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[Info-ZIP note, 981115: this file is based on PKWARE's appnote.txt of 15 February 1996, taking into account PKWARE's revised appnote.txt version of 01 May 1997. It has been unofficially corrected and extended by Info-ZIP without explicit permission by PKWARE. Although Info-ZIP believes the information to be accurate and complete, it is provided under a disclaimer similar to the PKWARE disclaimer below, differing only in the substitution of "Info-ZIP" for "PKWARE". In other words, use this information at your own risk, but we think it's correct. Specification info from PKWARE that was obviously wrong has been corrected silently (e.g. missing structure fields, wrong numbers As of PKZIPW 2.50, two new incompatibilities have been introduced by PKWARE; they are noted below. Note that the "NTFS tag" conflict is currently not real; PKZIPW 2.50 actually tags NTFS files as having come from a FAT file system, too.] Disclaimer ---------- Although PKWARE will attempt to supply current and accurate information relating to its file formats, algorithms, and the subject programs, the possibility of error can not be eliminated. PKWARE therefore expressly disclaims any warranty that the information contained in the associated materials relating to the subject programs and/or the format of the files created or accessed by the subject programs and/or the algorithms used by the subject programs, or any other matter, is current, correct or accurate as delivered. Any risk of damage due to any possible inaccurate information is assumed by the user of the information. Furthermore, the information relating to the subject programs and/or the file formats created or accessed by the subject programs and/or the algorithms used by the subject programs is subject to change without notice. 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: data descriptor signature 4 bytes (0x08074b50) 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 - FAT file system (DOS, OS/2, NT) + PKZIPW 2.50 VFAT, NTFS 1 - Amiga 2 - VMS (VAX or Alpha AXP) 3 - Unix 4 - VM/CMS 5 - Atari 6 - HPFS file system (OS/2, NT 3.x) 7 - Macintosh 8 - Z-System 9 - CP/M 10 - TOPS-20 [supposedly PKZIPW 2.50 NTFS] 11 - NTFS file system (NT) [used by Info-ZIP, only] 12 - SMS/QDOS 13 - Acorn RISC OS 14 - VFAT file system (Win95, NT) [Info-ZIP reservation, unused] 15 - MVS 16 - BeOS (BeBox or PowerMac) 17 - Tandem 18 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. 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. (Note: PKZIP version 2.04g for DOS only recognizes this bit for method 8 compression, newer versions of PKZIP recognize this bit for any compression method.) [Info-ZIP note: This bit was introduced by PKZIP 2.04 for DOS. In general, this feature can only be reliably used together with compression methods that allow intrinsic detection of the "end-of-compressed-data" condition. From the set of compression methods described in this Zip archive specification, only "deflate" meets this requirement. Especially, the method STORED does not work! The Info-ZIP tools recognize this bit regardless of the compression method; but, they rely on correctly set "compressed size" information in the central directory entry.] 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 9 - Reserved for enhanced Deflating 10 - PKWARE Data Compression Library Imploding 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. [Info-ZIP note: This feature is not yet supported by any PKWARE version of ZIP (at least not in PKZIP for DOS and PKZIP for Windows/WinNT). The Info-ZIP programs handle standard input differently: If input came from standard input, the filename is set to "-" (length one).] 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. [Info-ZIP discrepancy: If input came from standard input, the file name is set to "-" (without the quotes). As far as we know, the PKWARE specification for "input from stdin" is not supported by PKZIP/PKUNZIP for DOS, OS/2, Windows Windows NT.] 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 defined by PKWARE are: 0x0007 AV Info 0x0009 OS/2 extended attributes (also Info-ZIP) 0x000a PKWARE Win95/WinNT FileTimes [undocumented!] 0x000c PKWARE VAX/VMS (also Info-ZIP) 0x000d reserved for Unix The Header ID mappings defined by Info-ZIP and third parties are: 0x07c8 Info-ZIP Macintosh (old, J. Lee) 0x2605 ZipIt Macintosh (first version) 0x2705 ZipIt Macintosh v 1.3.5 and newer (w/o full filename) 0x334d Info-ZIP Macintosh (new, D. Haase's 'Mac3' field ) 0x4341 Acorn/SparkFS (David Pilling) 0x4453 Windows NT security descriptor (binary ACL) 0x4704 VM/CMS 0x470f MVS 0x4b46 FWKCS MD5 (third party, see below) 0x4c41 OS/2 access control list (text ACL) 0x4d49 Info-ZIP VMS (VAX or Alpha) 0x5356 AOS/VS (binary ACL) 0x5455 extended timestamp 0x5855 Info-ZIP Unix (original; also OS/2, NT, etc.) 0x6542 BeOS (BeBox, PowerMac, etc.) 0x756e ASi Unix 0x7855 Info-ZIP Unix (new) 0xfb4a SMS/QDOS 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. In the following descriptions, note that "Short" means two bytes, "Long" means four bytes, and "Long-Long" means eight bytes, regardless of their native sizes. Unless specifically noted, all integer fields should be interpreted as unsigned (non-negative) numbers. -OS/2 Extended Attributes Extra Field: ==================================== The following is the layout of the OS/2 extended attributes "extra" block. (Last Revision 19960922) Note: all fields stored in Intel low-byte/high-byte order. Local-header version: Value Size Description ----- ---- ----------- (OS/2) 0x0009 Short tag for this extra block type TSize Short total data size for this block BSize Long uncompressed EA data size CType Short compression type EACRC Long CRC value for uncompressed EA data (var.) variable compressed EA data Central-header version: Value Size Description ----- ---- ----------- (OS/2) 0x0009 Short tag for this extra block type TSize Short total data size for this block BSize Long size of uncompressed local EA data The value of CType is interpreted according to the "compression method" section above; i.e., 0 for stored, 8 for deflated, etc. The OS/2 extended attribute structure (FEA2LIST) is compressed and then stored in its entirety within this structure. There will only ever be one block of data in the variable-length field. -OS/2 Access Control List Extra Field: ==================================== The following is the layout of the OS/2 ACL extra block. (Last Revision 19960922) Local-header version: Value Size Description ----- ---- ----------- (ACL) 0x4c41 Short tag for this extra block type TSize Short total data size for this block BSize Long uncompressed ACL data size CType Short compression type EACRC Long CRC value for uncompressed ACL data (var.) variable compressed ACL data Central-header version: Value Size Description ----- ---- ----------- (ACL) 0x4c41 Short tag for this extra block type TSize Short total data size for this block BSize Long size of uncompressed local ACL data The value of CType is interpreted according to the "compression method" section above; i.e., 0 for stored, 8 for deflated, etc. The uncompressed ACL data consist of a text header of the form "ACL1:%hX,%hd\n", where the first field is the OS/2 ACCINFO acc_attr member and the second is acc_count, followed by acc_count strings of the form "%s,%hx\n", where the first field is acl_ugname (user group name) and the second acl_access. This block type will be extended for other operating systems as needed. -Windows NT Security Descriptor Extra Field: ========================================== The following is the layout of the NT Security Descriptor (another type of ACL) extra block. (Last Revision 19960922) Local-header version: Value Size Description ----- ---- ----------- (SD) 0x4453 Short tag for this extra block type TSize Short total data size for this block BSize Long uncompressed SD data size Version Byte version of uncompressed SD data format CType Short compression type EACRC Long CRC value for uncompressed SD data (var.) variable compressed SD data Central-header version: Value Size Description ----- ---- ----------- (SD) 0x4453 Short tag for this extra block type TSize Short total data size for this block BSize Long size of uncompressed local SD data The value of CType is interpreted according to the "compression method" section above; i.e., 0 for stored, 8 for deflated, etc. Version specifies how the compressed data are to be interpreted and allows for future expansion of this extra field type. Currently only version 0 is defined. For version 0, the compressed data are to be interpreted as a single valid Windows NT SECURITY_DESCRIPTOR data structure, in self-relative format. -PKWARE Win95/WinNT Extra Field: ============================== The following description covers PKWARE's undocumented Windows 95 & Windows NT extra field, introduced with the release of PKZIP for Windows 2.50. (Last Revision 19980425) This field has a fixed data size of 32 bytes and is only stored as local extra field. Value Size Description ----- ---- ----------- (WinNT) 0x000a Short Tag for this "extra" block type TSize Short Total Data Size for this block Unknwn1 Long ???? (all 0 ?) Unknwn2 Long ???? ModTime Long-Long 64-bit NTFS last-modified filetime AccTime Long-Long 64-bit NTFS last-access filetime CreTime Long-Long 64-bit NTFS creation filetime The NTFS filetimes are 64-bit unsigned integers, stored in Intel (least significant byte first) byte order. They determine the number of 1.0E-07 seconds (1/10th microseconds!) past WinNT "epoch", which is "01-Jan-1601 00:00:00 UTC". -PKWARE VAX/VMS Extra Field: ========================== The following is the layout of PKWARE's 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 Total Data Size for this 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. -Info-ZIP VMS Extra Field: ======================== The following is the layout of Info-ZIP's VMS attributes extra block for VAX or Alpha AXP. The local-header and central-header versions are identical. (Last Revision 19960922) Value Size Description ----- ---- ----------- (VMS2) 0x4d49 Short tag for this extra block type TSize Short total data size for this block ID Long block ID Flags Short info bytes BSize Short uncompressed block size Reserved Long (reserved) (var.) variable compressed VMS file-attributes block The block ID is one of the following unterminated strings: "VFAB" struct FAB "VALL" struct XABALL "VFHC" struct XABFHC "VDAT" struct XABDAT "VRDT" struct XABRDT "VPRO" struct XABPRO "VKEY" struct XABKEY "VMSV" version (e.g., "V6.1"; truncated at hyphen) "VNAM" reserved The lower three bits of Flags indicate the compression method. The currently defined methods are: 0 stored (not compressed) 1 simple "RLE" 2 deflated The "RLE" method simply replaces zero-valued bytes with zero-valued bits and non-zero-valued bytes with a "1" bit followed by the byte value. The variable-length compressed data contains only the data corre- sponding to the indicated structure or string. Typically multiple VMS2 extra fields are present (each with a unique block type). -Info-ZIP Macintosh Extra Field: ============================== The following is the layout of the (old) Info-ZIP resource-fork extra block for Macintosh. The local-header and central-header versions are identical. (Last Revision 19960922) Value Size Description ----- ---- ----------- (Mac) 0x07c8 Short tag for this extra block type TSize Short total data size for this block "JLEE" beLong extra-field signature FInfo 16 bytes Macintosh FInfo structure CrDat beLong HParamBlockRec fileParam.ioFlCrDat MdDat beLong HParamBlockRec fileParam.ioFlMdDat Flags beLong info bits DirID beLong HParamBlockRec fileParam.ioDirID VolName 28 bytes volume name (optional) All fields but the first two are in native Macintosh format (big-endian Motorola order, not little-endian Intel). The least significant bit of Flags is 1 if the file is a data fork, 0 other- wise. In addition, if this extra field is present, the filename has an extra 'd' or 'r' appended to indicate data fork or resource fork. The 28-byte VolName field may be omitted. -ZipIt Macintosh Extra Field (long): ================================== The following is the layout of the ZipIt extra block for Macintosh. The local-header and central-header versions are identical. (Last Revision 19970130) Value Size Description ----- ---- ----------- (Mac2) 0x2605 Short tag for this extra block type TSize Short total data size for this block "ZPIT" beLong extra-field signature FnLen Byte length of FileName FileName variable full Macintosh filename FileType Byte[4] four-byte Mac file type string Creator Byte[4] four-byte Mac creator string -ZipIt Macintosh Extra Field (short): =================================== The following is the layout of a shortened variant of the ZipIt extra block for Macintosh (without "full name" entry). This variant is used by ZipIt 1.3.5 and newer for entries that do not need a "full Mac filename" record. The local-header and central-header versions are identical. (Last Revision 19980903) Value Size Description ----- ---- ----------- (Mac2b) 0x2705 Short tag for this extra block type TSize Short total data size for this block "ZPIT" beLong extra-field signature FileType Byte[4] four-byte Mac file type string Creator Byte[4] four-byte Mac creator string -Info-ZIP Macintosh Extra Field (new): ==================================== The following is the layout of the (new) Info-ZIP extra block for Macintosh, designed by Dirk Haase. All values are in little-endian. (Last Revision 19981005) Local-header version: Value Size Description ----- ---- ----------- (Mac3) 0x334d Short tag for this extra block type ("M3") TSize Short total data size for this block BSize Long uncompressed finder attribute data size Flags Short info bits fdType Byte[4] Type of the File (4-byte string) fdCreator Byte[4] Creator of the File (4-byte string) (CType) Short compression type (CRC) Long CRC value for uncompressed MacOS data Attribs variable finder attribute data (see below) Central-header version: Value Size Description ----- ---- ----------- (Mac3) 0x334d Short tag for this extra block type ("M3") TSize Short total data size for this block BSize Long uncompressed finder attribute data size Flags Short info bits fdType Byte[4] Type of the File (4-byte string) fdCreator Byte[4] Creator of the File (4-byte string) The third bit of Flags in both headers indicates whether the LOCAL extra field is uncompressed (and therefore whether CType and CRC are omitted): Bits of the Flags: bit 0 if set, file is a data fork; otherwise unset bit 1 if set, filename will be not changed bit 2 if set, Attribs is uncompressed (no CType, CRC) bit 3 if set, date and times are in 64 bit if zero date and times are in 32 bit. bit 4 if set, timezone offsets fields for the native Mac times are omitted (UTC support deactivated) bits 5-15 reserved; Attributes: Attribs is a Mac-specific block of data in little-endian format with the following structure (if compressed, uncompress it first): Value Size Description ----- ---- ----------- fdFlags Short Finder Flags fdLocation.v Short Finder Icon Location fdLocation.h Short Finder Icon Location fdFldr Short Folder containing file FXInfo 16 bytes Macintosh FXInfo structure FXInfo-Structure: fdIconID Short fdUnused[3] Short unused but reserved 6 bytes fdScript Byte Script flag and number fdXFlags Byte More flag bits fdComment Short Comment ID fdPutAway Long Home Dir ID FVersNum Byte file version number may be not used by MacOS ACUser Byte directory access rights FlCrDat ULong date and time of creation FlMdDat ULong date and time of last modification FlBkDat ULong date and time of last backup These time numbers are original Mac FileTime values (local time!). Currently, date-time width is 32-bit, but future version may support be 64-bit times (see flags) CrGMTOffs Long(signed!) difference "local Creat. time - UTC" MdGMTOffs Long(signed!) difference "local Modif. time - UTC" BkGMTOffs Long(signed!) difference "local Backup time - UTC" These "local time - UTC" differences (stored in seconds) may be used to support timestamp adjustment after inter-timezone transfer. These fields are optional; bit 4 of the flags word controls their presence. Charset Short TextEncodingBase (Charset) valid for the following two fields FullPath variable Path of the current file. Zero terminated string (C-String) Currently coded in the native Charset. Comment variable Finder Comment of the current file. Zero terminated string (C-String) Currently coded in the native Charset. -Acorn SparkFS Extra Field: ========================= The following is the layout of David Pilling's SparkFS extra block for Acorn RISC OS. The local-header and central-header versions are identical. (Last Revision 19960922) Value Size Description ----- ---- ----------- (Acorn) 0x4341 Short tag for this extra block type TSize Short total data size for this block "ARC0" Long extra-field signature LoadAddr Long load address or file type ExecAddr Long exec address Attr Long file permissions Zero Long reserved; always zero The following bits of Attr are associated with the given file permissions: bit 0 user-writable ('W') bit 1 user-readable ('R') bit 2 reserved bit 3 locked ('L') bit 4 publicly writable ('w') bit 5 publicly readable ('r') bit 6 reserved bit 7 reserved -VM/CMS Extra Field: ================== The following is the layout of the file-attributes extra block for VM/CMS. The local-header and central-header versions are identical. (Last Revision 19960922) Value Size Description ----- ---- ----------- (VM/CMS) 0x4704 Short tag for this extra block type TSize Short total data size for this block flData variable file attributes data flData is an uncompressed fldata_t struct. -MVS Extra Field: =============== The following is the layout of the file-attributes extra block for MVS. The local-header and central-header versions are identical. (Last Revision 19960922) Value Size Description ----- ---- ----------- (MVS) 0x470f Short tag for this extra block type TSize Short total data size for this block flData variable file attributes data flData is an uncompressed fldata_t struct. -PKWARE Unix Extra Field: ======================== The following description covers PKWARE's undocumented Unix extra field, introduced with the release of PKZIP for Unix 2.50. (Last Revision 19980901) This field has a fixed data size of 12 bytes and is only stored as local extra field. Value Size Description ----- ---- ----------- (Unix0) 0x000d Short Tag for this "extra" block type TSize Short Total Data Size for this block ModTime Long time of last modification (UTC/GMT) AcTime Long time of last access (UTC/GMT) UID Short Unix user ID GID Short Unix group ID This field has exactly the same layout as Info-ZIP's abandoned "Unix1 timestamps & owner ID info" extra field; only the two tag bytes are different. -Extended Timestamp Extra Field: ============================== The following is the layout of the extended-timestamp extra block. (Last Revision 19970118) Local-header version: Value Size Description ----- ---- ----------- (time) 0x5455 Short tag for this extra block type TSize Short total data size for this block Flags Byte info bits (ModTime) Long time of last modification (UTC/GMT) (AcTime) Long time of last access (UTC/GMT) (CrTime) Long time of original creation (UTC/GMT) Central-header version: Value Size Description ----- ---- ----------- (time) 0x5455 Short tag for this extra block type TSize Short total data size for this block Flags Byte info bits (refers to local header!) (ModTime) Long time of last modification (UTC/GMT) The central-header extra field contains the modification time only, or no timestamp at all. TSize is used to flag its presence or absence. But note: If "Flags" indicates that Modtime is present in the local header field, it MUST be present in the central header field, too! This correspondence is required because the modification time value may be used to support trans-timezone freshening and updating operations with zip archives. The time values are in standard Unix signed-long format, indicating the number of seconds since 1 January 1970 00:00:00. The times are relative to Coordinated Universal Time (UTC), also sometimes referred to as Greenwich Mean Time (GMT). To convert to local time, the software must know the local timezone offset from UTC/GMT. The lower three bits of Flags in both headers indicate which time- stamps are present in the LOCAL extra field: bit 0 if set, modification time is present bit 1 if set, access time is present bit 2 if set, creation time is present bits 3-7 reserved for additional timestamps; not set Those times that are present will appear in the order indicated, but any combination of times may be omitted. (Creation time may be present without access time, for example.) TSize should equal (1 + 4*(number of set bits in Flags)), as the block is currently defined. Other timestamps may be added in the future. -Info-ZIP Unix Extra Field (type 1): ================================== The following is the layout of the old Info-ZIP extra block for Unix. It has been replaced by the extended-timestamp extra block (0x5455) and the Unix type 2 extra block (0x7855). (Last Revision 19970118) Local-header version: Value Size Description ----- ---- ----------- (Unix1) 0x5855 Short tag for this extra block type TSize Short total data size for this block ModTime Long time of last modification (UTC/GMT) AcTime Long time of last access (UTC/GMT) UID Short Unix user ID GID Short Unix group ID Central-header version: Value Size Description ----- ---- ----------- (Unix1) 0x5855 Short tag for this extra block type TSize Short total data size for this block ModTime Long time of last modification (GMT/UTC) AcTime Long time of last access (GMT/UTC) The file modification and access times are in standard Unix signed- long format, indicating the number of seconds since 1 January 1970 00:00:00. The times are relative to Coordinated Universal Time (UTC), also sometimes referred to as Greenwich Mean Time (GMT). To convert to local time, the software must know the local timezone offset from UTC/GMT. The modification time may be used by non-Unix systems to support inter-timezone freshening and updating of zip archives. The local-header extra block may optionally contain UID and GID info for the file. The local-header TSize value is the only indication of this. Note that Unix UIDs and GIDs are usually specific to a particular machine, and they generally require root access to restore. This extra field type is obsolete, but it has been in use since mid-1994. Therefore future archiving software should continue to support it. Some guidelines: An archive member should either contain the old "Unix1" extra field block or the new extra field types "time" and/or "Unix2". If both the old "Unix1" block type and one or both of the new block types "time" and "Unix2" are found, the "Unix1" block should be considered invalid and ignored. Unarchiving software should recognize both old and new extra field block types, but the info from new types overrides the old "Unix1" field. Archiving software should recognize "Unix1" extra fields for timestamp comparison but never create it for updated, freshened or new archive members. When copying existing members to a new archive, any "Unix1" extra field blocks should be converted to the new "time" and/or "Unix2" types. -Info-ZIP Unix Extra Field (type 2): ================================== The following is the layout of the new Info-ZIP extra block for Unix. (Last Revision 19960922) Local-header version: Value Size Description ----- ---- ----------- (Unix2) 0x7855 Short tag for this extra block type TSize Short total data size for this block UID Short Unix user ID GID Short Unix group ID Central-header version: Value Size Description ----- ---- ----------- (Unix2) 0x7855 Short tag for this extra block type TSize Short total data size for this block The data size of the central-header version is zero; it is used solely as a flag that UID/GID info is present in the local-header extra field. If additional fields are ever added to the local version, the central version may be extended to indicate this. Note that Unix UIDs and GIDs are usually specific to a particular machine, and they generally require root access to restore. -ASi Unix Extra Field: ==================== The following is the layout of the ASi extra block for Unix. The local-header and central-header versions are identical. (Last Revision 19960916) Value Size Description ----- ---- ----------- (Unix3) 0x756e Short tag for this extra block type TSize Short total data size for this block CRC Long CRC-32 of the remaining data Mode Short file permissions SizDev Long symlink'd size OR major/minor dev num UID Short user ID GID Short group ID (var.) variable symbolic link filename Mode is the standard Unix st_mode field from struct stat, containing user/group/other permissions, setuid/setgid and symlink info, etc. If Mode indicates that this file is a symbolic link, SizDev is the size of the file to which the link points. Otherwise, if the file is a device, SizDev contains the standard Unix st_rdev field from struct stat (includes the major and minor numbers of the device). SizDev is undefined in other cases. If Mode indicates that the file is a symbolic link, the final field will be the name of the file to which the link points. The file- name length can be inferred from TSize. [Note that TSize may incorrectly refer to the data size not counting the CRC; i.e., it may be four bytes too small.] -BeOS Extra Field: ================ The following is the layout of the file-attributes extra block for BeOS. (Last Revision 19970531) Local-header version: Value Size Description ----- ---- ----------- (BeOS) 0x6542 Short tag for this extra block type TSize Short total data size for this block BSize Long uncompressed file attribute data size Flags Byte info bits (CType) Short compression type (CRC) Long CRC value for uncompressed file attribs Attribs variable file attribute data Central-header version: Value Size Description ----- ---- ----------- (BeOS) 0x6542 Short tag for this extra block type TSize Short total data size for this block BSize Long size of uncompressed local EF block data Flags Byte info bits The least significant bit of Flags in both headers indicates whether the LOCAL extra field is uncompressed (and therefore whether CType and CRC are omitted): bit 0 if set, Attribs is uncompressed (no CType, CRC) bits 1-7 reserved; if set, assume error or unknown data Currently the only supported compression types are deflated (type 8) and stored (type 0); the latter is not used by Info-ZIP's Zip but is supported by UnZip. Attribs is a BeOS-specific block of data in big-endian format with the following structure (if compressed, uncompress it first): Value Size Description ----- ---- ----------- Name variable attribute name (null-terminated string) Type Long attribute type (32-bit unsigned integer) Size Long Long data size for this sub-block (64 bits) Data variable attribute data The attribute structure is repeated for every attribute. The Data field may contain anything--text, flags, bitmaps, etc. -SMS/QDOS Extra Field: ==================== The following is the layout of the file-attributes extra block for SMS/QDOS. The local-header and central-header versions are identical. (Last Revision 19960929) Value Size Description ----- ---- ----------- (QDOS) 0xfb4a Short tag for this extra block type TSize Short total data size for this block LongID Long extra-field signature (ExtraID) Long additional signature/flag bytes QDirect 64 bytes qdirect structure LongID may be "QZHD" or "QDOS". In the latter case, ExtraID will be present. Its first three bytes are "02\0"; the last byte is currently undefined. QDirect contains the file's uncompressed directory info (qdirect struct). Its elements are in native (big-endian) format: d_length beLong file length d_access byte file access type d_type byte file type d_datalen beLong data length d_reserved beLong unused d_szname beShort size of filename d_name 36 bytes filename d_update beLong time of last update d_refdate beLong file version number d_backup beLong time of last backup (archive date) -AOS/VS Extra Field: ================== The following is the layout of the extra block for Data General AOS/VS. The local-header and central-header versions are identical. (Last Revision 19961125) Value Size Description ----- ---- ----------- (AOSVS) 0x5356 Short tag for this extra block type TSize Short total data size for this block "FCI\0" Long extra-field signature Version Byte version of AOS/VS extra block (10 = 1.0) Fstat variable fstat packet AclBuf variable raw ACL data ($MXACL bytes) Fstat contains the file's uncompressed fstat packet, which is one of the following: normal fstat packet (P_FSTAT struct) DIR/CPD fstat packet (P_FSTAT_DIR struct) unit (device) fstat packet (P_FSTAT_UNIT struct) IPC file fstat packet (P_FSTAT_IPC struct) AclBuf contains the raw ACL data; its length is $MXACL. -FWKCS MD5 Extra Field: ===================== The following is the layout of the optional extra block used by the FWKCS utility. There is no local-header version; the following applies only to the central header. (Last Revision 19961207) Central-header version: Value Size Description ----- ---- ----------- (MD5) 0x4b46 Short tag for this extra block type TSize Short total data size for this block (19) "MD5" 3 bytes extra-field signature MD5hash 16 bytes 128-bit MD5 hash of uncompressed data The MD5 hash in this extra block is used to automatically identify files independent of their filenames; it is an an enhanced contents- signature. FWKCS provides an option to strip this extra field, if present, from a zipfile central directory. In adding this extra field, FWKCS preserves Zipfile Authenticity Verification; if stripping this extra field, FWKCS preserves all versions of AV through PKZIP version 2.04g. ``The MD5 algorithm is being placed in the public domain for review and possible adoption as a standard.'' (Ron Rivest, MIT Laboratory for Computer Science and RSA Data Security, Inc., April 1992, RFC 1321, 11.76-77). FWKCS, and FWKCS Contents_Signature System, are trademarks of Frederick W. Kantor. 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 directory 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 boundaries. Also, even though the central directory can span disk boundaries, 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. UnShrinking - Method 1 ---------------------- Shrinking is a Dynamic Ziv-Lempel-Welch compression algorithm with partial clearing. The initial code size is 9 bits, and the maximum code size is 13 bits. Shrinking differs from conventional Dynamic Ziv-Lempel-Welch implementations in several respects: 1) The code size is controlled by the compressor, and is not automatically increased when codes larger than the current code size are created (but not necessarily used). When the decompressor encounters the code sequence 256 (decimal) followed by 1, it should increase the code size read from the input stream to the next bit size. No blocking of the codes is performed, so the next code at the increased size should be read from the input stream immediately after where the previous code at the smaller bit size was read. Again, the decompressor should not increase the code size used until the sequence 256,1 is encountered. 2) When the table becomes full, total clearing is not performed. Rather, when the compressor emits the code sequence 256,2 (decimal), the decompressor should clear all leaf nodes from the Ziv-Lempel tree, and continue to use the current code size. The nodes that are cleared from the Ziv-Lempel tree are then re-used, with the lowest code value re-used first, and the highest code value re-used last. The compressor can emit the sequence 256,2 at any time. Expanding - Methods 2-5 ----------------------- The Reducing algorithm is actually a combination of two distinct algorithms. The first algorithm compresses repeated byte sequences, and the second algorithm takes the compressed stream from the first algorithm and applies a probabilistic compression method. The probabilistic compression stores an array of 'follower sets' S(j), for j=0 to 255, corresponding to each possible ASCII character. Each set contains between 0 and 32 characters, to be denoted as S(j)[0],...,S(j)[m], where m<32. The sets are stored at the beginning of the data area for a Reduced file, in reverse order, with S(255) first, and S(0) last. The sets are encoded as { N(j), S(j)[0],...,S(j)[N(j)-1] }, where N(j) is the size of set S(j). N(j) can be 0, in which case the follower set for S(j) is empty. Each N(j) value is encoded in 6 bits, followed by N(j) eight bit character values corresponding to S(j)[0] to S(j)[N(j)-1] respectively. If N(j) is 0, then no values for S(j) are stored, and the value for N(j-1) immediately follows. Immediately after the follower sets, is the compressed data stream. The compressed data stream can be interpreted for the probabilistic decompression as follows: let Last-Character <- 0. loop until done if the follower set S(Last-Character) is empty then read 8 bits from the input stream, and copy this value to the output stream. otherwise if the follower set S(Last-Character) is non-empty then read 1 bit from the input stream. if this bit is not zero then read 8 bits from the input stream, and copy this value to the output stream. otherwise if this bit is zero then read B(N(Last-Character)) bits from the input stream, and assign this value to I. Copy the value of S(Last-Character)[I] to the output stream. assign the last value placed on the output stream to Last-Character. end loop B(N(j)) is defined as the minimal number of bits required to encode the value N(j)-1. The decompressed stream from above can then be expanded to re-create the original file as follows: let State <- 0. loop until done read 8 bits from the input stream into C. case State of 0: if C is not equal to DLE (144 decimal) then copy C to the output stream. otherwise if C is equal to DLE then let State <- 1. 1: if C is non-zero then let V <- C. let Len <- L(V) let State <- F(Len). otherwise if C is zero then copy the value 144 (decimal) to the output stream. let State <- 0 2: let Len <- Len + C let State <- 3. 3: move backwards D(V,C) bytes in the output stream (if this position is before the start of the output stream, then assume that all the data before the start of the output stream is filled with zeros). copy Len+3 bytes from this position to the output stream. let State <- 0. end case end loop The functions F,L, and D are dependent on the 'compression factor', 1 through 4, and are defined as follows: For compression factor 1: L(X) equals the lower 7 bits of X. F(X) equals 2 if X equals 127 otherwise F(X) equals 3. D(X,Y) equals the (upper 1 bit of X) * 256 + Y + 1. For compression factor 2: L(X) equals the lower 6 bits of X. F(X) equals 2 if X equals 63 otherwise F(X) equals 3. D(X,Y) equals the (upper 2 bits of X) * 256 + Y + 1. For compression factor 3: L(X) equals the lower 5 bits of X. F(X) equals 2 if X equals 31 otherwise F(X) equals 3. D(X,Y) equals the (upper 3 bits of X) * 256 + Y + 1. For compression factor 4: L(X) equals the lower 4 bits of X. F(X) equals 2 if X equals 15 otherwise F(X) equals 3. D(X,Y) equals the (upper 4 bits of X) * 256 + Y + 1. Imploding - Method 6 -------------------- The Imploding algorithm is actually a combination of two distinct algorithms. The first algorithm compresses repeated byte sequences using a sliding dictionary. The second algorithm is used to compress the encoding of the sliding dictionary output, using multiple Shannon-Fano trees. The Imploding algorithm can use a 4K or 8K sliding dictionary size. The dictionary size used can be determined by bit 1 in the general purpose flag word; a 0 bit indicates a 4K dictionary while a 1 bit indicates an 8K dictionary. The Shannon-Fano trees are stored at the start of the compressed file. The number of trees stored is defined by bit 2 in the general purpose flag word; a 0 bit indicates two trees stored, a 1 bit indicates three trees are stored. If 3 trees are stored, the first Shannon-Fano tree represents the encoding of the Literal characters, the second tree represents the encoding of the Length information, the third represents the encoding of the Distance information. When 2 Shannon-Fano trees are stored, the Length tree is stored first, followed by the Distance tree. The Literal Shannon-Fano tree, if present is used to represent the entire ASCII character set, and contains 256 values. This tree is used to compress any data not compressed by the sliding dictionary algorithm. When this tree is present, the Minimum Match Length for the sliding dictionary is 3. If this tree is not present, the Minimum Match Length is 2. The Length Shannon-Fano tree is used to compress the Length part of the (length,distance) pairs from the sliding dictionary output. The Length tree contains 64 values, ranging from the Minimum Match Length, to 63 plus the Minimum Match Length. The Distance Shannon-Fano tree is used to compress the Distance part of the (length,distance) pairs from the sliding dictionary output. The Distance tree contains 64 values, ranging from 0 to 63, representing the upper 6 bits of the distance value. The distance values themselves will be between 0 and the sliding dictionary size, either 4K or 8K. The Shannon-Fano trees themselves are stored in a compressed format. The first byte of the tree data represents the number of bytes of data representing the (compressed) Shannon-Fano tree minus 1. The remaining bytes represent the Shannon-Fano tree data encoded as: High 4 bits: Number of values at this bit length + 1. (1 - 16) Low 4 bits: Bit Length needed to represent value + 1. (1 - 16) The Shannon-Fano codes can be constructed from the bit lengths using the following algorithm: 1) Sort the Bit Lengths in ascending order, while retaining the order of the original lengths stored in the file. 2) Generate the Shannon-Fano trees: Code <- 0 CodeIncrement <- 0 LastBitLength <- 0 i <- number of Shannon-Fano codes - 1 (either 255 or 63) loop while i >= 0 Code = Code + CodeIncrement if BitLength(i) <> LastBitLength then LastBitLength=BitLength(i) CodeIncrement = 1 shifted left (16 - LastBitLength) ShannonCode(i) = Code i <- i - 1 end loop 3) Reverse the order of all the bits in the above ShannonCode() vector, so that the most significant bit becomes the least significant bit. For example, the value 0x1234 (hex) would become 0x2C48 (hex). 4) Restore the order of Shannon-Fano codes as originally stored within the file. Example: This example will show the encoding of a Shannon-Fano tree of size 8. Notice that the actual Shannon-Fano trees used for Imploding are either 64 or 256 entries in size. Example: 0x02, 0x42, 0x01, 0x13 The first byte indicates 3 values in this table. Decoding the bytes: 0x42 = 5 codes of 3 bits long 0x01 = 1 code of 2 bits long 0x13 = 2 codes of 4 bits long This would generate the original bit length array of: (3, 3, 3, 3, 3, 2, 4, 4) There are 8 codes in this table for the values 0 thru 7. Using the algorithm to obtain the Shannon-Fano codes produces: Reversed Order Original Val Sorted Constructed Code Value Restored Length --- ------ ----------------- -------- -------- ------ 0: 2 1100000000000000 11 101 3 1: 3 1010000000000000 101 001 3 2: 3 1000000000000000 001 110 3 3: 3 0110000000000000 110 010 3 4: 3 0100000000000000 010 100 3 5: 3 0010000000000000 100 11 2 6: 4 0001000000000000 1000 1000 4 7: 4 0000000000000000 0000 0000 4 The values in the Val, Order Restored and Original Length columns now represent the Shannon-Fano encoding tree that can be used for decoding the Shannon-Fano encoded data. How to parse the variable length Shannon-Fano values from the data stream is beyond the scope of this document. (See the references listed at the end of this document for more information.) However, traditional decoding schemes used for Huffman variable length decoding, such as the Greenlaw algorithm, can be successfully applied. The compressed data stream begins immediately after the compressed Shannon-Fano data. The compressed data stream can be interpreted as follows: loop until done read 1 bit from input stream. if this bit is non-zero then (encoded data is literal data) if Literal Shannon-Fano tree is present read and decode character using Literal Shannon-Fano tree. otherwise read 8 bits from input stream. copy character to the output stream. otherwise (encoded data is sliding dictionary match) if 8K dictionary size read 7 bits for offset Distance (lower 7 bits of offset). otherwise read 6 bits for offset Distance (lower 6 bits of offset). using the Distance Shannon-Fano tree, read and decode the upper 6 bits of the Distance value. using the Length Shannon-Fano tree, read and decode the Length value. Length <- Length + Minimum Match Length if Length = 63 + Minimum Match Length read 8 bits from the input stream, add this value to Length. move backwards Distance+1 bytes in the output stream, and copy Length characters from this position to the output stream. (if this position is before the start of the output stream, then assume that all the data before the start of the output stream is filled with zeros). end loop Tokenizing - Method 7 -------------------- This method is not used by PKZIP. Deflating - Method 8 ----------------- The Deflate algorithm is similar to the Implode algorithm using a sliding dictionary of up to 32K with secondary compression from Huffman/Shannon-Fano codes. The compressed data is stored in blocks with a header describing the block and the Huffman codes used in the data block. The header format is as follows: Bit 0: Last Block bit This bit is set to 1 if this is the last compressed block in the data. Bits 1-2: Block type 00 (0) - Block is stored - All stored data is byte aligned. Skip bits until next byte, then next word = block length, followed by the ones compliment of the block length word. Remaining data in block is the stored data. 01 (1) - Use fixed Huffman codes for literal and distance codes. Lit Code Bits Dist Code Bits --------- ---- --------- ---- 0 - 143 8 0 - 31 5 144 - 255 9 256 - 279 7 280 - 287 8 Literal codes 286-287 and distance codes 30-31 are never used but participate in the huffman construction. 10 (2) - Dynamic Huffman codes. (See expanding Huffman codes) 11 (3) - Reserved - Flag a "Error in compressed data" if seen. Expanding Huffman Codes ----------------------- If the data block is stored with dynamic Huffman codes, the Huffman codes are sent in the following compressed format: 5 Bits: # of Literal codes sent - 257 (257 - 286) All other codes are never sent. 5 Bits: # of Dist codes - 1 (1 - 32) 4 Bits: # of Bit Length codes - 4 (4 - 19) The Huffman codes are sent as bit lengths and the codes are built as described in the implode algorithm. The bit lengths themselves are compressed with Huffman codes. There are 19 bit length codes: 0 - 15: Represent bit lengths of 0 - 15 16: Copy the previous bit length 3 - 6 times. The next 2 bits indicate repeat length (0 = 3, ... ,3 = 6) Example: Codes 8, 16 (+2 bits 11), 16 (+2 bits 10) will expand to 12 bit lengths of 8 (1 + 6 + 5) 17: Repeat a bit length of 0 for 3 - 10 times. (3 bits of length) 18: Repeat a bit length of 0 for 11 - 138 times (7 bits of length) The lengths of the bit length codes are sent packed 3 bits per value (0 - 7) in the following order: 16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15 The Huffman codes should be built as described in the Implode algorithm except codes are assigned starting at the shortest bit length, i.e. the shortest code should be all 0's rather than all 1's. Also, codes with a bit length of zero do not participate in the tree construction. The codes are then used to decode the bit lengths for the literal and distance tables. The bit lengths for the literal tables are sent first with the number of entries sent described by the 5 bits sent earlier. There are up to 286 literal characters; the first 256 represent the respective 8 bit character, code 256 represents the End-Of-Block code, the remaining 29 codes represent copy lengths of 3 thru 258. There are up to 30 distance codes representing distances from 1 thru 32k as described below. Length Codes ------------ Extra Extra Extra Extra Code Bits Length Code Bits Lengths Code Bits Lengths Code Bits Length(s) ---- ---- ------ ---- ---- ------- ---- ---- ------- ---- ---- --------- 257 0 3 265 1 11,12 273 3 35-42 281 5 131-162 258 0 4 266 1 13,14 274 3 43-50 282 5 163-194 259 0 5 267 1 15,16 275 3 51-58 283 5 195-226 260 0 6 268 1 17,18 276 3 59-66 284 5 227-257 261 0 7 269 2 19-22 277 4 67-82 285 0 258 262 0 8 270 2 23-26 278 4 83-98 263 0 9 271 2 27-30 279 4 99-114 264 0 10 272 2 31-34 280 4 115-130 Distance Codes -------------- Extra Extra Extra Extra Code Bits Dist Code Bits Dist Code Bits Distance Code Bits Distance ---- ---- ---- ---- ---- ------ ---- ---- -------- ---- ---- -------- 0 0 1 8 3 17-24 16 7 257-384 24 11 4097-6144 1 0 2 9 3 25-32 17 7 385-512 25 11 6145-8192 2 0 3 10 4 33-48 18 8 513-768 26 12 8193-12288 3 0 4 11 4 49-64 19 8 769-1024 27 12 12289-16384 4 1 5,6 12 5 65-96 20 9 1025-1536 28 13 16385-24576 5 1 7,8 13 5 97-128 21 9 1537-2048 29 13 24577-32768 6 2 9-12 14 6 129-192 22 10 2049-3072 7 2 13-16 15 6 193-256 23 10 3073-4096 The compressed data stream begins immediately after the compressed header data. The compressed data stream can be interpreted as follows: do read header from input stream. if stored block skip bits until byte aligned read count and 1's compliment of count copy count bytes data block otherwise loop until end of block code sent decode literal character from input stream if literal < 256 copy character to the output stream otherwise if literal = end of block break from loop otherwise decode distance from input stream move backwards distance bytes in the output stream, and copy length characters from this position to the output stream. end loop while not last block if data descriptor exists skip bits until byte aligned check data descriptor signature read crc and sizes endif Decryption ---------- The encryption used in PKZIP was generously supplied by Roger Schlafly. PKWARE is grateful to Mr. Schlafly for his expert help and advice in the field of data encryption. PKZIP encrypts the compressed data stream. Encrypted files must be decrypted before they can be extracted. Each encrypted file has an extra 12 bytes stored at the start of the data area defining the encryption header for that file. The encryption header is originally set to random values, and then itself encrypted, using three, 32-bit keys. The key values are initialized using the supplied encryption password. After each byte is encrypted, the keys are then updated using pseudo-random number generation techniques in combination with the same CRC-32 algorithm used in PKZIP and described elsewhere in this document. The following is the basic steps required to decrypt a file: 1) Initialize the three 32-bit keys with the password. 2) Read and decrypt the 12-byte encryption header, further initializing the encryption keys. 3) Read and decrypt the compressed data stream using the encryption keys. Step 1 - Initializing the encryption keys ----------------------------------------- Key(0) <- 305419896 Key(1) <- 591751049 Key(2) <- 878082192 loop for i <- 0 to length(password)-1 update_keys(password(i)) end loop Where update_keys() is defined as: update_keys(char): Key(0) <- crc32(key(0),char) Key(1) <- Key(1) + (Key(0) & 000000ffH) Key(1) <- Key(1) * 134775813 + 1 Key(2) <- crc32(key(2),key(1) >> 24) end update_keys Where crc32(old_crc,char) is a routine that given a CRC value and a character, returns an updated CRC value after applying the CRC-32 algorithm described elsewhere in this document. Step 2 - Decrypting the encryption header ----------------------------------------- The purpose of this step is to further initialize the encryption keys, based on random data, to render a plaintext attack on the data ineffective. Read the 12-byte encryption header into Buffer, in locations Buffer(0) thru Buffer(11). loop for i <- 0 to 11 C <- buffer(i) ^ decrypt_byte() update_keys(C) buffer(i) <- C end loop Where decrypt_byte() is defined as: unsigned char decrypt_byte() local unsigned short temp temp <- Key(2) | 2 decrypt_byte <- (temp * (temp ^ 1)) >> 8 end decrypt_byte After the header is decrypted, the last 1 or 2 bytes in Buffer should be the high-order word/byte of the CRC for the file being decrypted, stored in Intel low-byte/high-byte order, or the high-order byte of the file time if bit 3 of the general purpose bit flag is set. Versions of PKZIP prior to 2.0 used a 2 byte CRC check; a 1 byte CRC check is used on versions after 2.0. This can be used to test if the password supplied is correct or not. Step 3 - Decrypting the compressed data stream ---------------------------------------------- The compressed data stream can be decrypted as follows: loop until done read a character into C Temp <- C ^ decrypt_byte() update_keys(temp) output Temp end loop In addition to the above mentioned contributors to PKZIP and PKUNZIP, I would like to extend special thanks to Robert Mahoney for suggesting the extension .ZIP for this software. References: Fiala, Edward R., and Greene, Daniel H., "Data compression with finite windows", Communications of the ACM, Volume 32, Number 4, April 1989, pages 490-505. Held, Gilbert, "Data Compression, Techniques and Applications, Hardware and Software Considerations", John Wiley & Sons, 1987. Huffman, D.A., "A method for the construction of minimum-redundancy codes", Proceedings of the IRE, Volume 40, Number 9, September 1952, pages 1098-1101. Nelson, Mark, "LZW Data Compression", Dr. Dobbs Journal, Volume 14, Number 10, October 1989, pages 29-37. Nelson, Mark, "The Data Compression Book", M&T Books, 1991. Storer, James A., "Data Compression, Methods and Theory", Computer Science Press, 1988 Welch, Terry, "A Technique for High-Performance Data Compression", IEEE Computer, Volume 17, Number 6, June 1984, pages 8-19. Ziv, J. and Lempel, A., "A universal algorithm for sequential data compression", Communications of the ACM, Volume 30, Number 6, June 1987, pages 520-540. Ziv, J. and Lempel, A., "Compression of individual sequences via variable-rate coding", IEEE Transactions on Information Theory, Volume 24, Number 5, September 1978, pages 530-536.