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The 7z format is a modern archive format that provides high compression ratios and advanced features.
From DOC/7zFormat.txt:
7z archive can contain files compressed with any method.
The format supports:
Solid compression
Multiple compression methods
Strong AES-256 encryption
Large file support (16 EB limit)
Unicode filenames
File attributes and timestamps
Archive Structure A 7z archive consists of:
[SignatureHeader] [PackedStreams] // Compressed data [PackedHeaderStreams] // Compressed headers (optional) [Header] // Archive metadata
From 7z.h (lines 11-14):
#define k7zStartHeaderSize 0x 20 // 32 bytes #define k7zSignatureSize 6 const Byte k7zSignature [k7zSignatureSize] = { '7' , 'z' , 0x BC , 0x AF , 0x 27 , 0x 1C };
Structure (32 bytes):Offset Size Description ------ ---- ----------- 0 6 Signature: 37 7A BC AF 27 1C 6 1 ArchiveVersion (Major) 7 1 ArchiveVersion (Minor) 8 4 StartHeaderCRC 12 8 NextHeaderOffset 20 8 NextHeaderSize 28 4 NextHeaderCRC
From DOC/7zFormat.txt (lines 100-150):
Header Types: kEnd = 0x00 kHeader = 0x01 kArchiveProperties = 0x02 kAdditionalStreamsInfo = 0x03 kMainStreamsInfo = 0x04 kFilesInfo = 0x05 kPackInfo = 0x06 kUnpackInfo = 0x07 kSubStreamsInfo = 0x08 kSize = 0x09 kCRC = 0x0A kFolder = 0x0B kCodersUnpackSize = 0x0C kNumUnpackStream = 0x0D kEmptyStream = 0x0E kEmptyFile = 0x0F kAnti = 0x10 kName = 0x11 kCTime = 0x12 kATime = 0x13 kMTime = 0x14 kWinAttrib = 0x15 kComment = 0x16 kEncodedHeader = 0x17 kStartPos = 0x18 kDummy = 0x19
Compression Methods From DOC/Methods.txt, method IDs:
Common Methods Method ID Description Copy 00No compression LZMA 03 01 01LZMA algorithm LZMA2 21LZMA2 (default) PPMd 03 04 01Prediction by partial matching BZip2 04 02 02Burrows-Wheeler transform Deflate 04 01 08Deflate (ZIP)
Filters Filter ID Description BCJ 04x86 executable filter BCJ2 03 03 01 1Bx86 advanced filter ARM 07ARM executable filter ARM64 0AARM64 filter Delta 03Delta filter
Folder Structure From 7z.h (lines 42-51):
typedef struct { UInt32 NumCoders; // Number of compression methods UInt32 NumBonds; // Number of bonds between coders UInt32 NumPackStreams; UInt32 UnpackStream; UInt32 PackStreams [SZ_NUM_PACK_STREAMS_IN_FOLDER_MAX]; CSzBond Bonds [SZ_NUM_BONDS_IN_FOLDER_MAX]; CSzCoderInfo Coders [SZ_NUM_CODERS_IN_FOLDER_MAX]; } CSzFolder;
A folder represents a compression unit:
Can contain multiple files (solid compression)
Defines the compression method chain
Stores coder properties
From 7z.h (lines 24-30):
typedef struct { size_t PropsOffset; // Offset to properties in CodersData UInt32 MethodID; // Compression method ID Byte NumStreams; // Number of input/output streams Byte PropsSize; // Size of properties } CSzCoderInfo;
File Item From 7z.h (lines 56-66):
typedef struct { CNtfsFileTime MTime; // Modification time UInt64 Size ; // Uncompressed size UInt32 Crc; // CRC32 UInt32 Attrib; // File attributes Byte HasStream; // Has data stream Byte IsDir; // Is directory Byte IsAnti; // Anti-item (for updates) Byte CrcDefined; // CRC is valid Byte MTimeDefined; // MTime is valid Byte AttribDefined; // Attrib is valid } CSzFileItem;
File Attributes Windows attributes:
#define FILE_ATTRIBUTE_READONLY 0x 0001 #define FILE_ATTRIBUTE_HIDDEN 0x 0002 #define FILE_ATTRIBUTE_SYSTEM 0x 0004 #define FILE_ATTRIBUTE_DIRECTORY 0x 0010 #define FILE_ATTRIBUTE_ARCHIVE 0x 0020
Unix attributes (stored in high 16 bits):
UInt32 unixMode = (attrib >> 16 ) & 0x FFFF ;
Encryption 7zAES Method ID : 06 F1 07 01From DOC/Methods.txt (lines 172-173):
07 - [7z] 01 - 7zAES (AES-256 + SHA-256)
Properties structure :Byte[1] - Version (0x00) Byte[1] - Number of cycles (power of 2) Byte[1] - Salt size (0-16) Byte[*] - Salt data Byte[*] - Password verification data Byte[16] - IV
Key derivation :
Concatenate: Password + Salt
Hash with SHA-256, repeated 2^NumCycles times
Result: 32-byte AES key
When -mhe=on is used:
Archive headers are compressed
Compressed headers are encrypted with AES-256
Only signature header remains unencrypted
Password required to list files
Archive Database From 7z.h (lines 79-96):
typedef struct { UInt32 NumPackStreams; // Number of packed streams UInt32 NumFolders; // Number of folders UInt64 * PackPositions; // Positions of packed streams CSzBitUi32s FolderCRCs; // CRCs for folders size_t * FoCodersOffsets; // Offsets to coder data UInt32 * FoStartPackStreamIndex; // Start pack stream for folder UInt32 * FoToCoderUnpackSizes; // Unpack sizes Byte * FoToMainUnpackSizeIndex; // Main unpack size index UInt64 * CoderUnpackSizes; // Unpack sizes for all coders Byte * CodersData; // Coder properties data UInt64 RangeLimit; } CSzAr;
Property Limit Note File size 16 EB 2^64 bytes Archive size 16 EB 2^64 bytes Files per archive 2^32 4 billion Filename length 2^16 chars 64K Unicode chars Solid block size Unlimited Memory constrained Dictionary size 1.5 GB LZMA2 limit
Reading 7z Archives Opening Archive From DOC/7zC.txt (lines 93-107):
// 1. Declare variables CLookToRead2 lookStream; CSzArEx db; ISzAlloc allocImp; ISzAlloc allocTempImp; // 2. Initialize CRC table CrcGenerateTable (); // 3. Initialize database SzArEx_Init ( & db ); // 4. Open archive SRes res = SzArEx_Open ( & db , & lookStream.vt, & allocImp , & allocTempImp ); if (res == SZ_OK) { // Archive opened successfully // db.NumFiles contains file count }
// Extract file by index UInt32 fileIndex = 0 ; UInt32 blockIndex = 0x FFFFFFFF ; Byte * outBuffer = NULL ; size_t outBufferSize = 0 ; for ( UInt32 i = 0 ; i < db.NumFiles; i ++ ) { size_t offset = 0 ; size_t outSizeProcessed = 0 ; SRes res = SzArEx_Extract ( & db, & lookStream . vt , i, & blockIndex, & outBuffer, & outBufferSize, & offset, & outSizeProcessed, & allocImp, & allocTempImp ); if (res == SZ_OK) { // File data is at: outBuffer + offset // Size: outSizeProcessed } } // Free output buffer allocImp. Free ( & allocImp , outBuffer);
Creating 7z Archives Archive Handler From CPP/7zip/Archive/7z/7zHandler.h:
class CHandler : public IInArchive , public IOutArchive { public: // IInArchive STDMETHOD ( Open )( IInStream * stream , ...); STDMETHOD ( Extract )( const UInt32 * indices , UInt32 numItems , ...); // IOutArchive STDMETHOD ( UpdateItems )( ISequentialOutStream * outStream , ...); };
Update Operations // Update modes enum EUpdateMode { k_UpdateMode_Add , // Add new file k_UpdateMode_Update , // Update existing file k_UpdateMode_Delete , // Delete file k_UpdateMode_Rename // Rename file };
High Compression LZMA2 provides excellent compression ratios
Solid Compression Compress similar files together for better ratios
Strong Encryption AES-256 encryption with SHA-256 key derivation
Large File Support Support for files and archives up to 16 EB
Unicode Names Full Unicode support for filenames
Extensible Support for custom compression methods
Feature 7z ZIP TAR.GZ RAR Compression Excellent Good Good Excellent Speed Medium Fast Fast Medium Solid Yes No No Yes Encryption AES-256 AES-256 None AES-256 Header encryption Yes No No Yes Multi-threading Yes Partial No Yes Open source Yes Yes Yes No
Implementation Files Key source files:
Format handler : CPP/7zip/Archive/7z/7zHandler.cppArchive reader : C/7zArcIn.cDecoder : C/7zDec.cHeader structures : C/7z.hFormat constants : CPP/7zip/Archive/7z/7zHeader.h
Complete format specification:
DOC/7zFormat.txt - Full format specificationDOC/Methods.txt - Compression method IDsDOC/7zC.txt - ANSI-C decoder documentation
See Also