What is the B1 file format?
.b1 files are associated with B1 Free Archiver, a data archive application developed by B1 Team (Catalina Group Ltd.). The format is open and publicly documented, with a reference implementation available as B1 Pack, a Java library.
.b1 files store LZMA-compressed data and can hold any type of file, including:
- images,
- text documents,
- video clips,
- audio tracks,
- system files.
The B1 format supports multi-volume archive splitting, which is very useful for storing or transferring large amounts of data. Like ZIP archives, .b1 files are binary and can only be accessed with compatible archive software. The B1 standard also supports AES-256 encryption and password protection, so .b1 archives can be used not only for archiving or sharing data, but also for protecting content from unauthorized access or editing. Each file entry is verified with a CRC-32 checksum.
Beyond B1 Free Archiver itself, PeaZip can extract .b1 files. Note that 7-Zip does not natively support .b1. The B1 Free Archiver desktop application was last meaningfully updated around 2015, though the format specification remains open.
Security & safety
RISK: MEDIUMB1 archives can contain any file types including executables. Risk is comparable to any archive format - scan extracted files with antivirus before opening. AES-256 encryption is strong; password-protected B1 files cannot be cracked by brute force within reasonable time if a strong password was used.
Format details
in a nutshellPrograms that open B1 files
Windows2 apps
B1 Free Archiver
Free Install B1 Free Archiver (v1.7.120, last update ~2015, may be available via third-party mirrors). Double-click .b1 to extract. 
macOS2 apps
B1 Free Archiver
Free Legacy desktop version (no longer actively distributed from b1.org); use PeaZip instead. 
Linux2 apps
B1 Free Archiver
Free Legacy Linux build available from third-party sources; PeaZip is the better current option. 
Android1 app
B1 Free Archiver
Free Tap the .b1 file in B1 Archiver to extract. Mobile version may still be in Play Store. 
Technical details
deep spec| Magic bytes | `42 31 1E 00` - 4-byte signature (`B1` + `0x1E 0x00`) at offset 0 |
| Compression | LZMA (default); uncompressed storage also available |
| Encryption | AES-256, password-based (optional per archive) |
| Container | Proprietary binary stream of typed records: archive header, file entry records, optional encryption wrapper |
| Byte order | Little-endian |
| Checksum integrity | CRC-32 per file entry |
| Multi-volume support | Yes - archive splitting into multiple volumes is supported |
| Associated OS | Windows, macOS, Linux, Android, iOS |
| Reference implementation | B1 Pack (Java library + CLI); open-source |
| Entry record fields | Filename, uncompressed size, CRC-32, timestamps, and compression flags per entry |
| MIME type | `application/x-b1` |
| Desktop app status | B1 Free Archiver desktop last meaningfully updated circa 2015; developer shifted focus to online archiving tools |
| Released | 2011 |
| Latest version | B1 format v1 (stable; no major revisions documented since 2011) |
| Open standard | Yes · royalty-free |
| Specification | b1.org |
B1 conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about B1 files.
