Homework #2
CMPS 122, Spring 2004
| Assigned: | 12 April 2004 |
| Due: | 19 April 2004 at 10 AM |
Please read the homework guidelines for information about how to work on the assignment and how to submit it.
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Decrypt this binary file (hw2-1.crypt).
It was encrypted using AES with a 128-bit key. To make this problem tractable
in a reasonable amount of time, the key has only 24 bits that change, followed
by 104 bits that are all set to 0. For this problem, you need to write
the code that decodes the message, which is plain ASCII text. A compressed
tar file with
AES encryption and decryption code along with examples of how to use it
is available online. The file was encrypted with this
AES code (in CBC mode, with IV=0), so you should probably use this source
code to try and break it.
You may use any programming language you like to do this problem, but it's likely that only C or C++ will be fast enough to complete the assignment (this is brute force decryption...). For this assignment, please turn in any code you wrote (and you must write the code on your own) along with the key and decrypted message.
HINT: you don't need to decode the entire message to determine whether it's a valid ASCII & English message. How can you quickly (in the first few 128-bit blocks) tell whether an possible decryption is not valid? - The Rijndael algorithm uses a byte substitution table that comes from a formula applied to GF(28). Is it necessary to use that formula, or would any substitution table work? What restrictions are there on the form of the table?
- It's possible to write an encryption / decryption system that uses
"random" number generators. For this problem, you'll be writing code
that encrypts / decrypts using the Unix random number generators. The
key size is 128 bits, and the block size is 32 bits. Each ciphertext
block is calculated by XORing the plaintext block with the value returned
by the random number generator.
- Write code (in C) that encrypts and decrypts data using the Unix
random() and initstate() functions. The key should
be passed to initstate(),
and successive values obtained from random() should be XORed
with the plaintext to generate the ciphertext. You should write them
into a single program called randcrypt, which should be
called as follows:
randcrypt E|D key < input > output
In other words, randcrypt E 6789abcd < plain.txt > cipher.txt would encrypt plain.txt using the (hex) key 0x6789abcd and put the resulting ciphertext into cipher.txt. To decrypt cipher.txt, you would use randcrypt D 6789abcd < cipher.txt > decrypted.txt.
HINT: read the man pages for random() and related functions (they're in section 3). - Why is it not absolutely necessary to use CBC mode for this encryption algorithm?
- How might you go about decrypting a message encrypted with this technique? Is the algorithm really as strong as the 128-bit key?
- Write code (in C) that encrypts and decrypts data using the Unix
random() and initstate() functions. The key should
be passed to initstate(),
and successive values obtained from random() should be XORed
with the plaintext to generate the ciphertext. You should write them
into a single program called randcrypt, which should be
called as follows:
- Problem 9.3 in Stallings (page 280).
Last updated 11 Apr 2004 by Ethan L. Miller (elm at ucsc d0t edu)
