Message Digest : Cryptographic Hash Functions

Message Digest : Cryptographic Hash Functions

• 1.
  PCIT303: Cryptography andInformation Security Sanjivani Rural Education Society’s Sanjivani College of Engineering, Kopargaon-423603 (An Autonomous Institute Affiliated to Savitribai Phule Pune University, Pune) NACC ‘A’ Grade Accredited, ISO 9001:2015 Certified Department of Information Technology (UG Programme - NBAAccredited) Dr. M.A. Jawale Professor and Head, Dept. of IT
• 2.
  Unit III :Message Digest ▪ Cryptographic Hash Functions ▪ Applications of Cryptographic Hash Functions – a) Message Authentication, b) Digital Signatures, Unit-III: Part-I Message Digest Prof. Dr. Madhuri Jawale Department of Information Technology
• 3.
  Cryptographic Hash Functions ▪A hash function maps a variable-length message into a fixed-length hash value, or message digest. ▪ A hash function H accepts a variable-length block of data M as input and produces a fixed-size hash value h = H(M) ▪ A “good” hash function has the property that the results of applying the function to a large set of inputs will produce outputs that are evenly distributed and apparently random. ▪ In general terms, the principal object of a hash function is data integrity. ▪ A change to any bit or bits in M results, with high probability, in a change to the hash code. Unit-III: Part-I Message Digest Prof. Dr. Madhuri Jawale Department of Information Technology
• 4.
  Cryptographic Hash Functions ▪A cryptographic hash function is an algorithm for which it is computationally infeasible to find either a) a data object that maps to a pre-specified hash result (the one-way property) or b) two data objects that map to the same hash result (the collision-free property). ▪ Because of these characteristics, hash functions are often used to determine whether or not data has changed. Unit-III: Part-I Message Digest Prof. Dr. Madhuri Jawale Department of Information Technology
• 5.
  Cryptographic Hash Functions Unit-III:Part-I Message Digest Prof. Dr. Madhuri Jawale Department of Information Technology
• 6.
  Message Authentication ▪ Messageauthentication is a mechanism or service used to verify the integrity of a message. Message authentication assures that data received are exactly as sent (i.e., contain no modification, insertion, deletion, or replay). Unit-III: Part-I Message Digest Prof. Dr. Madhuri Jawale Department of Information Technology
• 7.
  Examples of theUse of a Hash Function for Message Authentication Unit-III: Part-I Message Digest Prof. Dr. Madhuri Jawale Department of Information Technology ▪ The message plus concatenated hash code is encrypted using symmetric encryption. Because only A and B share the secret key, the message must have come from A and has not been altered. ▪ Because encryption is applied to the entire message plus hash code, confidentiality is also provided.
• 8.
  Examples of theUse of a Hash Function for Message Authentication Unit-III: Part-I Message Digest Prof. Dr. Madhuri Jawale Department of Information Technology ▪ Only the hash code is encrypted, using symmetric encryption. This reduces the processing burden for those applications that do not require confidentiality.
• 9.
  Digital Signatures ▪ Anotherimportant application is the digital signature. ▪ In the case of the digital signature, the hash value of a message is encrypted with a user’s private key. Anyone who knows the user’s public key can verify the integrity of the message that is associated with the digital signature. ▪ In this case, an attacker who wishes to alter the message would need to know the user’s private key. Unit-III: Part-I Message Digest Prof. Dr. Madhuri Jawale Department of Information Technology
• 10.
  Digital Signatures Unit-III: Part-IMessage Digest Prof. Dr. Madhuri Jawale Department of Information Technology ▪ The hash code is encrypted, using public-key encryption with the sender’s private key. This provides authentication. It also provides a digital signature, because only the sender could have produced the encrypted hash code. In fact, this is the essence of the digital signature technique.
• 11.
  Digital Signatures Unit-III: Part-IMessage Digest Prof. Dr. Madhuri Jawale Department of Information Technology ▪ If confidentiality as well as a digital signature is desired, then the message plus the private-key-encrypted hash code can be encrypted using a symmetric secret key. This is a common technique.
• 12.
  Reference 1. Atul Kahate,“Cryptography and Network Security”, Second Edition, Chapter 4. Unit-III: Part-I Message Digest Prof. Dr. Madhuri Jawale Department of Information Technology