From Just Solve the File Format Problem
7z supports several compression schemes:
- LZMA (the default for 7-Zip prior to 9.30 alpha)
- LZMA2 (the default for 7-Zip 9.30 alpha +)
7z files begin with signature bytes
'7' 'z' BC AF 27 1C.
- Refer to the DOC/7zFormat.txt file in the source distribution