Convert LZ to RAR Online (An Open Integrity-Focused Format Meeting a Closed Proprietary One)

An Openly Documented Format Meeting One That Isn't

Lzip is a compression format created by Antonio Diaz Diaz and first released in 2008, and it's about as openly documented as a compression format gets: its structure is described in a published IETF Internet-Draft that also registers "application/lzip" as a formal media type, right down to the exact meaning of every byte in its header and trailer. A .lz file opens with the four-byte signature 0x4C 0x5A 0x49 0x50 ("LZIP"), followed by a version byte, a coded dictionary-size byte, LZMA-compressed data, and a trailer holding a CRC32, the original data size, and the compressed member size — what lzip's own documentation calls a "3-factor integrity check."

RAR sits at the opposite end of that spectrum. Created by Russian software developer Eugene Roshal, whose first name gives the format its "R," RAR has remained a proprietary format throughout its history, developed and sold commercially through WinRAR. RAR 5.0, introduced with WinRAR 5.0 in 2013, documents its container structure — block types, header layout, and archive-level flags — but the actual compression algorithm inside RAR has never been published by Roshal or WinRAR the way lzip's or 7z's have been; independent extraction tools like unrar and the open-source unarr project have had to reverse-engineer or license access to that part rather than read a specification for it.

Why RAR5's Own Checksum Choice Is Worth Knowing Before Converting

RAR5 introduced something specific and documented that's directly comparable to lzip's own integrity design: optional BLAKE2sp checksums for file data, a cryptographic hash strong enough that two different files producing the same BLAKE2sp value is effectively impossible, unlike the much weaker guarantees a plain CRC32 offers. If that option isn't turned on when a RAR5 archive is built, WinRAR falls back to an ordinary CRC32 instead, which is the same category of check ZIP and older RAR versions already use. Lzip, by comparison, always writes its 3-factor trailer — CRC32, data size, and member size together — on every single file it produces, with no equivalent opt-in step.

RAR5 also introduced a documented recovery-record improvement worth noting directly against lzip's own lziprecover tool: RAR's recovery records can now be as large as the archive itself and handle damage spread across multiple segments better than the older RAR4 format did, and WinRAR's own "Repair" command can use a partially corrupted recovery record to rebuild the archive structure and reach undamaged files inside it. That's a genuinely comparable goal to what lziprecover does for a damaged .lz file, even though the two tools are built around completely different underlying container designs.

What Moving From LZ to RAR Actually Changes

• Gain — native multi-file structure: RAR handles multiple files directly as separate entries, unlike a plain .lz file, which holds one compressed stream and needs tar first for more than one file.
• Gain — optional recovery records built into the archive itself: RAR5 can store a recovery record as large as the whole archive; lzip's recovery tooling, lziprecover, works from outside the file using its trailer format instead.
• Lose — lzip's openly published format specification: RAR's internal compression algorithm has never been documented publicly by its creator, unlike lzip's IETF Internet-Draft.
• Gain — split-volume archiving as a native feature: RAR has supported multi-volume archives for splitting large files across several parts since its earliest versions; lzip's format has no equivalent built in.
• Lose — the ability to create the new archive with free software alone: WinRAR is commercial software for creating RAR files, though free tools can extract them; lzip and its ecosystem are entirely free and open-source.
• Gain — RAR5's stronger optional BLAKE2sp checksum: when enabled, it offers a much stronger integrity guarantee per file than a plain CRC32, though it must be turned on rather than being automatic the way lzip's check is.

Who Can Create Each Format, and Who Can Only Read It

WinRAR is the only program that creates genuine RAR archives, since Roshal's compression algorithm is proprietary; free tools including 7-Zip and the open-source unrar utilities can extract RAR files but cannot build new ones. Windows 11 added native read support for RAR archives directly into File Explorer starting in 2023 through the integrated libarchive project, though creating RAR archives natively in Windows still isn't part of that built-in experience even with the later 24H2 update, which added write support only for 7z and TAR.

Lzip has the opposite problem: it's entirely free and open-source, and GNU tar has supported it transparently since version 1.23 through its own --lzip option, but the mainstream Windows tools that most people already have installed don't include it. 7-Zip's standard build needs the separate Lzip7z plugin, and WinRAR's own documented format list doesn't include lzip at all — meaning someone can freely build a .lz file with lzip itself, yet still struggle to find software that opens it on a typical Windows machine without installing something extra first.

That contrast is worth sitting with for a moment: a fully open, publicly specified format like lzip still needs a plugin hunt on Windows, while a fully closed, commercially licensed format like RAR opens for free in almost any archive tool because Roshal's own WinRAR extraction code has effectively become a de facto standard everyone else supports reading, even without access to how it actually compresses data internally.

Real Problems Reported Converting LZ Files to RAR

A recurring, documented complaint on 7-Zip's own discussion forums involves someone downloading a .tar.lz or plain .lz release from a Linux-centric project and finding stock 7-Zip doesn't recognize it, since lzip isn't built into the standard release — prompting a search for the separate Lzip7z plugin, or a decision to ask for (or convert to) a more universally opened format instead.

A second issue shows up specifically because RAR creation requires WinRAR, which is commercial software; someone wanting to build a genuine RAR archive from their .lz files, rather than just repackaging into something free tools handle, needs a WinRAR license (or its trial period) first — a licensing reality that doesn't apply to lzip or to most of the other formats this converter handles.

A third pattern involves lost per-file access with .tar.lz archives specifically: because the whole thing is one continuous lzip stream wrapped around a tar layer, pulling out a single file means decompressing everything first, which surprises people expecting to grab one document quickly — a limitation that disappears once the same content becomes a RAR archive with its own native per-file entries.

A fourth pattern involves version mismatches on the RAR side specifically: an archive built with the newer RAR5 container format won't open in copies of WinRAR or unrar old enough to predate 2013, since RAR5 introduced a genuinely different block structure from the RAR4 format that came before it, rather than just adding options onto the same layout. Checking which RAR version the receiving software actually supports avoids that particular surprise before sending a freshly converted archive anywhere.

LZ and RAR5 Compared Directly

Questions About Turning an LZ Archive Into RAR

Do I need to buy WinRAR to convert my LZ file to RAR?
To create a genuine RAR archive, yes — WinRAR is the only software that builds real RAR files, since Roshal's compression algorithm has never been published for other tools to implement independently.

Does RAR give me the same integrity protection as lzip?
Not automatically. Lzip's 3-factor trailer runs on every file by default, while RAR5's stronger BLAKE2sp checksum is an option that has to be turned on; without it, RAR5 falls back to an ordinary CRC32.

Why won't 7-Zip open my .lz file before I convert it?
Because the standard 7-Zip release doesn't include native lzip support at all. The separate Lzip7z plugin, or a tool that documents lzip support directly, needs to be installed first.

Can I split the RAR archive into multiple parts after converting?
Yes. RAR has supported multi-volume archives since its earliest versions, letting a large archive be split across several separate files, which lzip's format has no built-in equivalent for.

Is a .tar.lz file handled differently than a plain .lz file here?
Yes. A plain .lz file compresses one stream; .tar.lz wraps a tar archive of multiple files inside it. Converting either to RAR means decompressing back to the original files and rebuilding them as RAR's native multi-file entries.

Why has RAR's own compression algorithm never been published?
Because RAR has remained proprietary software developed and sold commercially through WinRAR since Eugene Roshal created it; while the RAR5 container structure is documented, the compression method inside it has stayed closed, unlike lzip's openly published IETF Internet-Draft.