Message Authentication and Hash Functions PPT PDF

Message Authentication and Hash Functions 1 Hash Functions Hash Functions h = H(M) – M is a variable-length message – h is a fixed-length hash value – H is a hash function The hash value is appended at t...

Message Authentication and Hash Functions 1 Hash Functions Hash Functions h = H(M) – M is a variable-length message – h is a fixed-length hash value – H is a hash function The hash value is appended at the source The receiver authenticates the message by recomputing the hash value Because the hash function itself is not considered to be secret, some means is required to protect the hash value Cryptography & Network Security H. Yoon Message Authentication and Hash Functions 2 Hash Functions Hash Function Requirements 1. H can be applied to any size data block 2. H produces fixed-length output 3. H(x) is relatively easy to compute for any given x 4. H is one-way, i.e., given h, it is computationally infeasible to find any x s.t. h = H(x) 5. H is weakly collision resistant: given x, it is computationally infeasible to find any y  x s.t. H(x) = H(y) 6. H is strongly collision resistant: it is computationally infeasible to find any x and y s.t. H(x) = H(y) Cryptography & Network Security H. Yoon Message Authentication and Hash Functions 3 Weak and Strong Collision Resistance Weak Collision Resistance Given an input X and a hashing function H(), it is very difficult to find another input X’ on which H(X) = H(X’). Strong Collision Resistance Given a hashing function H() and two arbitrary inputs X and Y, there exists an absolute minimum chance of H(X) being equal to H(Y). Cryptography & Network Security H. Yoon Message Authentication and Hash Functions 4 Authentication Functions Basic Uses of Hash Function Cryptography & Network Security H. Yoon Message Authentication and Hash Functions 5 Authentication Functions Basic Uses of Hash Function Cryptography & Network Security H. Yoon Message Authentication and Hash Functions 6 Authentication Functions Basic Uses of Hash Function Cryptography & Network Security H. Yoon Message Authentication and Hash Functions 7 Authentication Functions Basic Uses of Hash Function Cryptography & Network Security H. Yoon Message Authentication and Hash Functions 8 Authentication Functions Basic Uses of Hash Function Cryptography & Network Security H. Yoon Message Authentication and Hash Functions 9 Authentication Functions Basic Uses of Hash Function Cryptography & Network Security H. Yoon