The Four Levels
Level I challenges are similar to crossword puzzles from newspapers and can be solved with little cryptographic background. You might not even need a computer for solving them — all you need is a bit of clever thinking and probably a pen and paper. A program like CrypTool applied to a level I challenge can help reveal the answer within minutes or even seconds, if the necessary algorithms are already built in. Hence, if you are new to cryptography, but nonetheless interested in the mysterious topic of cryptanalysis, give the level I challenges a try.
Level II challenges require some background knowledge in cryptology and usually some computational power. Additionally, you may require tools that are not available in such convenient packages like CrypTool, OpenSSL or Sage. Therefore, you must first thoroughly understand the problem in order to write a computer program, which helps you getting along. It could take hours or even days to solve a level II challenge. Hence, if you consider yourself well-armed with cryptologic knowledge (such as if you are a university student in a cryptographic course), give the level II challenges a try.
Level III challenges require a thorough background in cryptanalysis and usually significant computational power as well. The problems in this level often represent current research topics that are believed to be very difficult to solve. Thus, practical solutions may not even exist and ready-to-run tools almost certainly do not. The methodology to solve some of these challenges may already be known, but it may require a huge amount of computational power. Challenges in this category mark the thin line between algorithms that are still secure and those that are not. Solving these challenges may take weeks or even several months. Hence, challenges in this level are intended for entire research groups with experts in cryptanalysis, programming, and distributed systems. Success cannot be guaranteed, but if you are the first to successfully solve one of these challenges, it will probably catch the attention of the scientific community. Of course, it still remains up to you to publish or present such scientific techniques and results.
Most level X challenges contain problems that have remained unsolved for a long time. The fact that they have been unsolved regardless of numerous attempts suggests these challenges being hard. However, a simple idea can probably reduce the difficulty of them to a simple level I challenge — currently it is just unknown if and how they can be solved. Of course such a challenge cannot be solved with a simple codeword, since even we (the respective author and the MysteryTwister team) do not know the solution (the plaintext, the original message or the approach). So if you discover a promising solution you should contact the MysteryTwister team. Furthermore, you can publish your findings in a scientific journal such as Cryptologia or Journal of Cryptology.