A printable PDF is available.
Homework 7 – Due Tuesday, March 27
- The basic Diffie-Hellman key exchange protocol is vulnerable to a
"man-in-the-middle" attack, as explained in the textbook. Describe
this attack.
- Consider a cryptographic hash function
f:{0,1}n -> {0,1}h that satisfies the preimage
resistance property and second preimage resistance property, even
though it only works on fixed-size input blocks. Joe needs a function
like this, but it has to work on pairs of n-bit inputs, so he
defines g:{0,1}n×{0,1}n -> {0,1}h as
g(x,y) = f(x XOR y) .
Is this function preimage resistant? Does it satisfy the second preimage resistance property? Justify both answers! - Prove that a hash function that satisfies the collision resistance property also satisfies the second preimage resistance property. (Hint: Write the statement you're trying to prove as an implication, and then prove the logical contrapositive.)