7z Archive Format Specification

This section contains the comprehensive specification for the 7z archive format as implemented by zesven.

Purpose

This specification provides a precise, unambiguous definition of the 7z archive format suitable for:

• Implementing compliant readers and writers
• Validating archive correctness
• Understanding format internals
• Ensuring interoperability with 7-Zip and other implementations

Scope

In scope:

• Binary format structure and layout
• Data encoding schemes
• Compression and filter method interfaces
• Encryption scheme
• File metadata representation
• Error conditions and handling

Out of scope:

• Compression algorithm internals (LZMA, LZMA2, etc.)
• Cryptographic primitive implementations
• Platform-specific extraction behaviors
• User interface considerations

Document Conventions

Requirement Keywords

This specification uses requirement level keywords as defined in RFC 2119:

Keyword	Meaning
MUST	Absolute requirement
MUST NOT	Absolute prohibition
SHOULD	Recommended but not required
SHOULD NOT	Not recommended but not prohibited
MAY	Optional

Byte Order

All multi-byte integers are little-endian unless explicitly stated otherwise.

Binary Notation

• Hexadecimal bytes: 0x7A or 7A
• Byte sequences: 37 7A BC AF 27 1C
• Binary literals: 0b10110100
• Bit ranges: bits 0-3 (inclusive, 0 is LSB)

Range Notation

All numeric ranges in this specification are inclusive on both ends unless otherwise noted:

• 0-255 means values from 0 to 255 inclusive (256 total values)
• 0x00-0x04 means values from 0x00 to 0x04 inclusive (5 total values)

For loop bounds in pseudocode:

• for i in 0..N means i takes values 0, 1, 2, ..., N-1 (exclusive upper bound, Python/Rust style)
• for i in 0 to N - 1 means the same range explicitly

Size Units

• All sizes are in bytes unless otherwise specified
• 1 KiB = 1024 bytes
• 1 MiB = 1024 KiB
• 1 GiB = 1024 MiB

Grammar Notation

Structure definitions use a BNF-like notation:

Structure ::= Field1 Field2 [OptionalField] Field3*
Field1 ::= BYTE
Field2 ::= NUMBER
OptionalField ::= 0x01 Data
Field3 ::= UINT32

Where:

• [...] denotes optional elements
• * denotes zero or more repetitions
• + denotes one or more repetitions
• | denotes alternatives

Document Map

Foundation (00-04)

Document	Description
Philosophy	Design principles, goals, invariants
Glossary	Canonical terminology definitions
Archive Structure	High-level archive layout
Signature Header	32-byte start header format
Data Encoding	NUMBER, BitField, and type encodings

Header Format (05-09)

Document	Description
Header Structure	Main and encoded header organization
Pack Info	Compressed stream information
Unpack Info	Folder and coder definitions
Substreams Info	Per-file data within folders
Files Info	File metadata and properties

Codecs and Filters (10-12)

Document	Description
Compression Methods	Compression algorithm interfaces
Filters	BCJ, Delta, and other preprocessors
Encryption	AES-256-SHA256 encryption scheme

Special Features (13-15)

Document	Description
Solid Archives	Solid block compression
Multi-Volume	Split archive handling
SFX Archives	Self-extracting archives

Metadata and Safety (16-18)

Document	Description
Timestamps & Attributes	Time and attribute formats
Security	Safety constraints and limits
Error Conditions	Error handling requirements

Reference Appendices

Document	Description
A: Property IDs	Complete property ID table
B: Method IDs	Complete compression method ID table
C: CRC Algorithm	CRC-32 specification
D: Compatibility	Interoperability notes

Reading Order

For implementers new to the 7z format:

1. Start with Philosophy for context
2. Read Glossary to understand terminology
3. Continue with Archive Structure for the big picture
4. Then proceed sequentially through the remaining documents

For quick reference:

• Property IDs: Appendix A
• Method IDs: Appendix B
• CRC algorithm: Appendix C

Version History

Version	Date	Changes
1.0.0	2025-01	Initial specification

Acknowledgments

This specification is derived from:

• Official 7-Zip source code by Igor Pavlov
• py7zr documentation by Hiroshi Miura
• Analysis of multiple open-source implementations

License

This specification is part of the zesven project and is licensed under MIT OR Apache-2.0.