COMP 165 - Fall 2016 - Homework 5
Due Wednesday, 26 October, 2016 in class
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(This is a version of Problem 8.6) Find an RC4 key that leaves the state array
unchanged after initialization, so S[i] still equals i for i = 0 to 255. Do you
think this is a problem? Calculate the first five k values produced for this key.
(This is a version of Problem 8.8) Alice and Bob agree privately on a 128-bit key
k, to be used with RC4. They would prefer to reuse this key, so they adopt the
following scheme to send each message:
Describe how the receiver (Alice or Bob) decrypts the message.
Generate a new random 80-bit value, v.
Use this value concatenated with k as the RC4 key for this message.
Send v concatenated with the RC4-encrypted message.
Alice and Bob occasionally change k, but perhaps not often enough.
How can Eve determine whether the same k is still being used
when Alice and Bob use the above scheme?
How many messages can be sent before the string (v, k) is expected to repeat?
(This is a version of Problem 11.5)
A hash function can be based on a symmetric encryption algorithm as follows:
Encrypt the first block, then for each subsequent block XOR it with the
previous encryption before encrypting it.
Thus H(B1, B2) = E( B2 XOR E(B1) ).
The encryption key must be public so that anyone can verify the hash.
Show how to find a C2 for any given C1 so that (C1, C2) has the same
hash value as (B1, B2). How much of a problem is this?