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


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

RAR File Format

Each Block has the following fields

Block 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

There are certain block types

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

Block Formats

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
  • 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_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

Bit Flags for 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)
  • Other bits in HEAD_FLAGS are reserved for internal use.
Operating System Indications
Byte Indicator Operating System
1 OS/2
2 Windows
3 Unix
4 Mas OS
5 BeOS

File Header (File in Archive)

File Header
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 'OS Indications' table for the flags used)
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
  • other new fields may appear here.

Bit Flags for Files in Archive
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)
Dictionary Bits
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



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.
    • A detailed list of information can be found here.
  • 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.


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:

  • RAR file unarchiver written in C
  • Easy implementation with a header file and the source code file
  • Information Link
  • Created by Eugene Roshal for opening up RAR files only
  • May not be used to reverse engineer the RAR file format and create RAR files
  • Source code provided for people to implement/integrate methods of opening RAR files
  • Additionally, implementations of UnRAR are available for a plethora of operating systems
  • Download Link
The Unarchiver
  • Utility made for Windows applications to open a multitude of files, including RAR files
  • Download Link

There is a lot more software to open RAR files, but have been omitted due to redundancy.

See Also