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Application Note

.ZIP File Format Specification

Note: This application note is also available as an on-line text file.


File:    APPNOTE - .ZIP File Format Specification
Version: 6.1.0 - NOTIFICATION OF CHANGE
Revised: 01/20/2004
Copyright (c) 1989 - 2004 PKWARE Inc., All Rights Reserved.

I. Purpose
----------

This specification is intended to define a cross-platform,
interoperable file format.  Since its first publication
in 1989, PKWARE has remained committed to ensuring the 
interoperability of the .ZIP file format through this
specification.  We trust that all .ZIP compatible vendors
and application developers that have adopted this format
will share and support this commitment.


II. 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 or omission 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.

If the version of this file is marked as a NOTIFICATION OF CHANGE,
the content defines an Early Feature Specification (EFS) change 
to the .ZIP file format that may be subject to modification prior 
to publication of the Final Feature Specification (FFS).  This
document may also contain information on Planned Feature 
Specifications (PFS) defining recognized future extensions.


III. General Format of a .ZIP file
----------------------------------

  Files stored in arbitrary order.  Large .ZIP files can span multiple
  diskette media or be split into user-defined segment sizes.  

  Overall .ZIP file format:

    [local file header 1]
    [file data 1]
    [data descriptor 1]
    . 
    .
    .
    [local file header n]
    [file data n]
    [data descriptor n]
    [central directory]
    [zip64 end of central directory record]
    [zip64 end of central directory locator] 
    [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
        file name length                2 bytes
        extra field length              2 bytes

        file name (variable size)
        extra field (variable size)

  B.  File data

      Immediately following the local header for a file
      is the compressed or stored data for the file. 
      The series of [local file header][file data][data
      descriptor] repeats for each file in the .ZIP archive. 

  C.  Data descriptor:

        crc-32                          4 bytes
        compressed size                 4 bytes
        uncompressed size               4 bytes

      This descriptor exists only if bit 3 of the general
      purpose bit flag is set (see below).  It is byte aligned
      and immediately follows the last byte of compressed data.
      This descriptor is used only when it was not possible to
      seek in the output .ZIP file, e.g., when the output .ZIP file
      was standard output or a non seekable device.  For Zip64 format
      archives, the compressed and uncompressed sizes are 8 bytes each.

  D.  Central directory structure:

      [file header 1]
      .
      .
      . 
      [file header n]
      [digital signature] 

      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
        file name 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

        file name (variable size)
        extra field (variable size)
        file comment (variable size)

      Digital signature:

        header signature                4 bytes  (0x05054b50)
        size of data                    2 bytes
        signature data (variable size)

  E.  Zip64 end of central directory record

        zip64 end of central dir 
        signature                       4 bytes  (0x06064b50)
        size of zip64 end of central
        directory record                8 bytes
        version made by                 2 bytes
        version needed to extract       2 bytes
        number of this disk             4 bytes
        number of the disk with the 
        start of the central directory  4 bytes
        total number of entries in the
        central directory on this disk  8 bytes
        total number of entries in the
        central directory               8 bytes
        size of the central directory   8 bytes
        offset of start of central
        directory with respect to
        the starting disk number        8 bytes
        zip64 extensible data sector    (variable size)

  F.  Zip64 end of central directory locator

        zip64 end of central dir locator 
        signature                       4 bytes  (0x07064b50)
        number of the disk with the
        start of the zip64 end of 
        central directory               4 bytes
        relative offset of the zip64
        end of central directory record 8 bytes
        total number of disks           4 bytes
        
  G.  End of central directory 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 directory on this disk  2 bytes
        total number of entries in
        the central directory           2 bytes
        size of the central directory   4 bytes
        offset of start of central
        directory with respect to
        the starting disk number        4 bytes
        .ZIP file comment length        2 bytes
        .ZIP file comment       (variable size)

  H.  Explanation of fields:

      version made by (2 bytes)

          The upper byte indicates the compatibility of the file
          attribute information.  If the external file attributes 
          are compatible with MS-DOS and can be read by PKZIP for 
          DOS version 2.04g then this value will be zero.  If these 
          attributes are not compatible, then this value will 
          identify the host system on which the attributes are 
          compatible.  Software can use this information to determine
          the line record format for text files etc.  The current
          mappings are:

          0 - MS-DOS and OS/2 (FAT / VFAT / FAT32 file systems)
          1 - Amiga                     2 - OpenVMS
          3 - Unix                      4 - VM/CMS
          5 - Atari ST                  6 - OS/2 H.P.F.S.
          7 - Macintosh                 8 - Z-System
          9 - CP/M                     10 - Windows NTFS
         11 - MVS (OS/390 - Z/OS)      12 - VSE
         13 - Acorn Risc               14 - VFAT
         15 - alternate MVS            16 - BeOS
         17 - Tandem                   18 - OS/400
         19 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.  This value is based on the
          specific features a ZIP program must support to be
          able to extract the file.  If multiple features are
          applied to a file, the minimum version should be 
          set to the feature having the highest value. New
          features or feature changes affecting the published
          format specification will be implemented using higher
          version numbers than the last published value to 
          avoid conflict.

          Current minimum feature versions are as defined below:

          1.0 - Default value
          1.1 - File is a volume label
          2.0 - File is a folder (directory)
          2.0 - File is compressed using Deflate compression
          2.0 - File is encrypted using traditional PKWARE encryption
          2.1 - File is compressed using Deflate64(tm)
          2.5 - File is compressed using PKWARE DCL Implode 
          2.7 - File is a patch data set 
          4.5 - File uses ZIP64 format extensions
          4.6 - File is compressed using BZIP2 compression*
          5.0 - File is encrypted using DES
          5.0 - File is encrypted using 3DES
          5.0 - File is encrypted using original RC2 encryption
          5.0 - File is encrypted using RC4 encryption
          5.1 - File is encrypted using AES encryption
          5.1 - File is encrypted using corrected RC2 encryption**
          5.2 - File is encrypted using corrected RC2-64 encryption**
          6.1 - File is encrypted using non-OAEP key wrapping***


          * Early 7.x (pre-7.2) versions of PKZIP incorrectly set the
          version needed to extract for BZIP2 compression to be 50
          when it should have been 46.

          ** Refer to the section on Strong Encryption Specification
          for additional information regarding RC2 corrections.

          *** Certificate encryption using non-OAEP key wrapping is the
          intended mode of operation for all versions beginning with 6.1.
          Support for OAEP key wrapping should only be used for
          backward compatibility when sending ZIP files to be opened by
          versions of PKZIP older than 6.1 (5.0 or 6.0).

          When using ZIP64 extensions, the corresponding value in the
          Zip64 end of central directory record should also be set.  
          This field currently supports only the value 45 to indicate
          ZIP64 extensions are present.

      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
                 3 Shannon-Fano trees were used to encode the
                 sliding dictionary output.  If clear, then 2
                 Shannon-Fano trees were used.

          (For Methods 8 and 9 - Deflating)
          Bit 2  Bit 1
            0      0    Normal (-en) compression option was used.
            0      1    Maximum (-exx/-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.)

          Bit 4: Reserved for use with method 8, for enhanced
                 deflating. 

          Bit 5: If this bit is set, this indicates that the file is 
                 compressed patched data.  (Note: Requires PKZIP 
                 version 2.70 or greater)

          Bit 6: Strong encryption.  If this bit is set, you should
                 set the version needed to extract value to at least
                 50 and you must also set bit 0.  If AES encryption
                 is used, the version needed to extract value must 
                 be at least 51.

          Bit 7: Currently unused.

          Bit 8: Currently unused.

          Bit 9: Currently unused.

          Bit 10: Currently unused.

          Bit 11: Currently unused.

          Bit 12: Reserved by PKWARE for enhanced compression.

          Bit 13: Reserved by PKWARE.

          Bit 14: Reserved by PKWARE.

          Bit 15: Reserved by PKWARE.

      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 - Enhanced Deflating using Deflate64(tm)
         10 - PKWARE Data Compression Library Imploding
         11 - Reserved by PKWARE
         12 - File is compressed using BZIP2 algorithm

      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.  If an archive is in zip64 format
          and the value in this field is 0xFFFFFFFF, the size will be
          in the corresponding 8 byte zip64 extended information 
          extra field.

      file name length: (2 bytes)
      extra field length: (2 bytes)
      file comment length: (2 bytes)

          The length of the file name, 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 file name length is set to zero.

      disk number start: (2 bytes)

          The number of the disk on which this file begins.  If an 
          archive is in zip64 format and the value in this field is 
          0xFFFF, the size will be in the corresponding 4 byte zip64 
          extended information extra field.

      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.

          Bits 1 and 2 are reserved for use by PKWARE.

      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.  If an archive is in zip64 format and the value
          in this field is 0xFFFFFFFF, the size will be in the 
          corresponding 8 byte zip64 extended information extra field.

      file name: (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 file name field.

      extra field: (Variable)

          This is for expansion.  If additional information
          needs to be stored for special needs or for specific 
          platforms, 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:

          0x0001        ZIP64 extended information extra field
          0x0007        AV Info
          0x0008        Reserved for future Unicode file name data (PFS)
          0x0009        OS/2
          0x000a        NTFS 
          0x000c        OpenVMS
          0x000d        Unix
          0x000f        Patch Descriptor
          0x0014        PKCS#7 Store for X.509 Certificates
          0x0015        X.509 Certificate ID and Signature for 
                        individual file
          0x0016        X.509 Certificate ID for Central Directory
          0x0017        Strong Encryption Header
          0x0018        Record Management Controls
          0x0065        IBM S/390 (Z390), AS/400 (I400) attributes 
                        - uncompressed
          0x0066        IBM S/390 (Z390), AS/400 (I400) attributes 
                        - compressed

          Third party mappings commonly used are:


          0x2605        ZipIt Macintosh
          0x2705        ZipIt Macintosh 1.3.5+
          0x07c8        Macintosh
          0x2805        ZipIt Macintosh 1.3.5+
          0x334d        Info-ZIP Macintosh
          0x4341        Acorn/SparkFS 
          0x4453        Windows NT security descriptor (binary ACL)
          0x4704        VM/CMS
          0x470f        MVS
          0x4b46        FWKCS MD5 (see below)
          0x4c41        OS/2 access control list (text ACL)
          0x4d49        Info-ZIP OpenVMS
          0x4f4c        Xceed original location extra field
          0x5356        AOS/VS (ACL)
          0x5455        extended timestamp
          0x554e        Xceed unicode extra field
          0x5855        Info-ZIP Unix (original, also OS/2, NT, etc)
          0x6542        BeOS/BeBox
          0x756e        ASi Unix
          0x7855        Info-ZIP Unix (new)
          0xfd4a        SMS/QDOS

          Detailed descriptions of Extra Fields defined by third 
          party mappings will be documented as information on
          these data structures is made available to PKWARE.  
          PKWARE does not guarantee the accuracy of any published
          third party data.

          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 file name, 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.

         -ZIP64 Extended Information Extra Field (0x0001):

          The following is the layout of the ZIP64 extended 
          information "extra" block. If one of the size or
          offset fields in the Local or Central directory
          record is too small to hold the required data,
          a ZIP64 extended information record is created.
          The order of the fields in the ZIP64 extended 
          information record is fixed, but the fields will
          only appear if the corresponding Local or Central
          directory record field is set to 0xFFFF or 0xFFFFFFFF.

          Note: all fields stored in Intel low-byte/high-byte order.

          Value      Size       Description
          -----      ----       -----------
  (ZIP64) 0x0001     2 bytes    Tag for this "extra" block type
          Size       2 bytes    Size of this "extra" block
          Original 
          Size       8 bytes    Original uncompresseed file size
          Compressed
          Size       8 bytes    Size of compressed data
          Relative Header
          Offset     8 bytes    Offset of local header record
          Disk Start
          Number     4 bytes    Number of the disk on which
                                this file starts 

          This entry in the Local header must include BOTH original
          and compressed file sizes.

         -OS/2 Extra Field (0x0009):

          The following is the layout of the OS/2 attributes "extra" 
          block.  (Last Revision  09/05/95)

          Note: all fields stored in Intel low-byte/high-byte order.

          Value       Size          Description
          -----       ----          -----------
  (OS/2)  0x0009      2 bytes       Tag for this "extra" block type
          TSize       2 bytes       Size for the following data block
          BSize       4 bytes       Uncompressed Block Size
          CType       2 bytes       Compression type
          EACRC       4 bytes       CRC value for uncompress block
          (var)       variable      Compressed block

          The OS/2 extended attribute structure (FEA2LIST) is 
          compressed and then stored in it's entirety within this 
          structure.  There will only ever be one "block" of data in 
          VarFields[].

         -NTFS Extra Field (0x000a):

          The following is the layout of the NTFS attributes 
          "extra" block. (Note: At this time the Mtime, Atime
          and Ctime values may be used on any WIN32 system.)  

          Note: all fields stored in Intel low-byte/high-byte order.

          Value      Size       Description
          -----      ----       -----------
  (NTFS)  0x000a     2 bytes    Tag for this "extra" block type
          TSize      2 bytes    Size of the total "extra" block
          Reserved   4 bytes    Reserved for future use
          Tag1       2 bytes    NTFS attribute tag value #1
          Size1      2 bytes    Size of attribute #1, in bytes
          (var.)     Size1      Attribute #1 data
          .
          .
          .
          TagN       2 bytes    NTFS attribute tag value #N
          SizeN      2 bytes    Size of attribute #N, in bytes
          (var.)     SizeN      Attribute #N data

          For NTFS, values for Tag1 through TagN are as follows:
          (currently only one set of attributes is defined for NTFS)

          Tag        Size       Description
          -----      ----       -----------
          0x0001     2 bytes    Tag for attribute #1 
          Size1      2 bytes    Size of attribute #1, in bytes
          Mtime      8 bytes    File last modification time
          Atime      8 bytes    File last access time
          Ctime      8 bytes    File creation time

         -OpenVMS Extra Field (0x000c):

          The following is the layout of the OpenVMS attributes 
          "extra" block.

          Note: all fields stored in Intel low-byte/high-byte order.

          Value      Size       Description
          -----      ----       -----------
  (VMS)   0x000c     2 bytes    Tag for this "extra" block type
          TSize      2 bytes    Size of the total "extra" block
          CRC        4 bytes    32-bit CRC for remainder of the block
          Tag1       2 bytes    OpenVMS attribute tag value #1
          Size1      2 bytes    Size of attribute #1, in bytes
          (var.)     Size1      Attribute #1 data
          .
          .
          .
          TagN       2 bytes    OpenVMS attribute tage value #N
          SizeN      2 bytes    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 
             OpenVMS C.  Neither of these values will ever be zero.

          2. No word alignment or padding is performed.

          3. A well-behaved PKZIP/OpenVMS 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.

         -UNIX Extra Field (0x000d):

          The following is the layout of the Unix "extra" block.
          Note: all fields are stored in Intel low-byte/high-byte 
          order.

          Value       Size          Description
          -----       ----          -----------
  (UNIX)  0x000d      2 bytes       Tag for this "extra" block type
          TSize       2 bytes       Size for the following data block
          Atime       4 bytes       File last access time
          Mtime       4 bytes       File last modification time
          Uid         2 bytes       File user ID
          Gid         2 bytes       File group ID
          (var)       variable      Variable length data field

          The variable length data field will contain file type 
          specific data.  Currently the only values allowed are
          the original "linked to" file names for hard or symbolic 
          links, and the major and minor device node numbers for
          character and block device nodes.  Since device nodes
          cannot be either symbolic or hard links, only one set of
          variable length data is stored.  Link files will have the
          name of the original file stored.  This name is NOT NULL
          terminated.  Its size can be determined by checking TSize -
          12.  Device entries will have eight bytes stored as two 4
          byte entries (in little endian format).  The first entry
          will be the major device number, and the second the minor
          device number.
          
         -PATCH Descriptor Extra Field (0x000f):

          The following is the layout of the Patch Descriptor "extra"
          block.

          Note: all fields stored in Intel low-byte/high-byte order.

          Value     Size     Description
          -----     ----     -----------
  (Patch) 0x000f    2 bytes  Tag for this "extra" block type
          TSize     2 bytes  Size of the total "extra" block
          Version   2 bytes  Version of the descriptor
          Flags     4 bytes  Actions and reactions (see below) 
          OldSize   4 bytes  Size of the file about to be patched 
          OldCRC    4 bytes  32-bit CRC of the file to be patched 
          NewSize   4 bytes  Size of the resulting file 
          NewCRC    4 bytes  32-bit CRC of the resulting file 

          Actions and reactions

          Bits          Description
          ----          ----------------
          0             Use for autodetection
          1             Treat as selfpatch
          2-3           RESERVED
          4-5           Action (see below)
          6-7           RESERVED
          8-9           Reaction (see below) to absent file 
          10-11         Reaction (see below) to newer file
          12-13         Reaction (see below) to unknown file
          14-15         RESERVED
          16-31         RESERVED

          Actions

          Action       Value
          ------       ----- 
          none         0
          add          1
          delete       2
          patch        3

          Reactions
 
          Reaction     Value
          --------     -----
          ask          0
          skip         1
          ignore       2
          fail         3

          Patch support is provided by PKPatchMaker(tm) technology and is 
          covered under U.S. Patents and Patents Pending.

         -PKCS#7 Store for X.509 Certificates (0x0014):

          Note: all fields stored in Intel low-byte/high-byte order.

          Value     Size     Description
          -----     ----     -----------
  (Store) 0x0014    2 bytes  Tag for this "extra" block type
          TSize     2 bytes  Size of the store data
          (var)     TSize    Data about the store


         -X.509 Certificate ID and Signature for individual file (0x0015):

          Note: all fields stored in Intel low-byte/high-byte order.

          Value     Size     Description
          -----     ----     -----------
  (CID)   0x0015    2 bytes  Tag for this "extra" block type
          TSize     2 bytes  Size of data that follows
          (var)     TSize    Data

         -X.509 Certificate ID and Signature for central directory (0x0016):

          Note: all fields stored in Intel low-byte/high-byte order.

          Value     Size     Description
          -----     ----     -----------
  (CDID)  0x0016    2 bytes  Tag for this "extra" block type
          TSize     2 bytes  Size of data that follows
          (var)     TSize    Data

         -Strong Encryption Header (0x0017) (EFS):

          Value     Size     Description
          -----     ----     -----------
          0x0017    2 bytes  Tag for this "extra" block type
          TSize     2 bytes  Size of data that follows
          Format    2 bytes  Format definition for this record
          AlgID     2 bytes  Encryption algorithm identifier
          Bitlen    2 bytes  Bit length of encryption key
          Flags     2 bytes  Processing flags
          CertData  TSize-8  Certificate decryption extra field data
                             (refer to the explanation for CertData
                              in the section describing the 
                              Certificate Processing Method under 
                              the Strong Encryption Specification)


         -Record Management Controls (0x0018):

          Value     Size     Description
          -----     ----     -----------
(Rec-CTL) 0x0018    2 bytes  Tag for this "extra" block type
          CSize     2 bytes  Size of total extra block data
          Tag1      2 bytes  Record control attribute 1
          Size1     2 bytes  Size of attribute 1, in bytes
          Data      Size1    Attribute 1 data
            .
            .
            .
          TagN      2 bytes  Record control attribute N
          SizeN     2 bytes  Size of attribute N, in bytes
          Data      SizeN    Attribute N data

         -MVS Extra Field (0x0065):

          The following is the layout of the MVS "extra" block.
          Note: Some fields are stored in Big Endian format.
          All text is in EBCDIC format unless otherwise specified.

          Value       Size          Description
          -----       ----          -----------
  (MVS)   0x0065      2 bytes       Tag for this "extra" block type
          TSize       2 bytes       Size for the following data block
          ID          4 bytes       EBCDIC "Z390" 0xE9F3F9F0 or
                                    "T4MV" for TargetFour
          (var)       TSize-4       Attribute data


         -OS/400 Extra Field (0x0065):

          The following is the layout of the OS/400 "extra" block.
          Note: Some fields are stored in Big Endian format.
          All text is in EBCDIC format unless otherwise specified.

          Value       Size          Description
          -----       ----          -----------
  (OS400) 0x0065      2 bytes       Tag for this "extra" block type
          TSize       2 bytes       Size for the following data block
          ID          4 bytes       EBCDIC "I400" 0xC9F4F0F0 or
                                    "T4MV" for TargetFour
          (var)       TSize-4       Attribute data


         -ZipIt Macintosh Extra Field (long) (0x2605):

          The following is the layout of the ZipIt extra block 
          for Macintosh. The local-header and central-header versions 
          are identical. This block must be present if the file is 
          stored MacBinary-encoded and it should not be used if the file 
          is not stored MacBinary-encoded.

          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, for files) (0x2705):

          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 of
          files (not directories) that do not have a MacBinary encoded
          file. The local-header and central-header versions are identical.

          Value         Size        Description
          -----         ----        -----------
  (Mac2b) 0x2705        Short       tag for this extra block type
          TSize         Short       total data size for this block (12)
          "ZPIT"        beLong      extra-field signature
          FileType      Byte[4]     four-byte Mac file type string
          Creator       Byte[4]     four-byte Mac creator string
          fdFlags       beShort     attributes from FInfo.frFlags,
                                    may be omitted
          0x0000        beShort     reserved, may be omitted


         -ZipIt Macintosh Extra Field (short, for directories) (0x2805):

          The following is the layout of a shortened variant of the
          ZipIt extra block for Macintosh used only for directory
          entries. This variant is used by ZipIt 1.3.5 and newer to 
          save some optional Mac-specific information about directories.
          The local-header and central-header versions are identical.

          Value         Size        Description
          -----         ----        -----------
  (Mac2c) 0x2805        Short       tag for this extra block type
          TSize         Short       total data size for this block (12)
          "ZPIT"        beLong      extra-field signature
          frFlags       beShort     attributes from DInfo.frFlags, may
                                    be omitted
          View          beShort     ZipIt view flag, may be omitted


          The View field specifies ZipIt-internal settings as follows:

          Bits of the Flags:
              bit 0           if set, the folder is shown expanded (open)
                              when the archive contents are viewed in ZipIt.
              bits 1-15       reserved, zero;


         -FWKCS MD5 Extra Field (0x4b46):

          The FWKCS Contents_Signature System, used in
          automatically identifying files independent of file name,
          optionally adds and uses an extra field to support the
          rapid creation of an enhanced contents_signature:

              Header ID = 0x4b46
              Data Size = 0x0013
              Preface   = 'M','D','5'
              followed by 16 bytes containing the uncompressed file's
              128_bit MD5 hash(1), low byte first.

          When FWKCS revises a .ZIP file central directory to add
          this extra field for a file, it also replaces the
          central directory entry for that file's uncompressed
          file length with a measured value.

          FWKCS provides an option to strip this extra field, if
          present, from a .ZIP file central directory. In adding
          this extra field, FWKCS preserves .ZIP file Authenticity
          Verification; if stripping this extra field, FWKCS
          preserves all versions of AV through PKZIP version 2.04g.

          FWKCS, and FWKCS Contents_Signature System, are
          trademarks of Frederick W. Kantor.

          (1) R. Rivest, RFC1321.TXT, MIT Laboratory for Computer
              Science and RSA Data Security, Inc., April 1992.
              ll.76-77: "The MD5 algorithm is being placed in the
              public domain for review and possible adoption as a
              standard."

      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. If an archive is in zip64 format
          and the value in this field is 0xFFFF, the size will 
          be in the corresponding 4 byte zip64 end of central 
          directory field.


      number of the disk with the start of the central
      directory: (2 bytes)

          The number of the disk on which the central
          directory starts. If an archive is in zip64 format
          and the value in this field is 0xFFFF, the size will 
          be in the corresponding 4 byte zip64 end of central 
          directory field.

      total number of entries in the central dir on 
      this disk: (2 bytes)

          The number of central directory entries on this disk.
          If an archive is in zip64 format and the value in 
          this field is 0xFFFF, the size will be in the 
          corresponding 8 byte zip64 end of central 
          directory field.

      total number of entries in the central dir: (2 bytes)

          The total number of files in the .ZIP file. If an 
          archive is in zip64 format and the value in this field
          is 0xFFFF, the size will be in the corresponding 8 byte 
          zip64 end of central directory field.

      size of the central directory: (4 bytes)

          The size (in bytes) of the entire central directory.
          If an archive is in zip64 format and the value in 
          this field is 0xFFFFFFFF, the size will be in the 
          corresponding 8 byte zip64 end of central 
          directory field.

      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. If an 
          archive is in zip64 format and the value in this 
          field is 0xFFFFFFFF, the size will be in the 
          corresponding 8 byte zip64 end of central 
          directory field.

      .ZIP file comment length: (2 bytes)

          The length of the comment for this .ZIP file.

      .ZIP file comment: (Variable)

          The comment for this .ZIP file.

      zip64 extensible data sector    (variable size)

          (currently reserved for use by PKWARE)


  I.  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 .ZIP file.

      5)  Spanned/Split archives created using PKZIP for Windows
          (V2.50 or greater), PKZIP Command Line (V2.50 or greater),
          or PKZIP Explorer will include a special spanning 
          signature as the first 4 bytes of the first segment of
          the archive.  This signature (0x08074b50) will be 
          followed immediately by the local header signature for
          the first file in the archive.  A special spanning
          marker may also appear in spanned/split archives if the
          spanning or splitting process starts but only requires
          one segement.  In this case the 0x08074b50 signature
          will be replaced with the temporary spanning marker
          signature of 0x30304b50.  Spanned/split archives
          created with this special signature are compatible with
          all versions of PKZIP from PKWARE.  Split archives can
          only be uncompressed by other versions of PKZIP that
          know how to create a split archive.

      6)  If one of the fields in the end of central directory
          record is too small to hold required data, the field
          should be set to -1 (0xFFFF or 0xFFFFFFFF) and the
          Zip64 format record should be created.

      7)  The end of central directory record and the
          Zip64 end of central directory locator record must
          reside on the same disk when splitting or spanning
          an archive.

IV. 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.

V. 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.

VI. 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

VII. Tokenizing - Method 7
--------------------------

This method is not used by PKZIP.

VIII. 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 - 256 (256 - 286)
           All other codes are never sent.
   5 Bits: # of Dist codes - 1           (1 - 32)
   4 Bits: # of Bit Length codes - 3     (3 - 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
   read crc and sizes
endif

IX. Enhanced Deflating - Method 9
---------------------------------

The Enhanced Deflating algorithm is similar to Deflate but
uses a sliding dictionary of up to 64K. Deflate64(tm) is supported
by the Deflate extractor. 

X. BZIP2 - Method 12
--------------------

BZIP2 is an open-source data compression algorithm developed by 
Julian Seward.  Information and source code for this algorithm
can be found on the internet.

XI. Traditional PKWARE Encryption
---------------------------------

The following information discusses the decryption steps
required to support traditional PKWARE encryption.  This
form of encryption is considered weak by todays standards
and its use is recommended only for situations with
low security needs or for compatiblity with older .ZIP 
applications.

XII. 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.  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


XIII. Strong Encryption Specification (EFS)
-------------------------------------------

Version 5.x of this specification introduced support for strong 
encryption algorithms.  These algorithms can be used with either 
a password or an X.509v3 digital certificate to encrypt each file. 
This format specification supports either password or certificate 
based encryption to meet the security needs of today, to enable 
interoperability between users within both PKI and non-PKI 
environments, and to ensure interoperability between different 
computing platforms that are running a ZIP program.  

Password based encryption is the most common form of encryption 
people are familiar with.  However, inherent weaknesses with 
passwords (e.g. susceptibility to dictionary/brute force attack) 
as well as password management and support issues make certificate 
based encryption a more secure and scalable option.  Industry 
efforts and support are defining and moving towards more advanced 
security solutions built around X.509v3 digital certificates and 
Public Key Infrastructures(PKI) because of the greater scalability, 
administrative options, and more robust security over traditional 
password-based encryption. 

Most standard encryption algorithms are supported with this
specification. Reference implementations for many of these 
algorithms are available from either commercial or open source 
distributors.  Readily available cryptographic toolkits make
implementation of the encryption features straight-forward.

The algorithms introduced in Version 5.0 of this specification 
include:

    RC2 40 bit, 64 bit, and 128 bit
    RC4 40 bit, 64 bit, and 128 bit
    DES
    3DES 112 bit and 168 bit
  
Version 5.1 adds support for the following:

    AES 128 bit, 192 bit, and 256 bit

The details of the strong encryption specification for
certificates remain under development as design and testing 
issues are worked out for the range of algorithms, encryption 
methods, certificate processing and cross-platform support 
necessary to meet the advanced security needs of .ZIP file 
users today and in the future. 

This feature specification is intended to support basic 
encryption needs of today, such as password support.  However
this specification is also designed to lay the foundation for 
future advanced security needs.

Single Password Symmetric Encryption Method:
-------------------------------------------

The Single Password Symmetric Encryption Method using strong 
encryption algorithms operates similarly to the traditional 
PKWARE encryption defined in this format.  Additional data 
structures are added to support the processing needs of the 
strong algorithms.

The Strong Encryption data structures are:

1. Bits 0 and 6 of the General Purpose bit flag in both local
and central header records.  Both bits set indicates strong 
encryption.


2. Extra Field 0x0017 in central header only.

     Fields to consider in this record are:

     Format - the data format identifier for this record.  The only
     value allowed at this time is the integer value 2.

     AlgId - integer identifier of the encryption algorithm from the
     following range

         0x6601 - DES
         0x6602 - RC2 (version needed to extract < 5.2)
         0x6603 - 3DES 168
         0x6609 - 3DES 112
         0x660E - AES 128 
         0x660F - AES 192 
         0x6610 - AES 256 
         0x6702 - RC2 (version needed to extract >= 5.2)
         0x6801 - RC4
         0xFFFF - Unknown algorithm

     Bitlen - Explicit bit length of key

          40
          56
          64
         112
         128
         168
         192
         256
   
     Flags - Processing flags needed for decryption

         0x0001 - Password is required to decrypt
         0x0002 - Certificates only
         0x0003 - Password or certificate required to decrypt

         Values > 0x0003 reserved for certificate processing


3. Decryption header record preceding compressed file data.

         -Decryption Header:

          Value     Size     Description
          -----     ----     -----------
          IVSize    2 bytes  Size of initialization vector (IV)
          IVData    IVSize   Initialization vector for this file
          Size      4 bytes  Size of remaining decryption header data
          Format    2 bytes  Format definition for this record
          AlgID     2 bytes  Encryption algorithm identifier
          Bitlen    2 bytes  Bit length of encryption key
          Flags     2 bytes  Processing flags
          ErdSize   2 bytes  Size of Encrypted Random Data
          ErdData   ErdSize  Encrypted Random Data
          Reserved1 4 bytes  Reserved certificate processing data
          Reserved2 (var)    Reserved for certificate processing data
          VSize     2 bytes  Size of password validation data
          VData     VSize-4  Password validation data
          VCRC32    4 bytes  Standard ZIP CRC32 of password validation data

     IVData - The size of the IV should match the algorithm block size.
              The IVData can be completely random data.  If the size of
              the randomly generated data does not match the block size
              it should be complemented with zero's or truncated as
              necessary.  If IVSize is 0,then IV = CRC32 + Uncompressed
              File Size (as a 64 bit little-endian, unsigned integer value).

     Format - the data format identifier for this record.  The only
     value allowed at this time is the integer value 3.

     AlgId - integer identifier of the encryption algorithm from the
     following range

         0x6601 - DES
         0x6602 - RC2 (version needed to extract < 5.2)
         0x6603 - 3DES 168
         0x6609 - 3DES 112
         0x660E - AES 128 
         0x660F - AES 192 
         0x6610 - AES 256 
         0x6702 - RC2 (version needed to extract >= 5.2)
         0x6801 - RC4
         0xFFFF - Unknown algorithm

     Bitlen - Explicit bit length of key

          40
          56
          64
         112
         128
         168
         192
         256
   
     Flags - Processing flags needed for decryption

         0x0001 - Password is required to decrypt
         0x0002 - Certificates only
         0x0003 - Password or certificate required to decrypt

         Values > 0x0003 reserved for certificate processing

     ErdData - Encrypted random data is used to generate a file
               session key for encrypting each file.  SHA1 is 
               used to calculate hash data used to derive keys.
               File session keys are derived from a master session
               key generated from the user-supplied password.
               If the Flags field in the decryption header contains 
               the value 0x4000, then the ErdData field must be 
               decrypted using 3DES.


     Reserved1 - Reserved for certificate processing, if value is
               zero, then Reserved2 data is absent.  See the explanation
               under the Certificate Processing Method for details on
               this data structure.

     Reserved2 - If present, the size of the Reserved2 data structure 
               is located by skipping the first 4 bytes of this field 
               and using the next 2 bytes as the remaining size.  See
               the explanation under the Certificate Processing Method
               for details on this data structure.

     VSize - This size value will always include the 4 bytes of the
             VCRC32 data and will be greater than 4 bytes.

     VData - Random data for password validation.  This data is VSize
             in length and VSize must be a multiple of the encryption
             block size.  VCRC32 is a checksum value of VData.  
             VData and VCRC32 are stored encrypted and start the
             stream of encrypted data for a file.

Strong Encryption is always applied to a file after compression. The
block oriented algorithms all operate in Cypher Block Chaining (CBC) 
mode.  The block size used for AES encryption is 16.  All other block
algorithms use a block size of 8.  Two ID's are defined for RC2 to 
account for a discrepancy found in the implementation of the RC2
algorithm in the cryptographic library on Windows XP SP1 and all 
earlier versions of Windows.

A pseudo-code representation of the encryption process is as follows:

Password = GetUserPassword()
RD  = Random()
ERD = Encrypt(RD,DeriveKey(SHA1(Password)))
For Each File
    IV = Random()
    VData = Random()
    FileSessionKey = DeriveKey(SHA1(IV + RD))
    Encrypt(VData + VCRC32 + FileData,FileSessionKey)
Done

The function names and parameter requirements will depend on
the choice of the cryptographic toolkit selected.  Almost any
toolkit supporting the reference implementations for each
algorithm can be used.  The RSA BSAFE(r), OpenSSL, and Microsoft
CryptoAPI libraries are all known to work well.  

Certificate Processing Method:
-----------------------------

The Certificate Processing Method for ZIP file encryption remains 
under development.  The information provided here serves as a guide
to those interested in certificate-based data decryption.  This
information may be subject to change in future versions of this
specification and is subject to change without notice.

OAEP Processing with Certificate-based Encryption:

Versions of PKZIP available during this development phase of the 
certificate processing method may set a value of 61 into the 
version needed to extract field for a file.  This indicates that 
non-OAEP key wrapping is used.  This affects certificate encryption 
only, and password encryption functions should not be affected by 
this value.  This means values of 61 may be found on files encrypted
with certificates only, or on files encrypted with both password
encryption and certificate encryption.  Files encrypted with both
methods can safely be decrypted using the password methods documented.

OAEP stands for Optimal Asymmetric Encryption Padding.  It is a
strengthening technique used for small encoded items such as decryption
keys.  This is commonly applied in cryptographic key-wrapping techniques
and is supported by PKCS #1.  Versions 5.0 and 6.0 of this specification 
were designed to support OAEP key-wrapping for certificate-based 
decryption keys for additional security.  

Support for private keys stored on Smart Cards or Tokens introduced
a conflict with this OAEP logic.  Most card and token products do 
not support the additional strengthening applied to OAEP key-wrapped 
data.  In order to resolve this conflict, versions 6.1 and above of this 
specification will no longer support OAEP when encrypting using 
digital certificates. 

Certificate Processing Data Fields:

The Certificate Processing Method of this specification defines the
following additional data fields:

Additional processing flags that can be present in the Flags field of both 
the 0x0017 field of the central directory Extra Field and the Decryption 
header record preceding compressed file data are:

         0x0007 - reserved for future use
         0x000F - reserved for future use
         0x0100 - Indicates non-OAEP key wrapping was used.  If this
                  this field is set, the version needed to extract must
                  be at least 61.  This means OAEP key wrapping is not
                  used when generating a Master Session Key using
                  ErdData.
         0x4000 - ErdData must be decrypted using 3DES-168, otherwise use the
                  same algorithm used for encrypting the file contents.
         0x8000 - reserved for future use

CertData - Extra Field 0x0017 record certificate data structure

The data structure used to store certificate data within the section
of the Extra Field defined by the CertData field of the 0x0017
record are as shown:

          Value     Size     Description
          -----     ----     -----------
          RCount    4 bytes  Number of recipients.  
          HashAlg   2 bytes  Hash algorithm identifier
          HSize     2 bytes  Hash size
          SRList    (var)    Simple list of recipients hashed public keys

          
     RCount    This defines the number intended recipients whose 
               public keys were used for encryption.  This identifies
               the number of elements in the SRList.

     HashAlg   This defines the hash algorithm used to calculate
               the public key hash of each public key used
               for encryption. This field currently supports
               only the following value for SHA-1

               0x8004 - SHA1

     HSize     This defines the size of a hashed public key.

     SRList    This is a variable length list of the hashed 
               public keys for each intended recipient.  Each 
               element in this list is HSize.  The total size of 
               SRList is determined using RCount * HSize.


Reserved1 - Certificate Decryption Header Reserved1 Data:

          Value     Size     Description
          -----     ----     -----------
          RCount    4 bytes  Number of recipients.  
          
     RCount    This defines the number intended recipients whose 
               public keys were used for encryption.  This defines
               the number of elements in the REList field defined below.


Reserved2 - Certificate Decryption Header Reserved2 Data Structures:


          Value     Size     Description
          -----     ----     -----------
          HashAlg   2 bytes  Hash algorithm identifier
          HSize     2 bytes  Hash size
          REList    (var)    List of recipient data elements


     HashAlg   This defines the hash algorithm used to calculate
               the public key hash of each public key used
               for encryption. This field currently supports
               only the following value for SHA-1

               0x8004 - SHA1

     HSize     This defines the size of a hashed public key
               defined in REHData.

     REList    This is a variable length of list of recipient data.  
               Each element in this list consists of a Recipient
               Element data structure as follows:


Recipient Element (REList) Data Structure:

          Value     Size     Description
          -----     ----     -----------
          RESize    2 bytes  Size of REHData + REKData
          REHData   HSize    Hash of recipients public key
          REKData   (var)    Simple key blob


     RESize    This defines the size of an individual REList 
               element.  This value is the combined size of the
               REHData field + REKData field.  REHData is defined by
               HSize.  REKData is variable and can be calculated
               for each REList element using RESize and HSize.

     REHData   Hashed public key for this recipient.

     REKData   Simple Key Blob.  The format of this data structure
               is identical to that defined in the Microsoft
               CryptoAPI and generated using the CryptExportKey()
               function.  The version of the Simple Key Blob
               supported at this time is 0x02 as defined by
               Microsoft.


Certificate Processing Differences:

The Certificate Processing Method of encryption differs from the
Single Password Symmetric Encryption Method as follows.  Instead
of using a user-defined password to generate a master session key,
cryptographically random data is used.  The key material is then
wrapped using standard key-wrapping techniques.  This key material
is wrapped using the public key of each recipient that will need
to decrypt the file using their corresponding private key.

This specification currently assumes digital certificates will follow
the X.509 V3 format for 1024 bit and higher RSA format digital
certificates.  Implementation of this Certificate Processing Method
requires supporting logic for key access and management.  This logic
is outside the scope of this specification.


License Agreement:
-----------------

The features set forth in this Section XIII (the "Strong Encryption 
Specification") are covered by a pending patent application. Portions of 
this Strong Encryption technology are available for use at no charge 
under the following terms and conditions. 

1. License Grant. 

   a. NOTICE TO USER. PLEASE READ THIS ENTIRE SECTION XIII.5 OF THE 
   APPNOTE (THE "AGREEMENT") CAREFULLY. BY USING ALL OR ANY PORTION OF THE 
   LICENSED TECHNOLOGY, YOU ACCEPT ALL THE TERMS AND CONDITIONS OF THIS 
   AGREEMENT AND YOU AGREE THAT THIS AGREEMENT IS ENFORCEABLE LIKE ANY 
   WRITTEN NEGOTIATED AGREEMENT SIGNED BY YOU. IF YOU DO NOT AGREE, DO NOT 
   USE THE LICENSED TECHNOLOGY.
 
   b. Definitions.
 
      i. "Licensed Technology" shall mean that proprietary technology now or 
      hereafter owned or controlled by PKWare, Inc. ("PKWARE") or any 
      subsidiary or affiliate  that covers or is necessary to be used to give 
      software the ability to a) extract and decrypt data from zip files 
      encrypted using any methods of data encryption and key processing which 
      are published in this APPNOTE or any prior APPNOTE, as supplemented by 
      any Additional Compatibility Information; and b) encrypt file contents 
      as part of .ZIP file processing using only the Single Password Symmetric 
      Encryption Method as published in this APPNOTE or any prior APPNOTE, as 
      supplemented by any Additional Compatibility Information.  For purposes 
      of this AGREEMENT, "Additional Compatibility Information" means, with 
      regard to any method of data encryption and key processing published in 
      this or any prior APPNOTE, any corrections, additions, or clarifications 
      to the information in such APPNOTE that are required in order to give 
      software the ability to successfully extract and decrypt zip files (or, 
      but solely in the case of the Single Password Symmetric Encryption Method, 
      to successfully encrypt zip files) in a manner interoperable with the 
      actual implementation of such method in any PKWARE product that is 
      documented or publicly described by PKWARE as being able to create, or 
      to extract and decrypt, zip files using that method.

      ii. "Licensed Products" shall mean any products you produce that 
      incorporate the Licensed Technology.
 
   c. License to Licensed Technology.
 
   PKWARE hereby grants to you a non-exclusive license to use the Licensed 
   Technology for the purpose of manufacturing, offering, selling and using 
   Licensed Products, which license shall extend to permit the practice of all 
   claims in any patent or patent application (collectively, "Patents") now or 
   hereafter owned or controlled by PKWARE in any jurisdiction in the world 
   that are infringed by implementation of the Licensed Technology.  You have 
   the right to sublicense rights you receive under the terms of this AGREEMENT 
   for the purpose of allowing sublicensee to manufacture, offer, sell and use 
   products that incorporate all or a portion of any of your Licensed Products, 
   but if you do, you agree to i) impose the same restrictions on any such 
   sublicensee as these terms impose on you and ii) notify the sublicensee, 
   by means chosen by you in your unfettered discretion, including a notice on 
   your web site, of the terms of this AGREEMENT and make available to each 
   sublicensee the full text of this APPNOTE.   Further, PKWARE hereby grants to 
   you a non-exclusive right to reproduce and distribute, in any form, copies of 
   this APPNOTE, without modification.  Notwithstanding anything to the contrary 
   in this AGREEMENT, you have the right to sublicense the rights, without any of 
   the restrictions described above or elsewhere in this AGREEMENT, to use, offer 
   to sell and sell Licensed Technology as incorporated in executable object code 
   or byte code forms of your Licensed Products.  Any sublicense to use the 
   Licensed Technology incorporated in a Licensed Product granted by you shall 
   survive the termination of this AGREEMENT for any reason.  PKWARE warrants that 
   this license shall continue to encumber the Licensed Technology regardless of 
   changes in ownership of the Licensed Technology.
 
   d. Proprietary Notices.
 
      i. With respect to any Licensed Product that is distributed by you either 
      in source code form or in the form of an object code library of externally 
      callable functions that has been designed by you for incorporation into third 
      party products, you agree to include, in the source code, or in the case of 
      an object code library, in accompanying documentation, a notice using the 
      words "patent pending" until a patent is issued to PKWARE covering any 
      portion of the Licensed Technology or PKWARE provides notice, by means 
      chosen by PKWARE in its unfettered discretion, that it no longer has any 
      patent pending covering any portion of the Licensed Technology.  With respect 
      to any Licensed Product, upon your becoming aware that at least one patent has 
      been granted covering the Licensed Technology, you agree to include in any 
      revisions made by you to the documentation (or any source code distributed 
      by you) the words "Pat. No.", or "Patent Number" and the patent number or 
      numbers of the applicable patent or patents.  PKWARE shall, from time to time, 
      inform you of the patent number or numbers of the patents covering the 
      Licensed Technology, by means chosen by PKWARE in its unfettered discretion, 
      including a notice on its web site.  It shall be a violation of the terms of 
      this AGREEMENT for you to sell Licensed Products without complying with the 
      foregoing marking provisions.
  
      ii. You acknowledge that the terms of this AGREEMENT do not grant you any 
      license or other right to use any PKWARE trademark in connection with the sale, 
      offering for sale, distribution and delivery of the Licensed Products, or in 
      connection with the advertising, promotion and offering of the Licensed Products. 
      You acknowledge PKWARE's ownership of the PKZIP trademark and all other marks 
      owned by PKWARE.
 
   e. Covenant of Compliance and Remedies.

   To the extent that you have elected to implement portions of the Licensed 
   Technology, you agree to use reasonable diligence to comply with those portions 
   of this Section XIII, as modified or supplemented by Additional Compatibility 
   Information available to you, describing the portions of the Licensed Technology 
   that you have elected to implement.  Upon reasonable request by PKWARE, you will 
   provide written notice to PKWARE identifying which version of this APPNOTE you 
   have relied upon for your implementation of any specified Licensed Product.
  
   If any substantial non-compliance with the terms of this AGREEMENT is determined 
   to exist, you will make such changes as necessary to bring your Licensed Products 
   into substantial compliance with the terms of this AGREEMENT.  If, within sixty 
   days of receipt of notice that a Licensed Product fails to comply with the terms 
   of this AGREEMENT, you fail to make such changes as necessary to bring your 
   Licensed Products into compliance with the terms of this AGREEMENT, PKWARE may 
   terminate your rights under this AGREEMENT.  PKWARE does not waive and expressly 
   reserves the right to pursue any and all additional remedies that are or may 
   become available to PKWARE.
 
   f. Warranty and Indemnification Regarding Exportation.
 
   You realize and acknowledge that, as between yourself and PKWARE, you are fully 
   responsible for compliance with the import and export laws and regulations of 
   any country in or to which you import or export any Licensed Products, and you 
   agree to hold PKWARE harmless from any claim of violation of any such import 
   or export laws.
 
   g. Patent Infringement.

   You agree that you will not bring or threaten to bring any action against PKWARE 
   for infringement of the claims of any patent owned or controlled by you solely 
   as a result of PKWARE's own implementation of the Licensed Technology.  As its 
   exclusive remedy for your breach of the foregoing agreement, PKWARE reserves 
   the right to suspend or terminate all rights granted under the terms of this 
   AGREEMENT if you bring or threaten to bring any such action against PKWARE, 
   effective immediately upon delivery of written notice of suspension or 
   termination to you. 

   h. Governing Law. 

   The license granted in this AGREEMENT shall be governed by and construed under 
   the laws of the State of Wisconsin and the United States. 

   i. Revisions and Notice.

   The license granted in this APPNOTE is irrevocable, except as expressly set 
   forth above.  You agree and understand that any changes which PKWARE determines 
   to make to this APPNOTE shall be posted at the same location as the current 
   APPNOTE or at a location which will be identified by means chosen by PKWARE, 
   including a notice on its web site, and shall be available for adoption by you 
   immediately upon such posting, or at such other time as PKWARE shall determine.  
   Any changes to the terms of the license published in a subsequent version of 
   this AGREEMENT shall be binding upon you only with respect to your products 
   that (i) incorporate any Licensed Technology (as defined in the subsequent 
   AGREEMENT) that is not otherwise included in the definition of Licensed 
   Technology under this AGREEMENT, or (ii) that you expressly identify are to 
   be licensed under the subsequent AGREEMENT, which identification shall be by 
   written notice with reference to the APPNOTE (version and release date or other 
   unique identifier) in which the subsequent AGREEMENT is published.  PKWARE 
   agrees to identify each change to this APPNOTE by using a unique version and 
   release date identifier or other unique identifier.
    
   j. Warranty by PKWARE

   PKWare, Inc. warrants that it has the right to grant the license hereunder.
         
XIV. Change Process
-------------------

In order for the .ZIP file format to remain a viable definition, this
specification should be considered as open for periodic review and
revision.  Although this format was originally designed with a 
certain level of extensibility, not all changes in technology
(present or future) were or will be necessarily considered in its
design.  If your application requires new definitions to the
extensible sections in this format, or if you would like to 
submit new data structures, please forward your request to
zipformat@pkware.com.  All submissions will be reviewed by the
ZIP File Specification Committee for possible inclusion into
future versions of this specification.  Periodic revisions
to this specification will be published to ensure interoperability. 
We encourage comments and feedback that may help improve clarity 
or content.

XV. Acknowledgements
--------------------

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.

XVI. 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.
 
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