RAR version 4.20 - Technical information
RAR version 4.20 - Technical information ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ THE ARCHIVE FORMAT DESCRIBED BELOW IS ONLY VALID FOR VERSIONS SINCE 1.50 ========================================================================== RAR archive file format ========================================================================== Archive file consists of variable length blocks. The order of these blocks may vary, but the first block must be a marker block followed by an archive header block. Each block begins with the following fields: HEAD_CRC 2 bytes CRC of total block or block part HEAD_TYPE 1 byte Block type HEAD_FLAGS 2 bytes Block flags HEAD_SIZE 2 bytes Block size ADD_SIZE 4 bytes Optional field - added block size 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. Declared block types: 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 Comment block is actually used only within other blocks and doesn't exist separately. Archive processing is made in the following manner: 1. Read and check marker block 2. Read archive header 3. Read or skip HEAD_SIZE-sizeof(MAIN_HEAD) bytes 4. If end of archive encountered then terminate archive processing, else read 7 bytes into fields HEAD_CRC, HEAD_TYPE, HEAD_FLAGS, HEAD_SIZE. 5. Check HEAD_TYPE. if HEAD_TYPE==0x74 read file header ( first 7 bytes already read ) read or skip HEAD_SIZE-sizeof(FILE_HEAD) bytes if (HEAD_FLAGS & 0x100) read or skip HIGH_PACK_SIZE*0x100000000+PACK_SIZE bytes else read or skip PACK_SIZE bytes else read corresponding HEAD_TYPE block: read HEAD_SIZE-7 bytes if (HEAD_FLAGS & 0x8000) read ADD_SIZE bytes 6. go to 4. ========================================================================== Block Formats ========================================================================== Marker block ( MARK_HEAD ) HEAD_CRC Always 0x6152 2 bytes HEAD_TYPE Header type: 0x72 1 byte HEAD_FLAGS Always 0x1a21 2 bytes HEAD_SIZE Block size = 0x0007 2 bytes The marker block is actually considered as a fixed byte sequence: 0x52 0x61 0x72 0x21 0x1a 0x07 0x00 Archive header ( MAIN_HEAD ) HEAD_CRC CRC of fields HEAD_TYPE to RESERVED2 2 bytes HEAD_TYPE Header type: 0x73 1 byte HEAD_FLAGS Bit flags: 2 bytes 0x0001 - Volume attribute (archive volume) 0x0002 - Archive comment present RAR 3.x uses the separate comment block and does not set this flag. 0x0004 - Archive lock attribute 0x0008 - Solid attribute (solid archive) 0x0010 - New volume naming scheme ('volname.partN.rar') 0x0020 - Authenticity information present RAR 3.x does not set this flag. 0x0040 - Recovery record present 0x0080 - Block headers are encrypted 0x0100 - First volume (set only by RAR 3.0 and later) other bits in HEAD_FLAGS are reserved for internal use HEAD_SIZE Archive header total size including archive comments 2 bytes RESERVED1 Reserved 2 bytes RESERVED2 Reserved 4 bytes File header (File in archive) HEAD_CRC CRC of fields from HEAD_TYPE to FILEATTR 2 bytes and file name HEAD_TYPE Header type: 0x74 1 byte HEAD_FLAGS Bit flags: 2 bytes 0x01 - file continued from previous volume 0x02 - file continued in next volume 0x04 - file encrypted with password 0x08 - file comment present RAR 3.x uses the separate comment block and does not set this flag. 0x10 - information from previous files is used (solid flag) (for RAR 2.0 and later) bits 7 6 5 (for RAR 2.0 and later) 0 0 0 - dictionary size 64 KB 0 0 1 - dictionary size 128 KB 0 1 0 - dictionary size 256 KB 0 1 1 - dictionary size 512 KB 1 0 0 - dictionary size 1024 KB 1 0 1 - dictionary size 2048 KB 1 1 0 - dictionary size 4096 KB 1 1 1 - file is directory 0x100 - 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. 0x200 - 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. 0x400 - the header contains additional 8 bytes after the file name, which are required to increase encryption security (so called 'salt'). 0x800 - Version flag. It is an old file version, a version number is appended to file name as ';n'. 0x1000 - Extended time field present. 0x8000 - 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) HEAD_SIZE File header full size including file name and comments 2 bytes PACK_SIZE Compressed file size 4 bytes UNP_SIZE Uncompressed file size 4 bytes HOST_OS Operating system used for archiving 1 byte 0 - MS DOS 1 - OS/2 2 - Windows 3 - Unix 4 - Mac OS 5 - BeOS FILE_CRC File CRC 4 bytes FTIME Date and time in standard MS DOS format 4 bytes UNP_VER RAR version needed to extract file 1 byte Version number is encoded as 10 * Major version + minor version. METHOD Packing method 1 byte 0x30 - storing 0x31 - fastest compression 0x32 - fast compression 0x33 - normal compression 0x34 - good compression 0x35 - best compression NAME_SIZE File name size 2 bytes ATTR File attributes 4 bytes HIGH_PACK_SIZE High 4 bytes of 64 bit value of compressed file size. 4 bytes Optional value, presents only if bit 0x100 in HEAD_FLAGS is set. HIGH_UNP_SIZE High 4 bytes of 64 bit value of uncompressed file size. 4 bytes Optional value, presents only if bit 0x100 in HEAD_FLAGS is set. FILE_NAME File name - string of NAME_SIZE bytes size SALT present if (HEAD_FLAGS & 0x400) != 0 8 bytes EXT_TIME present if (HEAD_FLAGS & 0x1000) != 0 variable size other new fields may appear here. ========================================================================== Application notes ========================================================================== 1. To process an SFX archive you need to skip the SFX module searching for the marker block in the archive. There is no marker block sequence (0x52 0x61 0x72 0x21 0x1a 0x07 0x00) in the SFX module itself. 2. The CRC is calculated using the standard polynomial 0xEDB88320. In case the size of the CRC is less than 4 bytes, only the low order bytes are used.
page revision: 0, last edited: 01 Mar 2014 09:23