input may vary but the length of the output given by Hash
functions is always fixed.
5. eg: The password taken in by say Gmail are not stored as the alphanumeric
characters in the password itself. Rather the passwords are hashed using some
some hash function and this hash function is stored by Gmail.
6. Some properties of a good hash function:
  a. No two inputs must have the same value as their digest/hash values.
  b. One should not be able to predict the input from a given hash value/digest.
7. The general procedure used by hash functions is as follows:
  a. Assume a 10 bit long message. A hash function takes two bits at a time and
  computes the hash value(length of which is always the same. refer point 3) for
  these two.
  b. Step 1 takes the first bit of the message and a seed value(provided by user)
  and finds the hash value for these.
  c. Step 2 takes the hash value from Step 1 and the second bit of the message
  and uses these as inputs to output a hash values
  d. This process continues till all the 10 bits from the message are encoded.

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