Tresor was a file and folder encryption application for Macintosh, written for Motorola 68k and PPC processors. It ran under Mac OS 9.0 - 10.6. It combined high cryptographic security and ease of use. Integrated compression functions allowed compression and encryption in a single pass, without any interim files. Filter functions allowed automation of complex encryption tasks without the use of risky scripts.
Tresor encrypted the complete content of any file using IDEA™ block encryption. It took a passphrase of arbitrary length and produced a 160-bit hash value using the Secure Hash Algorithm (SHA). It then took 128 bits of this hash value, using them as the encryption key for IDEA™. For decryption the same passphrase was needed again.
IDEA™ (International Data Encryption Algorithm) is a block encryption algorithm that uses data blocks of 64 bits and a key length of 128 bits. IDEA™ was developed at the Institute for Signal and Information Processing of the Swiss Federal Institute of Technology, Zurich, by James L. Massey and Xuejia Lai.
For use on older machines you may still download and license (from within the application) Tresor. But please be aware that it is not possible to decrypt Tresor files under Mac OS 10.7 or newer.
If you are looking for a more recent file encryption tool you may want to have a look at Zweifish.