Rar

RAR Archives (Roshal ARchive file format) are a proprietary format for storing information created by Eugene Roshal. The format is currently handled by Alexander Roshal, Eugene's brother.

Format

The file has the magic number of:

0x 52 61 72 21 1A 07 00

Which is a break down of the following to describe an Archive Header:

:* 0x6152 - HEAD_CRC

:* 0x72 - HEAD_TYPE

:* 0x1A21 - HEAD_FLAGS

:* 0x0007 - HEAD_SIZE

Older versions of the RAR file format have a magic number of :

0x 52 45 7E 5E

This format will not be discussed as documentation cannot be found.

RAR File Format

Each Block has the following fields:

Name	Size (bytes)	Description
HEAD_CRC	2	CRC of total block or block part
HEAD_TYPE	1	Block type
HEAD_FLAGS	2	Block flags
HEAD_SIZE	2	Block size
ADD_SIZE	4	Optional field - added block size

Block Fields

The field ADD_SIZE present only if (HEAD_FLAGS & 0x8000) != 0.

Total block size is HEAD_SIZE if (HEAD_FLAGS & 0x8000) == 0 and HEAD_SIZE+ADD_SIZE if the field ADD_SIZE is present - when (HEAD_FLAGS & 0x8000) != 0.

In each block the followings bits in HEAD_FLAGS have the same meaning:

0x4000 - if set, older RAR versions will ignore the block and remove it when the archive is updated. If clear, the block is copied to the new archive file when the archive is updated;
0x8000 - if set, ADD_SIZE field is present and the full block size is HEAD_SIZE+ADD_SIZE.

There are certain block types:

Head Type Signifier	Description
HEAD_TYPE=0x72	marker block
HEAD_TYPE=0x73	archive header
HEAD_TYPE=0x74	file header
HEAD_TYPE=0x75	old style comment header
HEAD_TYPE=0x76	old style authenticity information
HEAD_TYPE=0x77	old style subblock
HEAD_TYPE=0x78	old style recovery record
HEAD_TYPE=0x79	old style authenticity information
HEAD_TYPE=0x7a	subblock
HEAD_TYPE=0x7b	terminator

Block Types

Block Formats

There are several block formats that are contained within a RAR file. They are Marker Block, Archive Header, and File Header.

Marker Block (MARK_HEAD)

Field Name	Size (bytes)	Possibilities
HEAD_CRC	2	Always 0x6152
HEAD_TYPE	1	Header type: 0x72
HEAD_FLAGS	2	Always 0x1A21
HEAD_SIZE	2	Block size = 0x0007

MARK_HEAD

Note: the marker block is considered a fixed byte sequence (AKA, magic number) of: 0x52 0x61 0x72 0x21 0x1A 0x07 0x00 (which is seen as 'Rar! ').

Archive Header (MAIN_HEAD)

Field Name	Size (bytes)	Description
HEAD_CRC	2	CRC of fields HEAD_TYPE to RESERVED2
HEAD_TYPE	1	Header Type: 0x73
HEAD_FLAGS	2	Bit Flags (Please see 'Bit Flags for MAIN_HEAD' table for all possibilities).
HEAD_SIZE	2	Archive header total size including archive comments
RESERVED1	2	RESERVED
RESERVED2	4	RESERVED

MAIN_HEAD

Flag (0x)	Description
0001	Volume attribute (archive volume)
0002	Archive comment present RAR 3.x uses the separate comment block and does not set this flag.
0004	Archive lock attribute
0008	Solid attribute (solid archive)
0010	New volume naming scheme ('volname.partN.rar')
0020	Authenticity information present RAR 3.x does not set this flag.
0040	Recovery record present
0080	Block headers are encrypted
0100	First volume (set only by RAR 3.0 and later)

Bit Flags for MAIN_HEAD

Other bits in HEAD_FLAGS are reserved for internal use.

File Header (File in Archive)

Field Name	Size (bytes)	Description
HEAD_CRC	2	CRC of fields from HEAD_TYPE to FILEATTR and file name
HEAD_TYPE	1	Header Type: 0x74
HEAD_FLAGS	2	Bit Flags (Please see 'Bit Flags for File in Archive' table for all possibilities)
HEAD_SIZE	2	File header full size including file name and comments
PACK_SIZE	4	Compressed file size
UNP_SIZE	4	Uncompressed file size
HOST_OS	1	Operating system used for archiving (See the 'Operating System Indicators' table for the flags used)
FILE_CRC	4	File CRC
FTIME	4	Date and time in standard MS DOS format
UNP_VER	1	RAR version needed to extract file (Version number is encoded as 10 * Major version + minor version.)
METHOD	1	Packing method (Please see 'Packing Method' table for all possibilities
NAME_SIZE	2	File name size
ATTR	4	File attributes
HIGH_PACK_SIZE	4	High 4 bytes of 64-bit value of compressed file size. Optional value, presents only if bit 0x100 in HEAD_FLAGS is set.
HIGH_UNP_SIZE	4	High 4 bytes of 64-bit value of uncompressed file size. Optional value, presents only if bit 0x100 in HEAD_FLAGS is set.
FILE_NAME	NAME_SIZE bytes	File name - string of NAME_SIZE bytes size
SALT	8	present if (HEAD_FLAGS & 0x400) != 0
EXT_TIME	variable size	present if (HEAD_FLAGS & 0x1000) != 0

File Header

other new fields may appear here.

Flag (0x)	Description
01	File continued from previous volume
02	File continued in next volume
04	File encrypted with password
08	File comment present. RAR 3.x uses the separate comment block and does not set this flag.
10	Information from previous files is used (solid flag) (for RAR 2.0 and later)
Dictionary bits 7 6 5 (for RAR 2.0 and later)	Please see the 'Dictionary Bits' table for this descriptions
100	HIGH_PACK_SIZE and HIGH_UNP_SIZE fields are present. These fields are used to archive only very large files (larger than 2Gb), for smaller files these fields are absent.
200	FILE_NAME contains both usual and encoded Unicode name separated by zero. In this case NAME_SIZE field is equal to the length of usual name plus encoded Unicode name plus 1. If this flag is present, but FILE_NAME does not contain zero bytes, it means that file name is encoded using UTF-8.
400	The header contains additional 8 bytes after the file name, which are required to increase encryption security (so called 'salt').
800	Version flag. It is an old file version, a version number is appended to file name as ';n'.
1000	Extended time field present.
8000	This bit always is set, so the complete block size is HEAD_SIZE + PACK_SIZE (and plus HIGH_PACK_SIZE, if bit 0x100 is set)

Bit Flags for Files in Archive

Bits (7 6 5)	Description	Size (KB)
0 0 0	Dictionary Size	64
0 0 1	Dictionary Size	128
0 1 0	Dictionary Size	256
0 1 1	Dictionary Size	512
1 0 0	Dictionary Size	1024
1 0 1	Dictionary Size	2048
1 1 0	Dictionary Size	4096
1 1 1	file is a directory	N/A

Dictionary Bits

Byte Indicator	Operating System
0	MS DOS
1	OS/2
2	Windows
3	Unix
4	Mac OS
5	BeOS

Operating System Indicators

Byte Indicator	Method
0x30	Storing
0x31	Fastest Compression
0x32	Fast Compression
0x33	Normal Compression
0x34	Good Compression
0x35	Best Compression

Packing Method

Terminator (terminator)

There is often a terminating block present.

Field Name	Size (bytes)	Possibilities
HEAD_CRC	2	Always 0x3DC4
HEAD_TYPE	1	Header type: 0x7b
HEAD_FLAGS	2	Always 0x4000
HEAD_SIZE	2	Block size = 0x0007

terminator

As such, it is possible to use the terminator as a footer when carving, although, as always, file fragmentation should be considered.

0x C4 3D 7B 00 40 07 00

Metadata

Sub-formats

The RAR format is comprised of many sub-formats that have changed over the years. The different formats and their descriptions are as follows:

:* 1.3 (Does not have the RAR! signature)

:** There is difficulty finding information regarding this sub-format. Please update if you know something.

:* 1.5

:** Utilizes a proprietary compression method that is not available to the public.

:** Considered the root model of subsequent formats.

:* 2.0

:** Utilizes a proprietary compression method that is not available to the public.

:** Based off of version 1.5 of the RAR file format.

:* 3.0

:** Utilizes the PPMII and Lempel-Ziv (LZSS)] algorithms.

:** Encryption now uses cipher block chaining (CBC) instead of Advanced Encryption Standard (AES).

:** Based off of version 1.5 of the RAR file format.

Software

This only way to create a RAR file is using the Winrar software. There are several implementations of the process to open a RAR file (commonly known as the "unrar" process). Some of them are:

unrarLib

:* RAR file unarchiver written in C

:* Easy implementation with a header file and the source code file

:* Only works with RAR files up to version 2.0

:* Information Link

WinRAR

:* Only software that can create and open a RAR file

:* Distributed by a proprietary license

:* Works with all versions of RAR files

:* WinRAR executable for Windows

UnRAR

:* Created by Eugene Roshal for opening up RAR files only