A hash function maps data of arbitrary size to a fixed size value called a hash. Common hash functions include MD5 and SHA, with MD5 producing a 128-bit hash. While hashes were once used to securely store passwords, MD5 is now considered cryptographically broken due to collisions being found in its compression function. One-way signatures allow multiple users to generate linked signatures on the same message in a verifiable chain.

1. HASH FUNCTION
BY
STALIN RIJAL
DATE : 5-MAY-2018

2. CONTENT :
 HASH FUNCTION
 MD5
 ONE WAY SIGNATURE

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,.

4. EXAMPLE :

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.