MESSAGE DIGEST ALGORITHM:
Ø MD5 was developed by Rivest in 1991.
Ø The algorithm takes as input a message of arbitrary length and produces as output a 128-bit "fingerprint" or "message digest" of the input.
Ø It is conjectured that it is computationally infeasible to produce two messages having the same message digest, or to produce any message having a given prespecified target message digest
Ø The MD5 algorithm is intended for digital signature applications, where a large file must be "compressed" in a secure manner before being encrypted with a private (secret) key under a public-key cryptosystem such as RSA.
Ø MD5 algorithm is designed to be quite fast on 32-bit machines.
Ø In addition, the MD5 algorithm does not require any large substitution tables; the algorithm can be coded quite compactly.
PROPERTIES OF MESSAGE DIGEST:
Ø Every binary digit , bit , of input message data influences the content of the message digest. Otherwise some of the input data would not be protected from modification.
Ø If any bit of the input message is changed then each bit of the message digest has a probably of being changed of 0.5. This makes it difficult to crack the algorithm.
Ø It should be infeasible to find two messages with the same message digest. Otherwise ,an attacker could substitute a signed message or someone could repudiate a transaction by claiming to have signed a different message.
One-Way Hash Functions:
Ø Take a variable-length input M and produce
fixed-length output (hash value or message digest)
h=H(M)
Ø The idea is to fingerprint M
1. Given M easy to compute h
2. Given h very hard to compute M
3. One-bit change in M changes many bits in h
4. Good one-way hash function is collision-free:
given M it is very hard to find M` such that
H(M)=H(M’)
5. One-way hash function is public.
