CSE 539

Applied Cryptography

Assignment 1



Answer all questions, be brief

Please submit hard copy. Typed preferred.


  1. Explain the terms used to describe the security of the system

a)       Unconditionally secure

b)       Provably secure

c)       Computationally secure

d)       Secure enough

e)      Weak key


  1. DES uses 56-bit key and 3DES uses 168 bit key. Both DES and 3DES encrypt a 64-bit block of plaintext into a 64bit block of ciphertext.


Note that a 64bit input can only have 264 outputs and vice versa. Hence, for any block of ciphertext produced by DES or by 3DES there can be only 264 possible plaintexts.


Hence, we can say that a brute force attack on DES or 3DES would need to use only 264 attempts, and hence 3DES is not more than 64 bit strong. However, there is ample evidence in literature that 3DES is 168 bits strong. How is that possible? Please carefully justify your answer.


  1. “There is more to a key search attack than simply running through all the possible keys”. Explain



  1. Bob is a spy in a hostile foreign country and wants to send messages to Alice. He needs to use local communications channels in order to send messages. All the communication channels are monitored. If the authorities find any encrypted messages going through the channels they would force Bob to give out the secret key. Give the various ways in which a communication channel can be set up. (Assume both Bob and Alice agreed upon a key prior to this) .

  1. One advantage of public key algorithms is to use it to establish a secure session key for a private-key algorithm over an insecure channel. Assume Alice has a pair of public/private keys.

a.       Present a simple protocol, which allows Alice and Bob agree on a shared secret session key.

b.      Who determines the key in this protocol, Alice, Bob or both? Who should determine the key?


  1. Suppose Bob is using Merkle’s Puzzles to communicate with Alice and Mallory is listening in. Bob sends Alice a large number of puzzles. Why is it that Alice only needs to solve (or break) one puzzle, but Mallory has to break half of them on average?