ZIP Format - 7-Zip

ZIP is one of the most widely used archive formats, offering broad compatibility across all operating systems and applications.

Format Overview

The ZIP format provides:

Universal compatibility - Supported by virtually all platforms
Multiple compression methods - Deflate, BZip2, LZMA, and more
ZIP64 support - Files up to 16 exabytes
Strong encryption - AES-256 and WinZip AES
Split archives - Multi-volume support
Self-extracting - Create executable archives

Format Structure

According to source/CPP/7zip/Archive/Zip/ZipHeader.h, ZIP archives use the following structure:

[Local File Header + File Data + Data Descriptor] ...
[Central Directory Header] ...
[ZIP64 End of Central Directory Record]
[ZIP64 End of Central Directory Locator]
[End of Central Directory Record]

Signature Constants

From source/CPP/7zip/Archive/Zip/ZipHeader.h:13-23:

namespace NSignature {
  const UInt32 kLocalFileHeader   = 0x04034B50;
  const UInt32 kDataDescriptor    = 0x08074B50;
  const UInt32 kCentralFileHeader = 0x02014B50;
  const UInt32 kEcd               = 0x06054B50;  // End of Central Dir
  const UInt32 kEcd64             = 0x06064B50;  // ZIP64 ECD
  const UInt32 kEcd64Locator      = 0x07064B50;  // ZIP64 Locator
}

All multi-byte values in ZIP format use little-endian byte order.

Compression Methods

The ZIP format supports numerous compression methods (ZipHeader.h:37-68):

Standard Methods

Store (0) - No compression
Deflate (8) - Most common, good speed/ratio balance
Deflate64 (9) - Enhanced Deflate
BZip2 (12) - Better compression than Deflate
LZMA (14) - Excellent compression ratio

Advanced Methods

XZ (95) - LZMA2-based compression
PPMd (98) - Context modeling compression
Zstd (20, 93) - Modern fast compression
WavPack (97) - Audio compression
JPEG (96) - Image compression

Encryption Methods

ZipCrypto - Legacy weak encryption (not recommended)
WzAES (99) - AES encryption with authentication

ZipCrypto encryption is cryptographically weak and should not be used for sensitive data. Always use WzAES (AES-256) encryption for security.

Usage Examples

Create Standard ZIP Archive

7z a -tzip archive.zip files/

Uses Deflate compression by default.

Create ZIP with Maximum Compression

7z a -tzip -mm=Deflate -mx=9 archive.zip files/

Parameters:

-tzip - ZIP format
-mm=Deflate - Deflate compression method
-mx=9 - Maximum compression level

Create ZIP with LZMA Compression

7z a -tzip -mm=LZMA archive.zip files/

Provides better compression ratio but less compatibility.

ZIP archives with LZMA compression require 7-Zip or compatible software to extract. Standard ZIP utilities may not support this method.

Create Encrypted ZIP Archive

7z a -tzip -mem=AES256 -p archive.zip files/

Parameters:

-mem=AES256 - Use AES-256 encryption
-p - Prompt for password

Create Split ZIP Archive

7z a -tzip -v100m archive.zip files/

Creates: archive.zip, archive.z01, archive.z02, etc.

Create Self-Extracting Archive

7z a -tzip -sfx archive.exe files/

ZIP64 Format

ZIP64 extensions allow archives to exceed original ZIP limitations:

Feature	Original ZIP	ZIP64
Archive size	4 GB	16 EB
File size	4 GB	16 EB
Number of files	65,535	~4 billion

From ZipHeader.h:31-33:

const unsigned kEcd64_MainSize = 44;
const unsigned kEcd64_FullSize = 12 + kEcd64_MainSize;
const unsigned kEcd64Locator_Size = 20;

7-Zip automatically uses ZIP64 format when needed. You don’t need to specify it explicitly.

Version Requirements

Different ZIP features require minimum extraction versions (ZipHeader.h:72-82):

const Byte kExtractVersion_Default = 10;      // 1.0
const Byte kExtractVersion_Dir = 20;          // 2.0
const Byte kExtractVersion_Deflate = 20;      // 2.0
const Byte kExtractVersion_Deflate64 = 21;    // 2.1
const Byte kExtractVersion_Zip64 = 45;        // 4.5
const Byte kExtractVersion_BZip2 = 46;        // 4.6
const Byte kExtractVersion_Aes = 51;          // 5.1
const Byte kExtractVersion_LZMA = 63;         // 6.3

File Attributes

Host Operating Systems

ZIP format tracks the OS where the file was created (ZipHeader.h:155-180):

FAT (0) - MS-DOS, Windows
Unix (3) - Unix, Linux, macOS
NTFS (11) - Windows NT
VFAT (14) - Windows 95/NT
OSX (19) - macOS

Extra Fields

ZIP supports various extra data fields (ZipHeader.h:86-102):

Zip64 (0x01) - ZIP64 extended information
NTFS (0x0A) - NTFS timestamps and attributes
Unix Time (0x5455) - Unix timestamp
Unicode Name (0x7075) - UTF-8 filename
WzAES (0x9901) - AES encryption parameters

File Flags

From ZipHeader.h:136-142:

const unsigned kEncrypted = 1 << 0;              // Bit 0: Encrypted
const unsigned kDescriptorUsedMask = 1 << 3;    // Bit 3: Data descriptor
const unsigned kStrongEncrypted = 1 << 6;        // Bit 6: Strong encryption
const unsigned kUtf8 = 1 << 11;                  // Bit 11: UTF-8 encoding

Advanced Features

Preserve Unix Permissions

7z a -tzip -snl archive.zip files/

The -snl option stores symbolic links.

Exclude Files by Pattern

7z a -tzip archive.zip files/ -x!*.tmp -x!*.log

Update Existing Archive

7z u archive.zip newfile.txt

Only adds or updates changed files.

List Archive Contents

7z l archive.zip

Shows detailed information about archived files.

Test Archive Integrity

7z t archive.zip

Implementation Details

The ZIP handler is implemented in:

source/CPP/7zip/Archive/Zip/
├── ZipHandler.cpp      # Archive reading
├── ZipHandlerOut.cpp   # Archive creation
├── ZipIn.cpp          # Input processing
├── ZipOut.cpp         # Output processing
├── ZipHeader.h        # Format constants
├── ZipItem.h          # Item structures
└── ZipUpdate.cpp      # Archive updating

Compression Level Comparison

Deflate Compression

Level	Speed	Ratio	Use Case
-mx=1	Fastest	~40%	Quick backup
-mx=5	Fast	~50%	General use (default)
-mx=7	Medium	~55%	Better compression
-mx=9	Slow	~60%	Maximum Deflate

Method Comparison (1 GB mixed files)

Method	Time	Size	Compatibility
Store	10s	1000 MB	Universal
Deflate	60s	450 MB	Universal
Deflate64	75s	430 MB	Good
BZip2	120s	400 MB	Good
LZMA	180s	350 MB	Limited

Performance varies significantly based on data type. Text files compress better than binary data or already-compressed formats.

Compatibility Notes

Universal Compatibility

For maximum compatibility across all platforms:

7z a -tzip -mm=Deflate -mx=5 archive.zip files/

Windows Compatibility

Windows Explorer can open ZIP files with:

Store (0)
Deflate (8)
No encryption or WinZip AES

macOS/Linux Compatibility

Most Unix tools support:

Store, Deflate, Deflate64
BZip2 with proper libraries

Limitations

Be aware of these ZIP format limitations:

No solid compression (each file compressed independently)
Limited recovery information
Weak legacy encryption (ZipCrypto)
Some methods have limited tool support

Best Practices

For Sharing

Use Deflate compression with -mx=5 to -mx=7

For Security

Use AES-256 encryption: -mem=AES256 -p

For Size

Use LZMA method if recipients have 7-Zip

For Speed

Use Deflate with -mx=1 for fast compression

​Format Overview

​Format Structure

​Signature Constants

​Compression Methods

​Standard Methods

​Advanced Methods

​Encryption Methods

​Usage Examples

​Create Standard ZIP Archive

​Create ZIP with Maximum Compression

​Create ZIP with LZMA Compression

​Create Encrypted ZIP Archive

​Create Split ZIP Archive

​Create Self-Extracting Archive

​ZIP64 Format

​Version Requirements

​File Attributes

​Host Operating Systems

​Extra Fields

​File Flags

​Advanced Features

​Preserve Unix Permissions

​Exclude Files by Pattern

​Update Existing Archive

​List Archive Contents

​Test Archive Integrity

​Implementation Details

​Compression Level Comparison

​Deflate Compression

​Method Comparison (1 GB mixed files)

​Compatibility Notes

​Universal Compatibility

​Windows Compatibility

​macOS/Linux Compatibility

​Limitations

​Best Practices

For Sharing

For Security

For Size

For Speed

​See Also

Format Overview

Format Structure

Signature Constants

Compression Methods

Standard Methods

Advanced Methods

Encryption Methods

Usage Examples

Create Standard ZIP Archive

Create ZIP with Maximum Compression

Create ZIP with LZMA Compression

Create Encrypted ZIP Archive

Create Split ZIP Archive

Create Self-Extracting Archive

ZIP64 Format

Version Requirements

File Attributes

Host Operating Systems

Extra Fields

File Flags

Advanced Features

Preserve Unix Permissions

Exclude Files by Pattern

Update Existing Archive

List Archive Contents

Test Archive Integrity

Implementation Details

Compression Level Comparison

Deflate Compression

Method Comparison (1 GB mixed files)

Compatibility Notes

Universal Compatibility

Windows Compatibility

macOS/Linux Compatibility

Limitations

Best Practices

See Also