A portable C implementation of ipcrypt, a 32-bit block cipher that can be used to "anonymize" IP addresses in log files.
The cipher was designed by Jean-Philippe Aumasson.
Reference implementation in Python: ipcrypt.
WARNING: This is a toy cipher. You shouldn't use it for anything serious. See 1, 2, and 3.
2025 Update: This project has been deprecated in favor of ipcrypt 2, which is AES-based and supports IPv6 and non-deterministic encryption.