ZIP压缩文件数据结构解析二三

X. Enhanced Deflating - Method 9
--------------------------------
The Enhanced Deflating algorithm is similar to Deflate but
uses a sliding dictionary of up to 64K. Deflate64(tm) is supported
by the Deflate extractor.
XI. BZIP2 - Method 12
---------------------
BZIP2 is an open-source data compression algorithm developed by
Julian Seward.  Information and source code for this algorithm
can be found on the internet.
XII. Traditional PKWARE Encryption
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The following information discusses the decryption steps
required to support traditional PKWARE encryption.  This
form of encryption is considered weak by today's standards
and its use is recommended only for situations with
low security needs or for compatibility with older .ZIP
applications.
XIII. 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).