private ZipCrypto SetupCipher(string password)
{
ZipCrypto cipher = null;
// decrypt the file header data here if necessary.
if (Encryption == EncryptionAlgorithm.PkzipWeak)
{
if (password == null)
throw new BadPasswordException("This entry requires a password.");
cipher = new ZipCrypto();
cipher.InitCipher(password);
// Decrypt the header. This has a side effect of "further initializing the
// encryption keys" in the traditional zip encryption.
byte[] DecryptedHeader = cipher.DecryptMessage(_WeakEncryptionHeader, _WeakEncryptionHeader.Length);
// CRC check
// According to the pkzip spec, the final byte in the decrypted header
// is the highest-order byte in the CRC. We check it here.
if (DecryptedHeader[11] != (byte)((_Crc32 >> 24) & 0xff))
{
// In the case that bit 3 of the general purpose bit flag is set to indicate
// the presence of an 'Extended File Header', the last byte of the decrypted
// header is sometimes compared with the high-order byte of the lastmodified
// time, and not the CRC, to verify the password.
//
// This is not documented in the PKWare Appnote.txt.
// This was discovered this by analysis of the Crypt.c source file in the InfoZip library
// http://www.info-zip.org/pub/infozip/
if ((_BitField & 0x0008) != 0x0008)
{
throw new BadPasswordException("The password did not match.");
}
else if (DecryptedHeader[11] != (byte)((_TimeBlob >> 8) & 0xff))
{
throw new BadPasswordException("The password did not match.");
}
}
// We have a good password.
}
return cipher;
}
