Newsgroup: it.comp.lang.visual-basic
Titolo: Floppy Zippati

Supponiamo di avere un archivio pippo.ZIP (creato con l'opzione Multiple
Disk Spanning attivata) costituito da 3 floppy.
Inseriamo nel drive il secondo floppy.
Se apriamo il file pippo.ZIP (con pkunzip o winzip) verra' visualizzato il
seguente messaggio:
The diskette in drive A: does not contain a complete Zip file.
If it is part of a spanned archive set, please insert the last diskette of
the set into the drive and select OK; otherwise select Cancel.
Se facciamo lo stesso col primo dei 3 dischi, il messaggio sara':
The diskette in drive A: is the first diskette of a spanned archive set.
Please insert the last diskette of the spanned set into the drive and select
OK.
DOMANDA:
qualcuno ha idea di dove vada (pkunzip o winzip) a reperire tali
informazioni?
quello che dovrei ottenere e' sapere (da VB5) se il dischetto e' il primo,
uno intermedio o l'ultimo di un archivio, tutto questo senza andare a
scomodare con una shell pkunzip o winzip perche' il tutto deve restare
trasparente all'utente.
Roby, (quasi) sempre bloccato.

Qualcuno aveva problemi con 3 dischi zippati (non so chi, ho avuto problemi
con l'invio di un file di testo allegato: ma non è prorio possibile allegare
niente? neanche file ASCII?). Quel Qno voleva sapere come reperire
informazioni sui files .zip
Dato che non riesco ad allegare il file di cui sono entrato in possesso
(come non ne ho proprio idea, me lo sono trovato un giorno sull'HD), lo
incollo qui sotto come parte del messaggio.
Spero vi stia tutto si vostri HD (25K hehe)!
(tra parentesi se qualcuno riesce a tradurre in VB gli algoritmi di
compressione, per favore, me lo faccia sapere! Io li ho QUASI capiti, ma non
so come convertirli da pseudoC in VB)
Grazie e ciao.
-------------------------------------------------
General Format of a ZIP file
----------------------------
  Files stored in arbitrary order.  Large zipfiles can span multiple
  diskette media.
  Overall zipfile format:
    [local file header+file data] . . .
    [central directory] end of central directory record
  A.  Local file header:
 local file header signature     4 bytes  (0x04034b50)
 version needed to extract       2 bytes
 general purpose bit flag        2 bytes
 compression method              2 bytes
 last mod file time              2 bytes
 last mod file date              2 bytes
 crc-32                          4 bytes
 compressed size                 4 bytes
 uncompressed size               4 bytes
 filename length                 2 bytes
 extra field length              2 bytes
 filename (variable size)
 extra field (variable size)
  B.  Central directory structure:
      [file header] . . .  end of central dir record
      File header:
 central file header signature   4 bytes  (0x02014b50)
 version made by                 2 bytes
 version needed to extract       2 bytes
 general purpose bit flag        2 bytes
 compression method              2 bytes
 last mod file time              2 bytes
 last mod file date              2 bytes
 crc-32                          4 bytes
 compressed size                 4 bytes
 uncompressed size               4 bytes
 filename length                 2 bytes
 extra field length              2 bytes
 file comment length             2 bytes
 disk number start               2 bytes
 internal file attributes        2 bytes
 external file attributes        4 bytes
 relative offset of local header 4 bytes
 filename (variable size)
 extra field (variable size)
 file comment (variable size)
      End of central dir record:
 end of central dir signature    4 bytes  (0x06054b50)
 number of this disk             2 bytes
 number of the disk with the
 start of the central directory  2 bytes
 total number of entries in
 the central dir on this disk    2 bytes
 total number of entries in
 the central dir                 2 bytes
 size of the central directory   4 bytes
 offset of start of central
 directory with respect to
 the starting disk number        4 bytes
 zipfile comment length          2 bytes
 zipfile comment (variable size)
  C.  Explanation of fields:
      version made by
   The upper byte indicates the host system (OS) for the
   file.  Software can use this information to determine
   the line record format for text files etc.  The current
   mappings are:
   0 - MS-DOS and OS/2 (F.A.T. file systems)
   1 - Amiga                     2 - VMS
   3 - *nix                      4 - VM/CMS
   5 - Atari ST                  6 - OS/2 H.P.F.S.
   7 - Macintosh                 8 - Z-System
   9 - CP/M                      10 thru 255 - unused
   The lower byte indicates the version number of the
   software used to encode the file.  The value/10
   indicates the major version number, and the value
   mod 10 is the minor version number.
      version needed to extract
   The minimum software version needed to extract the
   file, mapped as above.
      general purpose bit flag:
   bit 0: If set, indicates that the file is encrypted.
   bit 1: If the compression method used was type 6,
   Imploding, then this bit, if set, indicates
   an 8K sliding dictionary was used.  If clear,
   then a 4K sliding dictionary was used.
   bit 2: If the compression method used was type 6,
   Imploding, then this bit, if set, indicates
   an 3 Shannon-Fano trees were used to encode the
   sliding dictionary output.  If clear, then 2
   Shannon-Fano trees were used.
   Note:  Bits 1 and 2 are undefined if the compression
   method is other than type 6 (Imploding).
   The upper three bits are reserved and used internally
   by the software when processing the zipfile.  The
   remaining bits are unused in version 1.0.
      compression method:
   (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
      date and time fields:
   The date and time are encoded in standard MS-DOS
   format.
      CRC-32:
   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.
      compressed size:
      uncompressed size:
   The size of the file compressed and uncompressed,
   respectively.
      filename length:
      extra field length:
      file comment length:
   The length of the filename, extra field, and comment
   fields respectively.  The combined length of any
   directory record and these three fields should not
   generally exceed 65,535 bytes.
      disk number start:
   The number of the disk on which this file begins.
      internal file attributes:
   The lowest bit of this field indicates, if set, that
   the file is apparently an ASCII or text file.  If not
   set, that the file apparently contains binary data.
   The remaining bits are unused in version 1.0.
      external file attributes:
   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.
      relative offset of local header:
   This is the offset from the start of the first disk on
   which this file appears, to where the local header should
   be found.
      filename:
   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.
      extra field:
   This is for future expansion.  If additional information
   needs to be stored in the future, it should be stored
   here.  Earlier versions of the software can then safely
   skip this file, and find the next file or header.  This
   field will be 0 length in version 1.0.
   In order to allow different programs and different types
   of information to be stored in the 'extra' field in .ZIP
   files, the following structure should be used for all
   programs storing data in this field:
   header1+data1 + header2+data2 . . .
   Each header should consist of:
     Header ID - 2 bytes
     Data Size - 2 bytes
   Note: all fields stored in Intel low-byte/high-byte order.
   The Header ID field indicates the type of data that is in
   the following data block.
   Header ID's of 0 thru 31 are reserved for use by PKWARE.
   The remaining ID's can be used by third party vendors for
   proprietary usage.
   The Data Size field indicates the size of the following
   data block. Programs can use this value to skip to the
   next header block, passing over any data blocks that are
   not of interest.
   Note: As stated above, the size of the entire .ZIP file
  header, including the filename, comment, and extra
  field should not exceed 64K in size.
   In case two different programs should appropriate the same
   Header ID value, it is strongly recommended that each
   program place a unique signature of at least two bytes in
   size (and preferably 4 bytes or bigger) at the start of
   each data area.  Every program should verify that it's
   unique signature is present, in addition to the Header ID
   value being correct, before assuming that it is a block of
   known type.
      file comment:
   The comment for this file.
      number of this disk:
   The number of this disk, which contains central
   directory end record.
      number of the disk with the start of the central directory:
   The number of the disk on which the central
   directory starts.
      total number of entries in the central dir on this disk:
   The number of central directory entries on this disk.
      total number of entries in the central dir:
   The total number of files in the zipfile.
      size of the central directory:
   The size (in bytes) of the entire central directory.
      offset of start of central directory with respect to
      the starting disk number:
   Offset of the start of the central direcory on the
   disk on which the central directory starts.
      zipfile comment length:
   The length of the comment for this zipfile.
      zipfile comment:
   The comment for this zipfile.
  D.  General notes:
      1)  All fields unless otherwise noted are unsigned and stored
   in Intel low-byte:high-byte, low-word:high-word order.
      2)  String fields are not null terminated, since the
   length is given explicitly.
      3)  Local headers should not span disk boundries.  Also, even
   though the central directory can span disk boundries, no
   single record in the central directory should be split
   across disks.
      4)  The entries in the central directory may not necessarily
   be in the same order that files appear in the zipfile.
 
UnShrinking
-----------
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 compresser emits the code
    sequence 256,2 (decimal), the decompressor should clear
    all leaf nodes from the Ziv-Lempel tree, and continue to
    use the current code size.  The nodes that are cleared
    from the Ziv-Lempel tree are then re-used, with the lowest
    code value re-used first, and the highest code value
    re-used last.  The compressor can emit the sequence 256,2
    at any time.
 
Expanding
---------
The Reducing algorithm is actually a combination of two
distinct algorithms.  The first algorithm compresses repeated
byte sequences, and the second algorithm takes the compressed
stream from the first algorithm and applies a probabilistic
compression method.
The probabilistic compression stores an array of 'follower
sets' S(j), for j=0 to 255, corresponding to each possible
ASCII character.  Each set contains between 0 and 32
characters, to be denoted as S(j)[0],...,S(j)[m], where m<32.
The sets are stored at the beginning of the data area for a
Reduced file, in reverse order, with S(255) first, and S(0)
last.
The sets are encoded as { N(j), S(j)[0],...,S(j)[N(j)-1] },
where N(j) is the size of set S(j).  N(j) can be 0, in which
case the follower set for S(j) is empty.  Each N(j) value is
encoded in 6 bits, followed by N(j) eight bit character values
corresponding to S(j)[0] to S(j)[N(j)-1] respectively.  If
N(j) is 0, then no values for S(j) are stored, and the value
for N(j-1) immediately follows.
Immediately after the follower sets, is the compressed data
stream.  The compressed data stream can be interpreted for the
probabilistic decompression as follows:
let Last-Character <- 0.
loop until done
    if the follower set S(Last-Character) is empty then
 read 8 bits from the input stream, and copy this
 value to the output stream.
    otherwise if the follower set S(Last-Character) is non-empty then
 read 1 bit from the input stream.
 if this bit is not zero then
     read 8 bits from the input stream, and copy this
     value to the output stream.
 otherwise if this bit is zero then
     read B(N(Last-Character)) bits from the input
     stream, and assign this value to I.
     Copy the value of S(Last-Character)[I] to the
     output stream.
    assign the last value placed on the output stream to
    Last-Character.
end loop
B(N(j)) is defined as the minimal number of bits required to
encode the value N(j)-1.
The decompressed stream from above can then be expanded to
re-create the original file as follows:
let State <- 0.
loop until done
    read 8 bits from the input stream into C.
    case State of
 0:  if C is not equal to DLE (144 decimal) then
  copy C to the output stream.
     otherwise if C is equal to DLE then
  let State <- 1.
 1:  if C is non-zero then
  let V <- C.
  let Len <- L(V)
  let State <- F(Len).
     otherwise if C is zero then
  copy the value 144 (decimal) to the output stream.
  let State <- 0
 2:  let Len <- Len + C
     let State <- 3.
 3:  move backwards D(V,C) bytes in the output stream
     (if this position is before the start of the output
     stream, then assume that all the data before the
     start of the output stream is filled with zeros).
     copy Len+3 bytes from this position to the output stream.
     let State <- 0.
    end case
end loop
The functions F,L, and D are dependent on the 'compression
factor', 1 through 4, and are defined as follows:
For compression factor 1:
    L(X) equals the lower 7 bits of X.
    F(X) equals 2 if X equals 127 otherwise F(X) equals 3.
    D(X,Y) equals the (upper 1 bit of X) * 256 + Y + 1.
For compression factor 2:
    L(X) equals the lower 6 bits of X.
    F(X) equals 2 if X equals 63 otherwise F(X) equals 3.
    D(X,Y) equals the (upper 2 bits of X) * 256 + Y + 1.
For compression factor 3:
    L(X) equals the lower 5 bits of X.
    F(X) equals 2 if X equals 31 otherwise F(X) equals 3.
    D(X,Y) equals the (upper 3 bits of X) * 256 + Y + 1.
For compression factor 4:
    L(X) equals the lower 4 bits of X.
    F(X) equals 2 if X equals 15 otherwise F(X) equals 3.
    D(X,Y) equals the (upper 4 bits of X) * 256 + Y + 1.
 
Imploding
---------
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 ouput,
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 succesfully 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
 
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 3, 32-bit keys.  The key values are
initialized using the supplied encryption password.  After each byte
is encrypted, the keys are then updated using psuedo-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 two bytes in Buffer
should be the high-order word of the CRC for the file being
decrypted, stored in Intel low-byte/high-byte order.  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 charcter into C
    Temp <- C ^ decrypt_byte()
    update_keys(temp)
    output Temp
end loop
In addition to the above mentioned contributors to PKZIP and PKUNZIP,
I would like to extend special thanks to Robert Mahoney for suggesting
the extension .ZIP for this software.
References:
    Storer, James A. "Data Compression, Methods and Theory",
       Computer Science Press, 1988
    Held, Gilbert  "Data Compression, Techniques and Applications,
      Hardware and Software Considerations"
       John Wiley & Sons, 1987

Per avere queste informazioni devi aprire i files in modo binario, e leggere
l'header.
Questo č l'HEADER. Buon lavoro... :-) Ciao
----------------------------------------------------------------------------
------------------------------------
FILE FORMAT
-----------
Files stored in arbitrary order.  Large zipfiles can span multiple
diskette media.
          Local File Header 1
                    file 1 extra field
                    file 1 comment
               file data 1
          Local File Header 2
                    file 2 extra field
                    file 2 comment
               file data 2
          .
          .
          .
          Local File Header n
                    file n extra field
                    file n comment
               file data n
     Central Directory
               central extra field
               central comment
          End of Central Directory
                    end comment
EOF
LOCAL FILE HEADER
-----------------
OFFSET LABEL       TYP  VALUE        DESCRIPTION
------ ----------- ---- ----------- ----------------------------------
00     ZIPLOCSIG   HEX  04034B50    ;Local File Header Signature
04     ZIPVER      DW   0000        ;Version needed to extract
06     ZIPGENFLG   DW   0000        ;General purpose bit flag
08     ZIPMTHD     DW   0000        ;Compression method
0A     ZIPTIME     DW   0000        ;Last mod file time (MS-DOS)
0C     ZIPDATE     DW   0000        ;Last mod file date (MS-DOS)
0E     ZIPCRC      HEX  00000000    ;CRC-32
12     ZIPSIZE     HEX  00000000    ;Compressed size
16     ZIPUNCMP    HEX  00000000    ;Uncompressed size
1A     ZIPFNLN     DW   0000        ;Filename length
1C     ZIPXTRALN   DW   0000        ;Extra field length
1E     ZIPNAME     DS   ZIPFNLN     ;filename
--     ZIPXTRA     DS   ZIPXTRALN   ;extra field
CENTRAL DIRECTORY STRUCTURE
---------------------------
OFFSET LABEL       TYP  VALUE        DESCRIPTION
------ ----------- ---- ----------- ----------------------------------
00     ZIPCENSIG   HEX  02014B50    ;Central file header signature
04     ZIPCVER     DB   00          ;Version made by
05     ZIPCOS      DB   00          ;Host operating system
06     ZIPCVXT     DB   00          ;Version needed to extract
07     ZIPCEXOS    DB   00          ;O/S of version needed for extraction
08     ZIPCFLG     DW   0000        ;General purpose bit flag
0A     ZIPCMTHD    DW   0000        ;Compression method
0C     ZIPCTIM     DW   0000        ;Last mod file time (MS-DOS)
0E     ZIPCDAT     DW   0000        ;Last mod file date (MS-DOS)
10     ZIPCCRC     HEX  00000000    ;CRC-32
14     ZIPCSIZ     HEX  00000000    ;Compressed size
18     ZIPCUNC     HEX  00000000    ;Uncompressed size
1C     ZIPCFNL     DW   0000        ;Filename length
1E     ZIPCXTL     DW   0000        ;Extra field length
20     ZIPCCML     DW   0000        ;File comment length
22     ZIPDSK      DW   0000        ;Disk number start
24     ZIPINT      DW   0000        ;Internal file attributes
       LABEL       BIT        DESCRIPTION
       ----------- --------- -----------------------------------------
       ZIPINT         0       if = 1, file is apparently an ASCII or
                              text file
                      0       if = 0, file apparently contains binary
                              data
                     1-7      unused in version 1.0.
26     ZIPEXT      HEX  00000000    ;External file attributes, host
                                    ;system dependent
2A     ZIPOFST     HEX  00000000    ;Relative offset of local header
                                    ;from the start of the first disk
                                    ;on which this file appears
2E     ZIPCFN      DS   ZIPCFNL     ;Filename or path - should not
                                    ;contain a drive or device letter,
                                    ;or a leading slash. All slashes
                                    ;should be forward slashes '/'
--     ZIPCXTR     DS   ZIPCXTL     ;extra field
--     ZIPCOM      DS   ZIPCCML     ;file comment
END OF CENTRAL DIR STRUCTURE
----------------------------
OFFSET LABEL       TYP  VALUE        DESCRIPTION
------ ----------- ---- ----------- ----------------------------------
00     ZIPESIG     HEX  06064B50    ;End of central dir signature
04     ZIPEDSK     DW   0000        ;Number of this disk
06     ZIPECEN     DW   0000        ;Number of disk with start central dir
08     ZIPENUM     DW   0000        ;Total number of entries in central dir
                                    ;on this disk
0A     ZIPECENN    DW   0000        ;total number entries in central dir
0C     ZIPECSZ     HEX  00000000    ;Size of the central directory
10     ZIPEOFST    HEX  00000000    ;Offset of start of central directory
                                    ;with respect to the starting disk
                                    ;number
14     ZIPECOML    DW   0000        ;zipfile comment length
16     ZIPECOM     DS   ZIPECOML    ;zipfile comment
ZIP VALUES LEGEND
-----------------
       HOST O/S
       VALUE  DESCRIPTION               VALUE  DESCRIPTION
       ----- -------------------------- ----- ------------------------
       0      MS-DOS and OS/2 (FAT)     5      Atari ST
       1      Amiga                     6      OS/2 1.2 extended file sys
       2      VMS                       7      Macintosh
       3      *nix                      8 thru
       4      VM/CMS                    255    unused
       GENERAL PURPOSE BIT FLAG
       LABEL       BIT        DESCRIPTION
       ----------- --------- -----------------------------------------
       ZIPGENFLG      0       If set, file is encrypted
          or          1       If file Imploded and this bit is set, 8K
       ZIPCFLG                sliding dictionary was used. If clear, 4K
                              sliding dictionary was used.
                      2       If file Imploded and this bit is set, 3
                              Shannon-Fano trees were used. If clear, 2
                              Shannon-Fano trees were used.
                     3-4      unused
                     5-7      used internaly by ZIP
       Note:  Bits 1 and 2 are undefined if the compression method is
              other than type 6 (Imploding).
       COMPRESSION METHOD
       NAME        METHOD  DESCRIPTION
       ----------- ------ --------------------------------------------
       Stored         0    No compression used
       Shrunk         1    LZW, 8K buffer, 9-13 bits with partial clearing
       Reduced-1      2    Probalistic compression, L(X) = lower 7 bits
       Reduced-2      3    Probalistic compression, L(X) = lower 6 bits
       Reduced-3      4    Probalistic compression, L(X) = lower 5 bits
       Reduced-4      5    Probalistic compression, L(X) = lower 4 bits
       Imploded       6    2 Shanno-Fano trees, 4K sliding dictionary
       Imploded       7    3 Shanno-Fano trees, 4K sliding dictionary
       Imploded       8    2 Shanno-Fano trees, 8K sliding dictionary
       Imploded       9    3 Shanno-Fano trees, 8K sliding dictionary
       EXTRA FIELD
       OFFSET LABEL       TYP  VALUE       DESCRIPTION
       ------ ----------- ---- ---------- ----------------------------
       00     EX1ID       DW   0000        ;0-31 reserved by PKWARE
       02     EX1LN       DW   0000
       04     EX1DAT      DS   EX1LN       ;Specific data for individual
       .                                   ;files. Data field should begin
       .                                   ;with a s/w specific unique ID
       EX1LN+4
              EXnID       DW   0000
              EXnLN       DW   0000
              EXnDAT      DS   EXnLN       ;entire header may not exceed 64k