TechNote,
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RAR version 4.01 - 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 theseblocks may vary, but the first block must be a marker block followed byan archive header block.Each block begins with the following fields:HEAD_CRC 2 bytes CRC of total block or block partHEAD_TYPE 1 byte Block typeHEAD_FLAGS 2 bytes Block flagsHEAD_SIZE 2 bytes Block sizeADD_SIZE 4 bytes Optional field - added block sizeField ADD_SIZE present only if (HEAD_FLAGS & 0x8000) != 0Total block size is HEAD_SIZE if (HEAD_FLAGS & 0x8000) == 0and 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 blockand remove it when the archive is updated.if clear, the block is copied to the new archivefile when the archive is updated;0x8000 - if set, ADD_SIZE field is present and the full blocksize is HEAD_SIZE+ADD_SIZE.Declared block types:HEAD_TYPE=0x72 marker blockHEAD_TYPE=0x73 archive headerHEAD_TYPE=0x74 file headerHEAD_TYPE=0x75 old style comment headerHEAD_TYPE=0x76 old style authenticity informationHEAD_TYPE=0x77 old style subblockHEAD_TYPE=0x78 old style recovery recordHEAD_TYPE=0x79 old style authenticity informationHEAD_TYPE=0x7a subblockComment block is actually used only within other blocks and doesn'texist separately.Archive processing is made in the following manner:1. Read and check marker block2. Read archive header3. Read or skip HEAD_SIZE-sizeof(MAIN_HEAD) bytes4. 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==0x74read file header ( first 7 bytes already read )read or skip HEAD_SIZE-sizeof(FILE_HEAD) bytesif (HEAD_FLAGS & 0x100)read or skip HIGH_PACK_SIZE*0x100000000+PACK_SIZE byteselseread or skip PACK_SIZE byteselseread corresponding HEAD_TYPE block:read HEAD_SIZE-7 bytesif (HEAD_FLAGS & 0x8000)read ADD_SIZE bytes6. go to 4.==========================================================================Block Formats==========================================================================Marker block ( MARK_HEAD )HEAD_CRC Always 0x61522 bytesHEAD_TYPE Header type: 0x721 byteHEAD_FLAGS Always 0x1a212 bytesHEAD_SIZE Block size = 0x00072 bytesThe marker block is actually considered as a fixed bytesequence: 0x52 0x61 0x72 0x21 0x1a 0x07 0x00Archive header ( MAIN_HEAD )HEAD_CRC CRC of fields HEAD_TYPE to RESERVED22 bytesHEAD_TYPE Header type: 0x731 byteHEAD_FLAGS Bit flags:2 bytes0x0001 - Volume attribute (archive volume)0x0002 - Archive comment presentRAR 3.x uses the separate comment blockand does not set this flag.0x0004 - Archive lock attribute0x0008 - Solid attribute (solid archive)0x0010 - New volume naming scheme ('volname.partN.rar')0x0020 - Authenticity information presentRAR 3.x does not set this flag.0x0040 - Recovery record present0x0080 - Block headers are encrypted0x0100 - First volume (set only by RAR 3.0 and later)other bits in HEAD_FLAGS are reserved forinternal useHEAD_SIZE Archive header total size including archive comments2 bytesRESERVED1 Reserved2 bytesRESERVED2 Reserved4 bytesFile header (File in archive)HEAD_CRC CRC of fields from HEAD_TYPE to FILEATTR2 bytes and file nameHEAD_TYPE Header type: 0x741 byteHEAD_FLAGS Bit flags:2 bytes0x01 - file continued from previous volume0x02 - file continued in next volume0x04 - file encrypted with password0x08 - file comment presentRAR 3.x uses the separate comment blockand 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 KB0 0 1 - dictionary size 128 KB0 1 0 - dictionary size 256 KB0 1 1 - dictionary size 512 KB1 0 0 - dictionary size 1024 KB1 0 1 - dictionary size 2048 KB1 1 0 - dictionary size 4096 KB1 1 1 - file is directory0x100 - HIGH_PACK_SIZE and HIGH_UNP_SIZE fieldsare present. These fields are used to archiveonly very large files (larger than 2Gb),for smaller files these fields are absent.0x200 - FILE_NAME contains both usual and encodedUnicode name separated by zero. In this caseNAME_SIZE field is equal to the lengthof usual name plus encoded Unicode name plus 1.If this flag is present, but FILE_NAME does notcontain zero bytes, it means that file nameis encoded using UTF-8.0x400 - the header contains additional 8 bytesafter the file name, which are required toincrease 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 completeblock 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 comments2 bytesPACK_SIZE Compressed file size4 bytesUNP_SIZE Uncompressed file size4 bytesHOST_OS Operating system used for archiving1 byte 0 - MS DOS1 - OS/22 - Windows3 - Unix4 - Mac OS5 - BeOSFILE_CRC File CRC4 bytesFTIME Date and time in standard MS DOS format4 bytesUNP_VER RAR version needed to extract file1 byteVersion number is encoded as10 * Major version + minor version.METHOD Packing method1 byte0x30 - storing0x31 - fastest compression0x32 - fast compression0x33 - normal compression0x34 - good compression0x35 - best compressionNAME_SIZE File name size2 bytesATTR File attributes4 bytesHIGH_PACK_SIZE High 4 bytes of 64 bit value of compressed file size.4 bytes Optional value, presents only if bit 0x100 in HEAD_FLAGSis 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_FLAGSis set.FILE_NAME File name - string of NAME_SIZE bytes sizeSALT present if (HEAD_FLAGS & 0x400) != 08 bytesEXT_TIME present if (HEAD_FLAGS & 0x1000) != 0variable sizeother new fields may appear here.==========================================================================Application notes==========================================================================1. To process an SFX archive you need to skip the SFX module searchingfor the marker block in the archive. There is no marker block sequence (0x520x61 0x72 0x21 0x1a 0x07 0x00) in the SFX module itself.2. The CRC is calculated using the standard polynomial 0xEDB88320. Incase the size of the CRC is less than 4 bytes, only the low order bytesare used.
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