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# ZIP Format Archiver implements key aspects of PKWARE'S ZIP v2.0. ``` General Format of a ZIP file ---------------------------- Files stored in arbitrary order. Large zipfiles can span multiple diskette media. Overall zipfile format: [local file header + file data + data_descriptor] . . . [central directory] end of central directory record A. Local file header: local file header signature 4 bytes (0x04034b50) version needed to extract 2 bytes general purpose bit flag 2 bytes compression method 2 bytes last mod file time 2 bytes last mod file date 2 bytes crc-32 4 bytes compressed size 4 bytes uncompressed size 4 bytes filename length 2 bytes extra field length 2 bytes filename (variable size) extra field (variable size) B. Data descriptor: crc-32 4 bytes compressed size 4 bytes uncompressed size 4 bytes This descriptor exists only if bit 3 of the general purpose bit flag is set (see below). It is byte aligned and immediately follows the last byte of compressed data. This descriptor is used only when it was not possible to seek in the output zip file, e.g., when the output zip file was standard output or a non seekable device. C. Central directory structure: [file header] . . . end of central dir record File header: central file header signature 4 bytes (0x02014b50) version made by 2 bytes version needed to extract 2 bytes general purpose bit flag 2 bytes compression method 2 bytes last mod file time 2 bytes last mod file date 2 bytes crc-32 4 bytes compressed size 4 bytes uncompressed size 4 bytes filename length 2 bytes extra field length 2 bytes file comment length 2 bytes disk number start 2 bytes internal file attributes 2 bytes external file attributes 4 bytes relative offset of local header 4 bytes filename (variable size) extra field (variable size) file comment (variable size) End of central dir record: end of central dir signature 4 bytes (0x06054b50) number of this disk 2 bytes number of the disk with the start of the central directory 2 bytes total number of entries in the central dir on this disk 2 bytes total number of entries in the central dir 2 bytes size of the central directory 4 bytes offset of start of central directory with respect to the starting disk number 4 bytes zipfile comment length 2 bytes zipfile comment (variable size) D. Explanation of fields: version made by (2 bytes) The upper byte indicates the host system (OS) for the file. Software can use this information to determine the line record format for text files etc. The current mappings are: 0 - MS-DOS and OS/2 (F.A.T. file systems) 1 - Amiga 2 - VAX/VMS 3 - *nix 4 - VM/CMS 5 - Atari ST 6 - OS/2 H.P.F.S. 7 - Macintosh 8 - Z-System 9 - CP/M 10 thru 255 - unused The lower byte indicates the version number of the software used to encode the file. The value/10 indicates the major version number, and the value mod 10 is the minor version number. version needed to extract (2 bytes) The minimum software version needed to extract the file, mapped as above. general purpose bit flag: (2 bytes) bit 0: If set, indicates that the file is encrypted. (For Method 6 - Imploding) bit 1: If the compression method used was type 6, Imploding, then this bit, if set, indicates an 8K sliding dictionary was used. If clear, then a 4K sliding dictionary was used. bit 2: If the compression method used was type 6, Imploding, then this bit, if set, indicates an 3 Shannon-Fano trees were used to encode the sliding dictionary output. If clear, then 2 Shannon-Fano trees were used. (For Method 8 - Deflating) bit 2 bit 1 0 0 Normal (-en) compression option was used. 0 1 Maximum (-ex) compression option was used. 1 0 Fast (-ef) compression option was used. 1 1 Super Fast (-es) compression option was used. Note: Bits 1 and 2 are undefined if the compression method is any other. (For method 8) bit 3: If this bit is set, the fields crc-32, compressed size and uncompressed size are set to zero in the local header. The correct values are put in the data descriptor immediately following the compressed data. The upper three bits are reserved and used internally by the software when processing the zipfile. The remaining bits are unused. compression method: (2 bytes) (see accompanying documentation for algorithm descriptions) 0 - The file is stored (no compression) 1 - The file is Shrunk 2 - The file is Reduced with compression factor 1 3 - The file is Reduced with compression factor 2 4 - The file is Reduced with compression factor 3 5 - The file is Reduced with compression factor 4 6 - The file is Imploded 7 - Reserved for Tokenizing compression algorithm 8 - The file is Deflated date and time fields: (2 bytes each) The date and time are encoded in standard MS-DOS format. If input came from standard input, the date and time are those at which compression was started for this data. CRC-32: (4 bytes) The CRC-32 algorithm was generously contributed by David Schwaderer and can be found in his excellent book "C Programmers Guide to NetBIOS" published by Howard W. Sams & Co. Inc. The 'magic number' for the CRC is 0xdebb20e3. The proper CRC pre and post conditioning is used, meaning that the CRC register is pre-conditioned with all ones (a starting value of 0xffffffff) and the value is post-conditioned by taking the one's complement of the CRC residual. If bit 3 of the general purpose flag is set, this field is set to zero in the local header and the correct value is put in the data descriptor and in the central directory. compressed size: (4 bytes) uncompressed size: (4 bytes) The size of the file compressed and uncompressed, respectively. If bit 3 of the general purpose bit flag is set, these fields are set to zero in the local header and the correct values are put in the data descriptor and in the central directory. filename length: (2 bytes) extra field length: (2 bytes) file comment length: (2 bytes) The length of the filename, extra field, and comment fields respectively. The combined length of any directory record and these three fields should not generally exceed 65,535 bytes. If input came from standard input, the filename length is set to zero. disk number start: (2 bytes) The number of the disk on which this file begins. internal file attributes: (2 bytes) The lowest bit of this field indicates, if set, that the file is apparently an ASCII or text file. If not set, that the file apparently contains binary data. The remaining bits are unused in version 1.0. external file attributes: (4 bytes) The mapping of the external attributes is host-system dependent (see 'version made by'). For MS-DOS, the low order byte is the MS-DOS directory attribute byte. If input came from standard input, this field is set to zero. relative offset of local header: (4 bytes) This is the offset from the start of the first disk on which this file appears, to where the local header should be found. filename: (Variable) The name of the file, with optional relative path. The path stored should not contain a drive or device letter, or a leading slash. All slashes should be forward slashes '/' as opposed to backwards slashes '\' for compatibility with Amiga and Unix file systems etc. If input came from standard input, there is no filename field. extra field: (Variable) This is for future expansion. If additional information needs to be stored in the future, it should be stored here. Earlier versions of the software can then safely skip this file, and find the next file or header. This field will be 0 length in version 1.0. In order to allow different programs and different types of information to be stored in the 'extra' field in .ZIP files, the following structure should be used for all programs storing data in this field: header1+data1 + header2+data2 . . . Each header should consist of: Header ID - 2 bytes Data Size - 2 bytes Note: all fields stored in Intel low-byte/high-byte order. The Header ID field indicates the type of data that is in the following data block. Header ID's of 0 thru 31 are reserved for use by PKWARE. The remaining ID's can be used by third party vendors for proprietary usage. The current Header ID mappings are: 0x0007 AV Info 0x0009 OS/2 0x000c VAX/VMS The Data Size field indicates the size of the following data block. Programs can use this value to skip to the next header block, passing over any data blocks that are not of interest. Note: As stated above, the size of the entire .ZIP file header, including the filename, comment, and extra field should not exceed 64K in size. In case two different programs should appropriate the same Header ID value, it is strongly recommended that each program place a unique signature of at least two bytes in size (and preferably 4 bytes or bigger) at the start of each data area. Every program should verify that its unique signature is present, in addition to the Header ID value being correct, before assuming that it is a block of known type. -VAX/VMS Extra Field: The following is the layout of the VAX/VMS attributes "extra" block. (Last Revision 12/17/91) Note: all fields stored in Intel low-byte/high-byte order. Value Size Description ----- ---- ----------- (VMS) 0x000c Short Tag for this "extra" block type TSize Short Size of the total "extra" block CRC Long 32-bit CRC for remainder of the block Tag1 Short VMS attribute tag value #1 Size1 Short Size of attribute #1, in bytes (var.) Size1 Attribute #1 data . . . TagN Short VMS attribute tage value #N SizeN Short Size of attribute #N, in bytes (var.) SizeN Attribute #N data Rules: 1. There will be one or more of attributes present, which will each be preceded by the above TagX & SizeX values. These values are identical to the ATR$C_XXXX and ATR$S_XXXX constants which are defined in ATR.H under VMS C. Neither of these values will ever be zero. 2. No word alignment or padding is performed. 3. A well-behaved PKZIP/VMS program should never produce more than one sub-block with the same TagX value. Also, there will never be more than one "extra" block of type 0x000c in a particular directory record. file comment: (Variable) The comment for this file. number of this disk: (2 bytes) The number of this disk, which contains central directory end record. number of the disk with the start of the central directory: (2 bytes) The number of the disk on which the central directory starts. total number of entries in the central dir on this disk: (2 bytes) The number of central directory entries on this disk. total number of entries in the central dir: (2 bytes) The total number of files in the zipfile. size of the central directory: (4 bytes) The size (in bytes) of the entire central directory. offset of start of central directory with respect to the starting disk number: (4 bytes) Offset of the start of the central direcory on the disk on which the central directory starts. zipfile comment length: (2 bytes) The length of the comment for this zipfile. zipfile comment: (Variable) The comment for this zipfile. D. General notes: 1) All fields unless otherwise noted are unsigned and stored in Intel low-byte:high-byte, low-word:high-word order. 2) String fields are not null terminated, since the length is given explicitly. 3) Local headers should not span disk boundries. Also, even though the central directory can span disk boundries, no single record in the central directory should be split across disks. 4) The entries in the central directory may not necessarily be in the same order that files appear in the zipfile. ``` *Source: http://www.pkware.com/documents/APPNOTE/APPNOTE-2.0.txt*