general format of a zip file

-------------------------------------------------
editorial note:
this version was downloaded from the file appnote.txt
from the pkzip website on july 13, 1998.
to obtain this file go to:  
http://www.pkware.com/download.html
  
then download --> appnote.zip
end of editorial note
-------------------------------------------------
disclaimer
----------
although pkware will attempt to supply current and accurate
information relating to its file formats, algorithms, and the
subject programs, the possibility of error can not be eliminated.
pkware therefore expressly disclaims any warranty that the
information contained in the associated materials relating to the
subject programs and/or the format of the files created or
accessed by the subject programs and/or the algorithms used by
the subject programs, or any other matter, is current, correct or
accurate as delivered.  any risk of damage due to any possible
inaccurate information is assumed by the user of the information.
furthermore, the information relating to the subject programs
and/or the file formats created or accessed by the subject
programs and/or the algorithms used by the subject programs is
subject to change without notice.
general format of a zip file
----------------------------
  files stored in arbitrary order.  large zipfiles can span multiple
  diskette media.
  overall zipfile format:
    [local file header + file data + data_descriptor] . . .
    [central directory] end of central directory record
  a.  local file header:
        local file header signature     4 bytes  (0x04034b50)
        version needed to extract       2 bytes
        general purpose bit flag        2 bytes
        compression method              2 bytes
        last mod file time              2 bytes
        last mod file date              2 bytes
        crc-32                          4 bytes
        compressed size                 4 bytes
        uncompressed size               4 bytes
        filename length                 2 bytes
        extra field length              2 bytes
        filename (variable size)
        extra field (variable size)
  b.  data descriptor:
        crc-32                          4 bytes
        compressed size                 4 bytes
        uncompressed size               4 bytes
      this descriptor exists only if bit 3 of the general
      purpose bit flag is set (see below).  it is byte aligned
      and immediately follows the last byte of compressed data.
      this descriptor is used only when it was not possible to
      seek in the output zip file, e.g., when the output zip file
      was standard output or a non seekable device.
  c.  central directory structure:
      [file header] . . .  end of central dir record
      file header:
        central file header signature   4 bytes  (0x02014b50)
        version made by                 2 bytes
        version needed to extract       2 bytes
        general purpose bit flag        2 bytes
        compression method              2 bytes
        last mod file time              2 bytes
        last mod file date              2 bytes
        crc-32                          4 bytes
        compressed size                 4 bytes
        uncompressed size               4 bytes
        filename length                 2 bytes
        extra field length              2 bytes
        file comment length             2 bytes
        disk number start               2 bytes
        internal file attributes        2 bytes
        external file attributes        4 bytes
        relative offset of local header 4 bytes
        filename (variable size)
        extra field (variable size)
        file comment (variable size)
      end of central dir record:
        end of central dir signature    4 bytes  (0x06054b50)
        number of this disk             2 bytes
        number of the disk with the
        start of the central directory  2 bytes
        total number of entries in
        the central dir on this disk    2 bytes
        total number of entries in
        the central dir                 2 bytes
        size of the central directory   4 bytes
        offset of start of central
        directory with respect to
        the starting disk number        4 bytes
        zipfile comment length          2 bytes
        zipfile comment (variable size)
  d.  explanation of fields:
      version made by (2 bytes)
          the upper byte indicates the 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 - vax/vms
          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 thru 255 - unused
          the lower byte indicates the version number of the
          software used to encode the file.  the value/10
          indicates the major version number, and the value
          mod 10 is the minor version number.
      version needed to extract (2 bytes)
          the minimum software version needed to extract the
          file, mapped as above.
      general purpose bit flag: (2 bytes)
          bit 0: if set, indicates that the file is encrypted.
          (for method 6 - imploding)
          bit 1: if the compression method used was type 6,
                 imploding, then this bit, if set, indicates
                 an 8k sliding dictionary was used.  if clear,
                 then a 4k sliding dictionary was used.
          bit 2: if the compression method used was type 6,
                 imploding, then this bit, if set, indicates
                 an 3 shannon-fano trees were used to encode the
                 sliding dictionary output.  if clear, then 2
                 shannon-fano trees were used.
          (for method 8 - deflating)
          bit 2  bit 1
            0      0    normal (-en) compression option was used.
            0      1    maximum (-ex) compression option was used.
            1      0    fast (-ef) compression option was used.
            1      1    super fast (-es) compression option was used.
          note:  bits 1 and 2 are undefined if the compression
                 method is any other.
          bit 3: if this bit is set, the fields crc-32, compressed size
                 and uncompressed size are set to zero in the local
                 header.  the correct values are put in the data descriptor
                 immediately following the compressed data.  (note: pkzip
                 version 2.04g for dos only recognizes this bit for method 8
                 compression, newer versions of pkzip recognize this bit
                 for any compression method.)
          the upper three bits are reserved and used internally
          by the software when processing the zipfile.  the
          remaining bits are unused.
      compression method: (2 bytes)
          (see accompanying documentation for algorithm
          descriptions)
          0 - the file is stored (no compression)
          1 - the file is shrunk
          2 - the file is reduced with compression factor 1
          3 - the file is reduced with compression factor 2
          4 - the file is reduced with compression factor 3
          5 - the file is reduced with compression factor 4
          6 - the file is imploded
          7 - reserved for tokenizing compression algorithm
          8 - the file is deflated
          9 - reserved for enhanced deflating
         10 - pkware date compression library imploding
      date and time fields: (2 bytes each)
          the date and time are encoded in standard ms-dos format.
          if input came from standard input, the date and time are
          those at which compression was started for this data.
      crc-32: (4 bytes)
          the crc-32 algorithm was generously contributed by
          david schwaderer and can be found in his excellent
          book "c programmers guide to netbios" published by
          howard w. sams & co. inc.  the 'magic number' for
          the crc is 0xdebb20e3.  the proper crc pre and post
          conditioning is used, meaning that the crc register
          is pre-conditioned with all ones (a starting value
          of 0xffffffff) and the value is post-conditioned by
          taking the one's complement of the crc residual.
          if bit 3 of the general purpose flag is set, this
          field is set to zero in the local header and the correct
          value is put in the data descriptor and in the central
          directory.
      compressed size: (4 bytes)
      uncompressed size: (4 bytes)
          the size of the file compressed and uncompressed,
          respectively.  if bit 3 of the general purpose bit flag
          is set, these fields are set to zero in the local header
          and the correct values are put in the data descriptor and
          in the central directory.
      filename length: (2 bytes)
      extra field length: (2 bytes)
      file comment length: (2 bytes)
          the length of the filename, extra field, and comment
          fields respectively.  the combined length of any
          directory record and these three fields should not
          generally exceed 65,535 bytes.  if input came from standard
          input, the filename length is set to zero.
      disk number start: (2 bytes)
          the number of the disk on which this file begins.
      internal file attributes: (2 bytes)
          the lowest bit of this field indicates, if set, that
          the file is apparently an ascii or text file.  if not
          set, that the file apparently contains binary data.
          the remaining bits are unused in version 1.0.
      external file attributes: (4 bytes)
          the mapping of the external attributes is
          host-system dependent (see 'version made by').  for
          ms-dos, the low order byte is the ms-dos directory
          attribute byte.  if input came from standard input, this
          field is set to zero.
      relative offset of local header: (4 bytes)
          this is the offset from the start of the first disk on
          which this file appears, to where the local header should
          be found.
      filename: (variable)
          the name of the file, with optional relative path.
          the path stored should not contain a drive or
          device letter, or a leading slash.  all slashes
          should be forward slashes '/' as opposed to
          backwards slashes '\' for compatibility with amiga
          and unix file systems etc.  if input came from standard
          input, there is no filename field.
      extra field: (variable)
          this is for future expansion.  if additional information
          needs to be stored in the future, it should be stored
          here.  earlier versions of the software can then safely
          skip this file, and find the next file or header.  this
          field will be 0 length in version 1.0.
          in order to allow different programs and different types
          of information to be stored in the 'extra' field in .zip
          files, the following structure should be used for all
          programs storing data in this field:
          header1+data1 + header2+data2 . . .
          each header should consist of:
            header id - 2 bytes
            data size - 2 bytes
          note: all fields stored in intel low-byte/high-byte order.
          the header id field indicates the type of data that is in
          the following data block.
          header id's of 0 thru 31 are reserved for use by pkware.
          the remaining id's can be used by third party vendors for
          proprietary usage.
          the current header id mappings defined by pkware are:
          0x0007        av info
          0x0009        os/2
          0x000c        vax/vms
          0x000d        reserved for unix
          several third party mappings commonly used are:
          0x4b46        fwkcs md5 (see below)
          0x07c8        macintosh
          0x4341        acorn/sparkfs 
          0x4453        windows nt security descriptor (binary acl)
          0x4704        vm/cms
          0x470f        mvs
          0x4c41        os/2 access control list (text acl)
          0x4d49        info-zip vms (vax or alpha)
          0x5455        extended timestamp
          0x5855        info-zip unix (original, also os/2, nt, etc)
          0x6542        beos/bebox
          0x756e        asi unix
          0x7855        info-zip unix (new)
          0xfd4a        sms/qdos
          the data size field indicates the size of the following
          data block. programs can use this value to skip to the
          next header block, passing over any data blocks that are
          not of interest.
          note: as stated above, the size of the entire .zip file
                header, including the filename, comment, and extra
                field should not exceed 64k in size.
          in case two different programs should appropriate the same
          header id value, it is strongly recommended that each
          program place a unique signature of at least two bytes in
          size (and preferably 4 bytes or bigger) at the start of
          each data area.  every program should verify that its
          unique signature is present, in addition to the header id
          value being correct, before assuming that it is a block of
          known type.
         -os/2 extra field:
          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        short           tag for this "extra" block type
          tsize         short           size for the following data block
          bsize         long            uncompressed block size
          ctype         short           compression type
          eacrc         long            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[].
         -vax/vms extra field:
          the following is the layout of the vax/vms attributes "extra"
          block.  (last revision 12/17/91)
          note: all fields stored in intel low-byte/high-byte order.
          value         size            description
          -----         ----            -----------
  (vms)   0x000c        short           tag for this "extra" block type
          tsize         short           size of the total "extra" block
          crc           long            32-bit crc for remainder of the block
          tag1          short           vms attribute tag value #1
          size1         short           size of attribute #1, in bytes
          (var.)        size1           attribute #1 data
          .
          .
          .
          tagn          short           vms attribute tage value #n
          sizen         short           size of attribute #n, in bytes
          (var.)        sizen           attribute #n data
          rules:
          1. there will be one or more of attributes present, which will
             each be preceded by the above tagx & sizex values.  these
             values are identical to the atr$c_xxxx and atr$s_xxxx constants
             which are defined in atr.h under vms c.  neither of these values
             will ever be zero.
          2. no word alignment or padding is performed.
          3. a well-behaved pkzip/vms program should never produce more than
             one sub-block with the same tagx value.  also, there will never
             be more than one "extra" block of type 0x000c in a particular
             directory record.
          - fwkcs md5 extra field:
          the fwkcs contents_signature system, used in
          automatically identifying files independent of filename,
          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 zipfile central directory to add
          this extra field for a file, it also replaces the
          central directory entry for that file's uncompressed
          filelength with a measured value.
          fwkcs provides an option to strip this extra field, if
          present, from a zipfile central directory. in adding
          this extra field, fwkcs preserves zipfile authenticity
          verification; if stripping this extra field, fwkcs
          preserves all versions of av through pkzip version 2.04g.
          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.
      number of the disk with the start of the central directory: (2 bytes)
          the number of the disk on which the central
          directory starts.
      total number of entries in the central dir on this disk: (2 bytes)
          the number of central directory entries on this disk.
      total number of entries in the central dir: (2 bytes)
          the total number of files in the zipfile.
      size of the central directory: (4 bytes)
          the size (in bytes) of the entire central directory.
      offset of start of central directory with respect to
      the starting disk number:  (4 bytes)
          offset of the start of the central directory on the
          disk on which the central directory starts.
      zipfile comment length: (2 bytes)
          the length of the comment for this zipfile.
      zipfile comment: (variable)
          the comment for this zipfile.
  d.  general notes:
      1)  all fields unless otherwise noted are unsigned and stored
          in intel low-byte:high-byte, low-word:high-word order.
      2)  string fields are not null terminated, since the
          length is given explicitly.
      3)  local headers should not span disk boundaries.  also, even
          though the central directory can span disk boundaries, no
          single record in the central directory should be split
          across disks.
      4)  the entries in the central directory may not necessarily
          be in the same order that files appear in the zipfile.
unshrinking - method 1
----------------------
shrinking is a dynamic ziv-lempel-welch compression algorithm
with partial clearing.  the initial code size is 9 bits, and
the maximum code size is 13 bits.  shrinking differs from
conventional dynamic ziv-lempel-welch implementations in several
respects:
1)  the code size is controlled by the compressor, and is not
    automatically increased when codes larger than the current
    code size are created (but not necessarily used).  when
    the decompressor encounters the code sequence 256
    (decimal) followed by 1, it should increase the code size
    read from the input stream to the next bit size.  no
    blocking of the codes is performed, so the next code at
    the increased size should be read from the input stream
    immediately after where the previous code at the smaller
    bit size was read.  again, the decompressor should not
    increase the code size used until the sequence 256,1 is
    encountered.
2)  when the table becomes full, total clearing is not
    performed.  rather, when the compressor emits the code
    sequence 256,2 (decimal), the decompressor should clear
    all leaf nodes from the ziv-lempel tree, and continue to
    use the current code size.  the nodes that are cleared
    from the ziv-lempel tree are then re-used, with the lowest
    code value re-used first, and the highest code value
    re-used last.  the compressor can emit the sequence 256,2
    at any time.

expanding - methods 2-5
-----------------------
the reducing algorithm is actually a combination of two
distinct algorithms.  the first algorithm compresses repeated
byte sequences, and the second algorithm takes the compressed
stream from the first algorithm and applies a probabilistic
compression method.
the probabilistic compression stores an array of 'follower
sets' s(j), for j=0 to 255, corresponding to each possible
ascii character.  each set contains between 0 and 32
characters, to be denoted as s(j)[0],...,s(j)[m], where m<32.
the sets are stored at the beginning of the data area for a
reduced file, in reverse order, with s(255) first, and s(0)
last.
the sets are encoded as { n(j), s(j)[0],...,s(j)[n(j)-1] },
where n(j) is the size of set s(j).  n(j) can be 0, in which
case the follower set for s(j) is empty.  each n(j) value is
encoded in 6 bits, followed by n(j) eight bit character values
corresponding to s(j)[0] to s(j)[n(j)-1] respectively.  if
n(j) is 0, then no values for s(j) are stored, and the value
for n(j-1) immediately follows.
immediately after the follower sets, is the compressed data
stream.  the compressed data stream can be interpreted for the
probabilistic decompression as follows:
let last-character <- 0.
loop until done
    if the follower set s(last-character) is empty then
        read 8 bits from the input stream, and copy this
        value to the output stream.
    otherwise if the follower set s(last-character) is non-empty then
        read 1 bit from the input stream.
        if this bit is not zero then
            read 8 bits from the input stream, and copy this
            value to the output stream.
        otherwise if this bit is zero then
            read b(n(last-character)) bits from the input
            stream, and assign this value to i.
            copy the value of s(last-character)[i] to the
            output stream.
    assign the last value placed on the output stream to
    last-character.
end loop
b(n(j)) is defined as the minimal number of bits required to
encode the value n(j)-1.
the decompressed stream from above can then be expanded to
re-create the original file as follows:
let state <- 0.
loop until done
    read 8 bits from the input stream into c.
    case state of
        0:  if c is not equal to dle (144 decimal) then
                copy c to the output stream.
            otherwise if c is equal to dle then
                let state <- 1.
        1:  if c is non-zero then
                let v <- c.
                let len <- l(v)
                let state <- f(len).
            otherwise if c is zero then
                copy the value 144 (decimal) to the output stream.
                let state <- 0
        2:  let len <- len + c
            let state <- 3.
        3:  move backwards d(v,c) bytes in the output stream
            (if this position is before the start of the output
            stream, then assume that all the data before the
            start of the output stream is filled with zeros).
            copy len+3 bytes from this position to the output stream.
            let state <- 0.
    end case
end loop
the functions f,l, and d are dependent on the 'compression
factor', 1 through 4, and are defined as follows:
for compression factor 1:
    l(x) equals the lower 7 bits of x.
    f(x) equals 2 if x equals 127 otherwise f(x) equals 3.
    d(x,y) equals the (upper 1 bit of x) * 256 + y + 1.
for compression factor 2:
    l(x) equals the lower 6 bits of x.
    f(x) equals 2 if x equals 63 otherwise f(x) equals 3.
    d(x,y) equals the (upper 2 bits of x) * 256 + y + 1.
for compression factor 3:
    l(x) equals the lower 5 bits of x.
    f(x) equals 2 if x equals 31 otherwise f(x) equals 3.
    d(x,y) equals the (upper 3 bits of x) * 256 + y + 1.
for compression factor 4:
    l(x) equals the lower 4 bits of x.
    f(x) equals 2 if x equals 15 otherwise f(x) equals 3.
    d(x,y) equals the (upper 4 bits of x) * 256 + y + 1.
imploding - method 6
--------------------
the imploding algorithm is actually a combination of two distinct
algorithms.  the first algorithm compresses repeated byte
sequences using a sliding dictionary.  the second algorithm is
used to compress the encoding of the sliding dictionary output,
using multiple shannon-fano trees.
the imploding algorithm can use a 4k or 8k sliding dictionary
size. the dictionary size used can be determined by bit 1 in the
general purpose flag word; a 0 bit indicates a 4k dictionary
while a 1 bit indicates an 8k dictionary.
the shannon-fano trees are stored at the start of the compressed
file. the number of trees stored is defined by bit 2 in the
general purpose flag word; a 0 bit indicates two trees stored, a
1 bit indicates three trees are stored.  if 3 trees are stored,
the first shannon-fano tree represents the encoding of the
literal characters, the second tree represents the encoding of
the length information, the third represents the encoding of the
distance information.  when 2 shannon-fano trees are stored, the
length tree is stored first, followed by the distance tree.
the literal shannon-fano tree, if present is used to represent
the entire ascii character set, and contains 256 values.  this
tree is used to compress any data not compressed by the sliding
dictionary algorithm.  when this tree is present, the minimum
match length for the sliding dictionary is 3.  if this tree is
not present, the minimum match length is 2.
the length shannon-fano tree is used to compress the length part
of the (length,distance) pairs from the sliding dictionary
output.  the length tree contains 64 values, ranging from the
minimum match length, to 63 plus the minimum match length.
the distance shannon-fano tree is used to compress the distance
part of the (length,distance) pairs from the sliding dictionary
output. the distance tree contains 64 values, ranging from 0 to
63, representing the upper 6 bits of the distance value.  the
distance values themselves will be between 0 and the sliding
dictionary size, either 4k or 8k.
the shannon-fano trees themselves are stored in a compressed
format. the first byte of the tree data represents the number of
bytes of data representing the (compressed) shannon-fano tree
minus 1.  the remaining bytes represent the shannon-fano tree
data encoded as:
    high 4 bits: number of values at this bit length + 1. (1 - 16)
    low  4 bits: bit length needed to represent value + 1. (1 - 16)
the shannon-fano codes can be constructed from the bit lengths
using the following algorithm:
1)  sort the bit lengths in ascending order, while retaining the
    order of the original lengths stored in the file.
2)  generate the shannon-fano trees:
    code <- 0
    codeincrement <- 0
    lastbitlength <- 0
    i <- number of shannon-fano codes - 1   (either 255 or 63)
    loop while i >= 0
        code = code + codeincrement
        if bitlength(i) <> lastbitlength then
            lastbitlength=bitlength(i)
            codeincrement = 1 shifted left (16 - lastbitlength)
        shannoncode(i) = code
        i <- i - 1
    end loop
3)  reverse the order of all the bits in the above shannoncode()
    vector, so that the most significant bit becomes the least
    significant bit.  for example, the value 0x1234 (hex) would
    become 0x2c48 (hex).
4)  restore the order of shannon-fano codes as originally stored
    within the file.
example:
    this example will show the encoding of a shannon-fano tree
    of size 8.  notice that the actual shannon-fano trees used
    for imploding are either 64 or 256 entries in size.
example:   0x02, 0x42, 0x01, 0x13
    the first byte indicates 3 values in this table.  decoding the
    bytes:
            0x42 = 5 codes of 3 bits long
            0x01 = 1 code  of 2 bits long
            0x13 = 2 codes of 4 bits long
    this would generate the original bit length array of:
    (3, 3, 3, 3, 3, 2, 4, 4)
    there are 8 codes in this table for the values 0 thru 7.  using the
    algorithm to obtain the shannon-fano codes produces:
                                  reversed     order     original
val  sorted   constructed code      value     restored    length
---  ------   -----------------   --------    --------    ------
0:     2      1100000000000000        11       101          3
1:     3      1010000000000000       101       001          3
2:     3      1000000000000000       001       110          3
3:     3      0110000000000000       110       010          3
4:     3      0100000000000000       010       100          3
5:     3      0010000000000000       100        11          2
6:     4      0001000000000000      1000      1000          4
7:     4      0000000000000000      0000      0000          4
the values in the val, order restored and original length columns
now represent the shannon-fano encoding tree that can be used for
decoding the shannon-fano encoded data.  how to parse the
variable length shannon-fano values from the data stream is beyond the
scope of this document.  (see the references listed at the end of
this document for more information.)  however, traditional decoding
schemes used for huffman variable length decoding, such as the
greenlaw algorithm, can be successfully applied.
the compressed data stream begins immediately after the
compressed shannon-fano data.  the compressed data stream can be
interpreted as follows:
loop until done
    read 1 bit from input stream.
    if this bit is non-zero then       (encoded data is literal data)
        if literal shannon-fano tree is present
            read and decode character using literal shannon-fano tree.
        otherwise
            read 8 bits from input stream.
        copy character to the output stream.
    otherwise                   (encoded data is sliding dictionary match)
        if 8k dictionary size
            read 7 bits for offset distance (lower 7 bits of offset).
        otherwise
            read 6 bits for offset distance (lower 6 bits of offset).
        using the distance shannon-fano tree, read and decode the
          upper 6 bits of the distance value.
        using the length shannon-fano tree, read and decode
          the length value.
        length <- length + minimum match length
        if length = 63 + minimum match length
            read 8 bits from the input stream,
            add this value to length.
        move backwards distance+1 bytes in the output stream, and
        copy length characters from this position to the output
        stream.  (if this position is before the start of the output
        stream, then assume that all the data before the start of
        the output stream is filled with zeros).
end loop
tokenizing - method 7
--------------------
this method is not used by pkzip.
deflating - method 8
-----------------
the deflate algorithm is similar to the implode algorithm using
a sliding dictionary of up to 32k with secondary compression
from huffman/shannon-fano codes.
the compressed data is stored in blocks with a header describing
the block and the huffman codes used in the data block.  the header
format is as follows:
   bit 0: last block bit     this bit is set to 1 if this is the last
                             compressed block in the data.
   bits 1-2: block type
      00 (0) - block is stored - all stored data is byte aligned.
               skip bits until next byte, then next word = block length,
               followed by the ones compliment of the block length word.
               remaining data in block is the stored data.
      01 (1) - use fixed huffman codes for literal and distance codes.
               lit code    bits             dist code   bits
               ---------   ----             ---------   ----
                 0 - 143    8                 0 - 31      5
               144 - 255    9
               256 - 279    7
               280 - 287    8
               literal codes 286-287 and distance codes 30-31 are never
               used but participate in the huffman construction.
      10 (2) - dynamic huffman codes.  (see expanding huffman codes)
      11 (3) - reserved - flag a "error in compressed data" if seen.
expanding huffman codes
-----------------------
if the data block is stored with dynamic huffman codes, the huffman
codes are sent in the following compressed format:
   5 bits: # of literal codes sent - 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
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
in addition to the above mentioned contributors to pkzip and pkunzip,
i would like to extend special thanks to robert mahoney for suggesting
the extension .zip for this software.
references:
    fiala, edward r., and greene, daniel h., "data compression with
       finite windows",  communications of the acm, volume 32, number 4,
       april 1989, pages 490-505.
    held, gilbert, "data compression, techniques and applications,
                    hardware and software considerations",
       john wiley & sons, 1987.
    huffman, d.a., "a method for the construction of minimum-redundancy
       codes", proceedings of the ire, volume 40, number 9, september 1952,
       pages 1098-1101.
    nelson, mark, "lzw data compression", dr. dobbs journal, volume 14,
       number 10, october 1989, pages 29-37.
    nelson, mark, "the data compression book",  m&t books, 1991.
    storer, james a., "data compression, methods and theory",
       computer science press, 1988
    welch, terry, "a technique for high-performance data compression",
       ieee computer, volume 17, number 6, june 1984, pages 8-19.
    ziv, j. and lempel, a., "a universal algorithm for sequential data
       compression", communications of the acm, volume 30, number 6,
       june 1987, pages 520-540.
    ziv, j. and lempel, a., "compression of individual sequences via
       variable-rate coding", ieee transactions on information theory,
       volume 24, number 5, september 1978, pages 530-536.

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