Guide · zip → 7z

ZIP Archive Format: The Complete Technical Guide

✓ ISO/IEC Verified · 6 min · Updated October 19, 2026 · By Pablo Cirre

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What Is ZIP?

ZIP is a lossless data compression and archiving format created by Phil Katz and Gary Conway in 1989. It was designed as an open format to replace ARC and PKZIP became the dominant implementation. The specification was published by PKWARE and has been extended many times; the current version is APPNOTE.TXT 6.3.10 (2023). ZIP is now natively supported by every major operating system — Windows, macOS, Linux, iOS, Android — making it the most universally compatible archive format on earth.

ZIP is unusual among archive formats in that it stores files independently — each file in the archive is compressed and stored with its own metadata, without requiring the entire archive to be decoded to extract a single file. This design trades compression ratio for random access.

ZIP Architecture: Internal Structure

A ZIP file is structured as follows:

[Local file header 1] [File data 1]
[Local file header 2] [File data 2]
...
[Local file header N] [File data N]
[Data descriptor (optional)]
[Archive decryption header (optional)]
[Central directory]
  [Central directory entry 1]
  [Central directory entry 2]
  ...
[End of central directory record]

The central directory at the end of the file is the key innovation: it is an index of all files in the archive with their offsets. ZIP readers first seek to the end of the file, find the end-of-central-directory record, then jump to each file's offset as needed. This enables:

Random access: extract file N without reading files 1 through N-1.
Streaming append: add files to the end and update the central directory.
Self-extracting archives: prepend an EXE header before the ZIP data — the central directory offset is still found from the end.

Compression Methods

Each file in a ZIP archive can use a different compression method:

Method	Code	Description
Stored	0	No compression — raw bytes
Shrunk	1	LZW variant (obsolete)
Imploded	6	Old PKZIP algorithm (obsolete)
Deflated	8	zlib/DEFLATE — the standard, ~95% of ZIPs
Deflate64	9	Extended window (PKWARE proprietary)
BZIP2	12	Better ratio than DEFLATE, slower
LZMA	14	7-Zip LZMA — good ratio, rarely used in ZIP
Zstandard	93	Modern, fast, good ratio (recent addition)
XZ	95	LZMA2 (recent addition)

Stored is used for already-compressed data (JPEG, PNG, MP4, PDF) — re-compressing them wastes CPU without saving space.

Deflated is the universal default — supported everywhere, reasonable ratio, fast.

ZIP64 Extensions

The original ZIP format used 32-bit fields, limiting:

File size to 4 GB (2³² − 1 bytes)
Archive size to 4 GB
Number of files to 65,535

ZIP64 (introduced in PKZIP 4.5) extends these fields to 64 bits:

File size up to 18 EB (2⁶⁴ − 1 bytes)
Archive size up to 18 EB
Number of files up to 4.29 billion

ZIP64 is triggered automatically by modern tools when any limit is exceeded. Most ZIP readers support it, but some legacy tools (including Windows XP Explorer) do not.

Encryption in ZIP

ZIP supports two encryption schemes:

Traditional PKZIP encryption (ZipCrypto): a stream cipher using a 96-bit key derived from the password. Severely weak by modern standards — known-plaintext attacks can recover the key with ~12 bytes of known content. Do not use for sensitive data.

AES encryption (WinZip/APPNOTE AES extension): AES-128 or AES-256 with PBKDF2 key derivation and HMAC-SHA1 authentication. Secure when using a strong password. Uses compression method = 99 as the marker.

# zip with AES-256 on Linux (requires zip >= 3.0)
zip -e --password "strong-password" archive.zip files/

# 7-zip AES-256 in ZIP container
7z a -tzip -mem=AES256 -p"strong-password" archive.zip files/

Note: the file names and directory structure in a ZIP archive are NOT encrypted even with AES encryption — only the file contents are. Use 7z format with AES for full encryption including filenames.

Multi-Volume (Split) Archives

ZIP supports splitting archives across multiple files:

archive.z01  (first part)
archive.z02
archive.z03
archive.zip  (last part — contains the central directory)

# Create split archive (25 MB parts)
zip -s 100m archive.zip largefile.iso

# Combine and extract
zip --fix archive.zip
unzip archive.zip

ZIP in Software Distribution

ZIP is the container of choice for distributing multi-file software packages because:

JAR (Java Archive): a ZIP containing Java .class files, resources, and META-INF/MANIFEST.MF.
WAR (Web Application Archive): a ZIP containing a Java web app.
APK (Android Package): a ZIP containing the Android app's DEX bytecode, resources, and signature.
DOCX / XLSX / PPTX: Office Open XML — a ZIP containing XML files and assets.
EPUB: a ZIP containing XHTML, CSS, and images.
ODF (.odt, .ods, .odp): OpenDocument Format — a ZIP with XML files.
XPI: Firefox extensions.
VSIX: Visual Studio extensions.
wheel (.whl): Python package distribution format.

To inspect these: rename to .zip or use unzip -l file.docx.

Command-Line Usage

# Create ZIP
zip archive.zip file1.txt file2.txt
zip -r archive.zip directory/         # recursive
zip -9 archive.zip files/             # max compression

# Extract
unzip archive.zip
unzip archive.zip -d /output/dir/    # to specific directory
unzip -p archive.zip file.txt        # pipe to stdout

# List contents
unzip -l archive.zip

# Test integrity
unzip -t archive.zip

# Add/update file in existing ZIP
zip archive.zip newfile.txt

# Delete file from ZIP (requires zipnote or repack)
zip -d archive.zip unwanted.txt

Cross-platform: Windows 10+ has native zip/unzip via File Explorer and PowerShell:

Compress-Archive -Path .\files\ -DestinationPath archive.zip
Expand-Archive -Path archive.zip -DestinationPath .\output\

ZIP Vulnerabilities

Zip Slip: a path traversal vulnerability where archive entries contain ../ sequences, causing extraction to write files outside the target directory. Always sanitize entry paths before extraction.

Zip Bomb: a ZIP containing a small compressed file that expands to enormous size (e.g., 42.zip is 42 KB compressed → 4.5 PB uncompressed). Mitigate by tracking uncompressed size during extraction.

Password recovery: ZipCrypto-encrypted archives can be brute-forced rapidly. John the Ripper and Hashcat support ZIP password cracking.

ZIP vs. Other Archive Formats

Format	Compression	Encryption	Random access	Solid archive	Unicode filenames
ZIP	Deflate/etc.	ZipCrypto, AES	Yes	No	UTF-8 (modern)
7z	LZMA2	AES-256	Yes (partial)	Yes	Yes
TAR.GZ	DEFLATE	No (separate)	No	Yes	Yes
TAR.BZ2	BZIP2	No	No	Yes	Yes
RAR	RAR4/5	AES-256	Yes	Yes	Yes
XZ	LZMA2	No	No	Yes	N/A

ZIP's universal compatibility makes it the default for consumer distribution; 7z/XZ win on compression ratio.

Best Practices

Use Stored method for already-compressed files (JPEG, MP4, ZIP) to avoid wasting CPU.
Use AES-256 encryption (not ZipCrypto) for any sensitive archive.
Enable ZIP64 when files or archives may exceed 4 GB.
Sanitize entry paths during extraction to prevent Zip Slip attacks.
Set a size limit during extraction to prevent Zip Bomb denial-of-service.
Use -r recursive when zipping directories to preserve the full tree.
Preserve permissions with zip -X or use TAR for Unix permission sets.
Use UTF-8 flag in modern tools to avoid filename encoding issues across platforms.
Test integrity with unzip -t before distributing archives.
For maximum compression of non-ZIP-native content, consider 7z LZMA2 instead.

Archive format conversions used most often:

Frequently Asked Questions

ZIP stores each file independently with its own compression and stores a central directory index at the end of the archive. To extract a single file, the ZIP reader reads the central directory (at the end), gets the byte offset of the desired file, seeks directly to it, and decompresses only that file. TAR.GZ wraps a TAR archive (which has no index) inside a single gzip stream. To reach file N in a TAR.GZ, the reader must decompress the entire gzip stream from the beginning up to the point where file N starts. For random-access retrieval of individual files, ZIP is far faster; for solid compression (best ratio for many small files), TAR.GZ/XZ win.

7z (LZMA2) typically beats both ZIP and RAR by 10–30% for text-heavy content, but is slower to create. RAR compresses slightly better than ZIP and supports recovery records (good for damaged archives) but the format is proprietary. ZIP is the universal lingua franca — every OS opens it natively. Use 7z for long-term storage, ZIP for sharing.

No. ZipCrypto uses a weak stream cipher with a 96-bit key derived from the password using a flawed algorithm. If an attacker knows 12 or more bytes of any file in the archive (e.g., the standard header of a known file type), they can perform a known-plaintext attack to recover the encryption key without knowing the password — tools like pkcrack can do this in seconds or minutes. For secure encryption, use AES-256 in ZIP (supported by 7-Zip, WinZip, modern Windows) or switch to the 7z format which also encrypts filenames.

Yes, when the file contains anything sensitive. Standard ZIP "encryption" (ZipCrypto) is broken and trivially crackable. Use AES-256 (ZIP 2.0+, supported by 7-Zip, WinRAR, modern Windows). Verify the encryption type when creating: many tools default to ZipCrypto for compatibility. Better still, use 7z which uses AES-256 by default.

A Zip Bomb is a malicious ZIP file that contains a small compressed payload that expands to enormous size — the classic "42.zip" is 42 KB compressed but contains ~4.5 petabytes of data (nested ZIP files of all-zeros). If your code extracts ZIP files without limits, a Zip Bomb can exhaust disk space or memory. Protect your application by: (1) reading the uncompressed size field before extraction and rejecting files above a threshold; (2) tracking total bytes written and aborting if it exceeds a limit; (3) limiting the nesting depth for ZIP-within-ZIP scenarios; (4) using libraries with built-in Zip Bomb protection like Python's zipfile with allowZip64 and size checks.

Original ZIP: 4 GB total + 65,535 files. ZIP64 (transparent extension): up to 16 EB and unlimited files. Modern tools (Windows Explorer, macOS Finder, 7-Zip, Linux unzip) all support ZIP64 automatically when needed. The only catch: very old tools (pre-2007) cannot open ZIP64 archives.

Use ZIP when: the recipient is on Windows (ZIP is natively supported without extra tools), you need random file access within the archive, or you are packaging a format that expects ZIP (DOCX, APK, JAR, EPUB). Use TAR (typically with gzip, bzip2, or xz compression) when: you are on Linux/Unix and need to preserve file permissions, ownership, symlinks, and extended attributes; you want solid compression (single compression stream over all files) for better ratio on many small files; or you are creating backups. TAR.XZ or TAR.ZST offer significantly better compression than ZIP for the same content.

Some content does not compress well: already-compressed media (JPG, MP4, MP3), encrypted files, random binary data. Compression algorithms exploit redundancy — these formats already removed it. To shrink them you would need lossy re-encoding, not archiving. Save archives for text, source code, uncompressed media (BMP, WAV, RAW), and document files.

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