Unlike all other Unix compressors, orz's format doesn't give any reliable way to sniff it in a maybe-compressed file. While in some contexts (private data, files with .orz suffix) the format is already known, there are also cases where programs assume it's possible to find out transport compression by reading the start of the header. And eg. libarchive/bsdtar have no other mode but sniffing.

I see that you haven't committed to a stable bitstream yet -- at least, the decompressor gives a warning when trying to uncompress a file made with an earlier version. Thus, adding such a magic might still be acceptable to you.

A proper magic would be:

• at least 32-bits in length
• not all in ASCII (unlike current version number)
• unlikely to happen in unrelated files