Chroma can also play levels from two similar games, XOR and Enigma. It features a "warts and all" reimplementation based on a reverse engineering of the game engines, meaning that, unlike various other conversions, solutions found with Chroma are guaranteed to match those for the original game. With the option to use the original graphics too, Chroma offers a truly authentic playing experience.
XOR was published by Logotron Longman in 1987. It features fifteen mazes of increasingly fiendish puzzles built from surreal elements such as chickens, fish, bombs and dolls. It was released on a variety of platforms, including the BBC Micro, Amstrad CPC, Commodore 64, ZX Spectrum and Atari ST. A later release for the Amiga included a second set of fifteen further mazes.
Enigma was devised by an anonymous author, and subsequently reimplemented by Simon Tatham in 2000. It features twelve levels of falling blocks, exploding bombs, and pushing stuff around - a mixture of Boulderdash, Sokoban and XOR.
If you haven't done so already, you'll need to install Chroma. Then choose one of the following:
XOR and Enigma levels for Linux and other operating systems (unpack the contents into ~/.chroma)
XOR and Enigma levels for Windows (unzip the contents into C:\Program Files\Chroma, adding to the existing directories)
Please note that, unlike Chroma, the XOR levels are not free or open source software - copyright is retained by the original authors. They are being made available here only in the absence of any complaints from said authors regarding their presence, nor that of the various other copies and conversions of XOR that exist elsewhere on the Internet. Copyright has been waived on the Enigma levels, which have been granted to the public domain.
For a completely open source solution to this problem, have a look at the convert2chroma.pl script, which comes with the Chroma source code, and can extract levels and graphics data from all the aforementioned versions of XOR provided you have the appropriate disc images for it to use.
If you're interested in the technical details of the XOR game engine, you may enjoy this disassembly of the BBC version.
There's a separate Hall of Fame for XOR and Enigma.