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+Revised: 03/01/1999
+
+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
+ 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.)
+
+ 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: Currently unused.
+
+ 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 - 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.
+
+ 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.
+
+ 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
+ 0x000a NTFS
+ 0x000c VAX/VMS
+ 0x000d Unix
+ 0x000f Patch Descriptor
+
+ 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 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[].
+
+ -UNIX Extra Field:
+
+ 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.
+
+ -VAX/VMS Extra Field:
+
+ The following is the layout of the VAX/VMS 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 VMS attribute tag value #1
+ Size1 2 bytes Size of attribute #1, in bytes
+ (var.) Size1 Attribute #1 data
+ .
+ .
+ .
+ TagN 2 bytes VMS 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
+ 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.
+
+ -NTFS Extra Field:
+
+ The following is the layout of the NTFS attributes
+ "extra" block.
+
+ 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 tage 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
+
+ -PATCH Descriptor Extra Field:
+
+ 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
+
+ - 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.