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. When encrypting the central directory, if the
          local header is not in ZIP64 format and general purpose 
          bit flag 13 is set indicating masking, the value stored 
          in the Local Header will be zero. 

      compressed size: (4 bytes)
      uncompressed size: (4 bytes)

          The size of the file compressed and uncompressed,
          respectively.  When a decryption header is present it will
          be placed in front of the file data and the value of the
          compressed file size will include the bytes of the decryption
          header.  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.  When encrypting the central directory, if the
          local header is not in ZIP64 format and general purpose bit 
          flag 13 is set indicating masking, the value stored for the 
          uncompressed size in the Local Header will be zero. 

      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)

          Bits 1 and 2 are reserved for use by PKWARE.

          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.

          The 0x0002 bit of this field indicates, if set, that a 
          4 byte variable record length control field precedes each 
          logical record indicating the length of the record. The 
          record length control field is stored in little-endian byte
          order.  This flag is independent of text control characters, 
          and if used in conjunction with text data, includes any 
          control characters in the total length of the record. This 
          value is provided for mainframe data transfer support.

      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.  If encrypting
          the central directory and general purpose bit flag 13 is set 
          indicating masking, the file name stored in the Local Header 
          will not be the actual file name.  A masking value consisting 
          of a unique hexadecimal value will be stored.  This value will 
          be sequentially incremented for each file in the archive. See
          the section on the Strong Encryption Specification for details 
          on retrieving the encrypted file name. 

      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 extended language encoding data (PFS)
                        (see APPENDIX D)
          0x0009        OS/2
          0x000a        NTFS 
          0x000c        OpenVMS
          0x000d        UNIX
          0x000e        Reserved for file stream and fork descriptors
          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
          0x0019        PKCS#7 Encryption Recipient Certificate List
          0x0065        IBM S/390 (Z390), AS/400 (I400) attributes 
                        - uncompressed
          0x0066        Reserved for IBM S/390 (Z390), AS/400 (I400) 
                        attributes - compressed
          0x4690        POSZIP 4690 (reserved) 

          Third party mappings commonly used are:


          0x07c8        Macintosh
          0x2605        ZipIt Macintosh
          0x2705        ZipIt Macintosh 1.3.5+
          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)
          0xa220        Microsoft Open Packaging Growth Hint
          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 uncompressed 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 size fields. If encrypting the 
          central directory and bit 13 of the general purpose bit
          flag is set indicating masking, the value stored in the
          Local Header for the original file size will be zero.


         -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