3. HASH FUNCTION :
A hash function is any function that can be used to map data of
arbitrary size to data of fixed size.
The values returned by a hash function are called hash values,
hash codes, digests, or simply hashes.
A hash value is a numeric value of a fixed length that uniquely
identifies data.
There are different hash functions MD4,MD5,SHA,SHS,etc,.
5. PROPERTIES :
The hash size is always fixed i.e. output of hash has fixed size.
Hash function is non reversible i.e. mathematically it cant be
reversed.
Hash results cant collide. Means if I have two different inputs the
output cant be same.
If we change even one bit of the string(or input) from right of left
entire hash should change.
deterministic so the same message always results in the same hash.
6. MD5 (MESSAGE DIGEST 5) :
It takes as input a message of arbitrary length and produces
as output a 128 bit “fingerprint” or “message digest” of the
input.
The MD5 algorithm is intended for digital signature applications,
where a large file must be “compressed” in secure manner before
being encrypted with private-key under a public-key.
MD5 is used to store hash passwords and verify when user login.
An intruder can see the hashes of passwords, even if he accessed
the password.
7. DRAWBACKS :
In 1993 a hash collision was found in MD5 compression function.
In 2009 a preimage attack against MD5 was published that breaks
MD5’s preimage resistance.
In 2012, the most infamous attack happened by the Flame Malware.
The CMU software Engineering Instute considers MD5
“cryptographically broken and unsuitable for further use”.
8. ONE WAY SIGNATURE :
Method of generating chain of signature on the same message
by different users.
Each signature acts as a link of chain and any link can be
added easily to the chain.
Computationally infeasible to remove any intermediate link
from chain without removing all the links.
They are similar to proxy signature.
They are completely non interactive and stateless.