**Introduction**

In hash function H accepts a variable length block of input data called as ‘M’ and produces the fixed size hash value can be represented as h = M(H).

Figure: Block Diagram of hash function |

When hash
function provides security, this is called cryptographic hash functions. Hash
function protects the integrity of the message. If encryption process is
applying on message with hash function, it is also providing authentication and
confidentiality.

A hash
function provides a property that has function applied on variable amount of
data (M) and then it produces the fixed amount of output data. If any bit or
bits changes in the data, then whole hash function output data will also
change. Cryptographic has function is one-way function, which is
practically infeasible to invert. The most popular hashing algorithm
is MD5 and SHA.

**Properties of hash Function**

** Compression:** As per
compression properties, output of the hash function is much smaller than the
size of input.

** Pre-image
resistance:** Pre-image resistance means difficult to find the
input from given hash function output. i.e., x=H(m). So if x is given, it is
difficult to message m.

** Weak
Collision Resistance:** Given message m1, weak collision
resistance means that it is difficult to produce another
message m2 such that H(m1)=H(m2). i.e, it means it is infeasible to find two
different messages with the same hash value.

** Strong
Collision Resistance:** Strong collision resistance means that is
difficult to find any two different messages that hash to the same value. i.e.,
it means it is hard to find m1 & m2 such that same hash
value H(m1) = H(m2).

**Characteristics of hash Function**

1. It is
quick to calculate hash value (h) for any given message. i.e., x = H(m).

2. Hash
function (H) can be applied to variable length of data block.

3. A
small change in a message should change the hash value.

4. Hash
function has one-way property; it is impossible to generate message from given hash
value.

5. The
hash function uses all the input data.

6. The
hash function "uniformly" distributes the data across the entire set
of possible hash values.

7. The
hash function generates very different hash values for similar message.

**To learn more about Introduction of Hash Function & its properties, Click here**

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