Private repository for Christian Stigen Larsen.
$ python sdes.py
The last message should be run with pypy, otherwise it will be too slow. I could make a version in C just to show, or go deep into the SDES algorithm to try to break it in a smarter way to save time, but the input text and keyspace is so low that the most practically effective way is to just brute-force it.
$ python dec.py # message 1
$ pypy dec2.py # message 2
Strategy used: Brute-force 10-bit and 20-bit keys. Every time a key decrypts to a non-printable character (i.e., outside of the ASCII range 32-126), it is removed from the list of key candidates. Thus, the keyspace is reduced quite quickly. It seems to be very effective, but for the 20-bit key it takes two seconds with pypy. For a larger keyspace (52 bit for example), it would still be viable, but may need to move to C, and for larger keys, need a smarter approach.
For fun, I optimized this in several steps. The original run took 1.5 minutes with stock Python 2.7, and 2.6s JIT-compiled through pypy. I was able to make the brute-forcing algorithm smarter by reducing the time complexity from something like O(n^2) to something more like O(n), where n is the number of bits in the key (20).
After that, the runtime dropped first to 30s for Python 2.7 and 1.6s for pypy, then some more optimizations got it down to 10-11s for Python, 0.8s for pypy. To push the envelope, I implemented SDES in C++. With the unoptimized brute-force algorithm, it finds the key in less than 0.3s. I can probably get it below a hundred milliseconds, but at the cost of memory usage. I don't think it's worth it to spend more time on this, but it may be good later on.
Python 2.7 or 3.3+ with standard libraries such as zlib.
The C++ implementation of SDES requires a C++ compiler such as clang++ or g++ (LLVM and GCC, respectively).
- (Cornell Cryptography)[https://www.math.cornell.edu/~mec/2003-2004/cryptography/subs/hints.html}
- (Google 10000 English Words)[https://github.com/first20hours/google-10000-english]
- (SDES explanation)[http://mercury.webster.edu/aleshunas/COSC%205130/G-SDES.pdf]