7Z file format: coders and method IDs

Each coder in a folder names its method with an ID of 1 to 15 bytes. The register is Methods.txt in the 7-Zip source. Most archives use only LZMA2 or LZMA, sometimes with a branch filter and 7zAES; the rest of the register is there because 7-Zip’s codecs are shared between its archive handlers, and a 7z archive may name any of them.

Methods 7z archives use

ID	Method	Properties
`00`	Copy (stored, no compression)	None
`03`	Delta filter	1 byte: distance − 1 (1 to 256 bytes)
`04`	BCJ x86 filter (new ID form)	None
`05`	PPC branch filter, big-endian	None
`06`	IA64 branch filter	None
`07`	ARM branch filter, little-endian	None
`08`	ARM Thumb branch filter, little-endian	None
`09`	SPARC branch filter	None
`0A`	ARM64 branch filter	Optional; 7-Zip accepts a 4-byte start offset
`0B`	RISC-V branch filter	Optional, as ARM64
`21`	LZMA2	1 byte: dictionary size
`03 01 01`	LZMA	5 bytes: lc, lp, pb and dictionary size
`03 03 01 03`	BCJ x86 filter	None
`03 03 01 1B`	BCJ2 x86 filter, four packed streams	None
`03 03 02 05`	PPC branch filter, big-endian	None
`03 03 03 01`	Alpha branch filter	None
`03 03 04 01`	IA64 branch filter	None
`03 03 05 01`	ARM branch filter, little-endian	None
`03 03 06 05`	M68 branch filter, big-endian	None
`03 03 07 01`	ARM Thumb branch filter, little-endian	None
`03 03 08 05`	SPARC branch filter	None
`03 04 01`	PPMd (variant H)	5 bytes: order and memory size
`03 7F 01`	Experimental method	–
`04 02 02`	BZip2	None
`04 01 08`	Deflate	None
`04 01 09`	Deflate64	None
`06 F1 07 01`	7zAES: AES-256 with a SHA-256 key	Cycles, salt and IV

The one-byte filter IDs (04 to 0B) are the newer form; 7-Zip still writes the four-byte 03 03 … IDs for the older filters, and readers accept both.

The rest of the register

ID	Method
`02 03 02`	Swap2: swaps bytes in 2-byte units
`02 03 04`	Swap4: swaps bytes in 4-byte units
`04 00`	Reserved
`04 01 00`	ZIP Copy (not used; use `00`)
`04 01 01`	ZIP Shrink
`04 01 06`	ZIP Implode
`04 01 0A`	ZIP Imploding (method 10)
`04 01 0C`	ZIP BZip2 (not used; use `04 02 02`)
`04 01 0E`	ZIP LZMA (LZMA as stored in ZIP)
`04 01 5D`	ZIP Zstandard
`04 01 5F`	ZIP xz
`04 01 60`	ZIP JPEG
`04 01 61`	ZIP WavPack
`04 01 62`	ZIP PPMd (variant I as stored in ZIP)
`04 01 63`	ZIP WinZip AES
`04 03 01`	RAR 1.x
`04 03 02`	RAR 2.x
`04 03 03`	RAR 3.x and 4.x
`04 03 05`	RAR 5
`04 04 01`	ARJ methods 1, 2 and 3
`04 04 02`	ARJ method 4
`04 05`	Z (Unix compress)
`04 06`	LZH
`04 07`	Reserved for 7z
`04 08`	CAB
`04 09 01`	NSIS Deflate
`04 09 02`	NSIS BZip2
`04 F7 0x xx`	Reserved for external codecs
`04 F7 10 01`	LZHAM
`04 F7 11 01`	Zstandard (Tino Reichardt’s 7-Zip build)
`04 F7 11 02`	Brotli
`04 F7 11 04`	LZ4
`04 F7 11 05`	LZ5
`04 F7 11 06`	Lizard
`04 F7 12 01`	WavPack2
`04 F7 12 FE`	eSplitter
`04 F7 12 FF`	RawSplitter
`04 F7 13 01`	Transpose
`06 F0 01 00` to `04`	AES-128 in ECB, CBC, CFB, OFB and CTR modes
`06 F0 01 40` to `44`	AES-192 in the same five modes
`06 F0 01 80` to `84`	AES-256 in the same five modes
`06 F0 01 C0` to `C4`	AES with the key size given elsewhere, same five modes
`06 F1 01 01`	ZipCrypto (traditional ZIP encryption)
`06 F1 03 02`	RAR 2.x encryption (named only “RAR” in the register)
`06 F1 03 03`	RAR 2.9 AES: AES-128 with a modified SHA-1 key
`06 F1 07 02` to `FF`	Reserved for 7z

The register also lists a few more private ranges kept for other developers; IDs not shown here have no public description, and a reader that does not know an ID cannot unpack that folder.

LZMA properties

Offset	Size	Meaning
`0`	1	`(pb × 5 + lp) × 9 + lc`: `lc` literal context bits 0 to 8, `lp` literal position bits 0 to 4, `pb` position bits 0 to 4. The usual value `5D` means lc 3, lp 0, pb 2.
`1`	4	Dictionary size in bytes, little-endian.

These are the same five bytes that start a .lzma file. In 7z the uncompressed size is not stored with them; it comes from the folder’s unpack size.

LZMA2 properties

One byte, 0 to 40. The dictionary size is (2 | (b & 1)) << (b / 2 + 11), so 0 is 4 KiB, 1 is 6 KiB, 2 is 8 KiB and so on, doubling every two steps; 40 means 4 GiB − 1. 18 (24) is 16 MiB. The LZMA2 chunk format is the same as in .xz files.

PPMd properties

Offset	Size	Meaning
`0`	1	Model order, 2 to 32.
`1`	4	Memory size in bytes, little-endian.

7zAES properties

The key is made by hashing the password (UTF-16LE) and the salt with SHA-256 2^cycles times, adding a 64-bit counter each round. Data is AES-256 in CBC mode. The property layout is from 7-Zip’s source, as the format description does not give it.

Byte	Bits	Meaning
`0`	0 to 5	`NumCyclesPower`: log2 of the number of SHA-256 rounds, normally 19.
`0`	6	An IV follows (adds 1 to the IV size).
`0`	7	A salt follows (adds 1 to the salt size).
`1`	4 to 7	Added to the salt size. Present only when bit 6 or 7 of byte 0 is set.
`1`	0 to 3	Added to the IV size.
2 …		The salt bytes, then the IV bytes (up to 16; a shorter IV is padded with zeros).

When the end header is encrypted too, the file names cannot be listed without the password.