[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.
