Convert TZ to RAR Online (Moving a Public-Domain 1980s Algorithm Into a Proprietary One)

Two Formats That Took Opposite Paths on Patents and Openness

A TZ file is a tar archive — the Unix bundling format from Seventh Edition Unix in 1979 — compressed with the Unix compress utility, whose Lempel-Ziv-Welch algorithm was implemented by Spencer Thomas in 1984 and finalized in 1985. RAR is a completely different kind of format: Eugene Roshal designed its compression algorithm from scratch in 1993 and released WinRAR in 1995, and RARLAB has never published RAR's internal compression algorithm as an open specification the way compress's LZW method is documented in the POSIX standard.

The two formats' histories actually ran in opposite directions on the openness question. Compress's LZW algorithm was covered by two U.S. patents, 4,464,650 and 4,558,302, both originally assigned to Sperry Corporation, and Unisys — Sperry's successor — spent years actively enforcing licensing terms around LZW starting in the early 1990s, most visibly against the GIF image format. Those patents have since expired, leaving LZW fully in the public domain today. RAR started proprietary in 1993 and has stayed that way the entire time; nothing about its compression internals was ever patent-protected and then freed, because RARLAB simply never published the algorithm as an open specification to begin with.

What RAR's Proprietary Compression Does That Plain LZW Never Could

Compress's LZW coding builds a single adaptive dictionary of byte sequences, capped at a maximum code width set by the -b option — 9 to 16 bits per the POSIX specification — with no additional entropy-coding pass layered on top of it. RAR uses a proprietary LZ-based algorithm combined with context-based prediction modeling and arithmetic coding as a genuine second stage, and the RAR5 format, the default since WinRAR 5.0, supports a sliding dictionary of up to 1 GB — several orders of magnitude beyond LZW's roughly 65,000-entry ceiling at a 16-bit maximum width.

That combination of a vastly larger search window and an added arithmetic-coding stage is why a RAR archive rebuilt from the same files a TZ holds routinely comes out meaningfully smaller — the gap isn't marginal, it reflects roughly a decade and a half of algorithm development between the two designs, plus LZW's complete lack of any secondary compression pass to squeeze out further redundancy the way RAR's arithmetic coding does.

RAR also applies specific preprocessing filters to certain data types, such as executable code, ahead of its general compression stage, letting it exploit structure in that content that a generic dictionary coder like LZW was never built to recognize. Whether this filtering makes a visible difference depends heavily on what the original TZ archive actually contains.

There's also a behavioral difference in how each format handles a long stream of data once its dictionary fills up. Classic compress implementations monitor their own compression ratio as they work, and when that ratio drops enough, the algorithm clears its entire dictionary and starts rebuilding it from scratch partway through the file, discarding everything it had learned up to that point. RAR's much larger sliding window has no equivalent forced reset in ordinary operation — it keeps referencing far-back data as it moves through the stream instead of throwing away its accumulated context, which matters directly for a TZ archive holding many bundled files, since tar's concatenation of similar files is exactly the kind of long, repetitive stream where LZW's periodic resets cost the most compression opportunity.

What Trading LZW for RAR's Algorithm Gains and What It Gives Up

• Gain — recovery records that can repair a damaged archive: RAR's recovery-record feature, part of the format since its original 1993 design, can reconstruct portions of an archive after physical damage or a bad transfer, something compress and tar have no equivalent for at all.
• Gain — a substantially smaller archive on most content: RAR generally outperforms plain LZW by a wide margin, thanks to its far larger dictionary and its added arithmetic-coding stage.
• Lose — free creation of new archives: WinRAR is shareware, and creating new RAR files in most tools requires a paid license, while compress and tar have always been free.
• Lose — an openly documented compression specification: compress's LZW method is described in the POSIX standard; RAR's compression internals have never been published as an open specification the same way.
• Gain — optional AES-256 encryption with filename protection: compress has no password-protection mechanism whatsoever, while RAR can encrypt both file contents and, if selected, the archive's own file list.
• Unchanged — every extracted file's actual content: both LZW and RAR's compression are lossless, so what comes out of either archive afterward is identical, byte for byte.

Where Each Side of This Conversion Actually Runs

Reading the source TZ file requires a genuine Unix or Linux system with compress-compatible tooling — the ncompress package, or gzip's own uncompress/zcat commands where included — since neither Windows nor macOS opens .tz natively out of the box. 7-Zip and similar third-party tools can also extract Z-compressed data on Windows, per 7-Zip's own documented supported-formats list, which places Z among the formats it reads but never writes.

Creating the resulting RAR file needs WinRAR itself, since RARLAB's own software remains the primary tool for building new RAR archives, with a paid license required beyond its trial period. Opening that RAR afterward is broadly possible with the free unrar utility on Linux, 7-Zip's own RAR-reading support, or WinRAR's trial mode on any platform — RAR extraction is far more widely and freely available than RAR creation, a real, documented asymmetry in the format's own tooling ecosystem.

Neither format opens natively anywhere without extra software: not in Windows File Explorer, not in macOS Finder, and not in a default Linux install for RAR specifically, even though tar and basic compress-decoding tools are far more likely to already be present on a Unix-like system than WinRAR or unrar ever are by default.

Real Reasons This Specific Conversion Gets Requested

A frequently reported scenario involves someone finding a decades-old .tar.Z archive from an old backup, FTP mirror, or archived Unix project and needing to hand it to a colleague who works primarily in Windows-centric RAR-based workflows — RAR's recovery-record feature specifically appeals here, since a fragile old archive being moved onto new media benefits from a format that can self-repair minor damage, unlike compress or plain tar.

A second real pattern involves splitting a large decompressed TZ archive's contents across multiple volumes for distribution, a built-in RAR feature with no equivalent in compress or tar's own design, both of which only ever handle a single continuous stream. A third scenario involves simply shrinking an old archive as much as possible before long-term storage, where RAR's far larger dictionary and its arithmetic-coding stage give a real, measurable size reduction over the aging LZW encoding the file currently uses.

LZW's Public-Domain Simplicity Set Against RAR's Proprietary Design

Questions About Moving an Old Compress Archive Into RAR

Do I need to pay for anything to convert my TZ file to RAR?
Reading the source TZ file is free with tools like ncompress or 7-Zip's extraction support, but creating the new RAR archive typically requires a licensed copy of WinRAR, since RAR is a commercially licensed, proprietary format.

Is compress's LZW algorithm still patent-protected the way RAR's algorithm has always been proprietary?
No, not anymore. The two U.S. patents that once covered LZW, 4,464,650 and 4,558,302, have both expired, leaving the algorithm fully in the public domain, while RAR's internal algorithm has remained undocumented and proprietary since RARLAB created it.

Will my RAR file be noticeably smaller than the original TZ?
Usually, yes, and often by a wide margin. RAR's dictionary can reach up to 1 GB in the RAR5 format and adds an arithmetic-coding stage that compress's plain LZW encoding has no equivalent for at all.

What is a RAR recovery record, and does a TZ archive have anything similar?
It's extra data RAR can embed to reconstruct parts of an archive after damage or a bad download. Neither compress nor tar was designed with any comparable self-repair mechanism, so a TZ archive has nothing like it.

Can I open the resulting RAR file on the same Unix system I extracted the TZ file on?
Generally yes, using the free unrar utility, though it typically isn't part of a default install and needs to be added through the system's package manager, unlike the compress-decoding tools a Unix system is more likely to already have.