This program is a nifty little tool written for the consummate spy or paranoid hacker who wants to transmit "secret" information across open, text-based networks (like community boards, forums, SMS, or even social media). It has also proven to be a relatively reliable way to store small files with a moderate degree of security. It hopes to be extensible, maintainable, and ultimately portable.
The program can read from a file (or standard input, if omitted) and can write to a file (or standard output, if omitted). The format of the input and output can be binary, base-16, or base-64 encoded. The program will prompt for a passphrase if a key file is omitted.
The stream cipher is a state machine with a permutation vector of 256 bytes indexed with feedback from each plaintext and cipher-text character. The vector is "perturbed" first with a key, and then each byte is fed into the machine. The cipher function is nonlinear and has several moving indices. The original cipher appeared somewhere as an experiment on sci.crypt.research in the late 90s. The source has been maintained by the author in one form or another since 1999 (ported to C, C++, and Delphi).
There are some practical limitations to the algorithm and the application.
- A key must be no more than 256 characters in length (2048 bits).
- Strength in numbers: The cipher is most secure when all 256 bytes are used.
- Encoding the cipher-text as base 16 or base 64 increases the overall size by at least 2:1 and 4:3, respectively, plus additional space for newlines in the output column.
- Checksums and error-correction data are not present in the output stream.
- The user is not protected from transposition or typographical errors in the transmission of the message. Care must be taken to ensure that the message is reassembled correctly before decoding.
- The program doesn't handle or generate "incorrect-key" errors.
To encode README.md from base 256 and output to README.md.crypt in base 64
$ zigma encode in=README.md out=README.md.crypt in.fmt=256 out.fmt=64
To decode README.md.crypt from base 64 and output to README.md in base 256
$ zigma decode in=README.md.crypt out=README.md in.fmt=64 out.fmt=256