Introduction

• To compress a file with encryption, set the password with the CZipArchive::SetPassword() method before compression. You will need to set the same password with this method before decompressing this file later. You can set different passwords for different files.
• To clear the password, call the CZipArchive::SetPassword() method without arguments or with an empty string as an argument.
• You should avoid using characters other than ASCII in the password string. There may be problems with extracting such an archive on a system with a different code page. If you need to, you can control the code page when settings a password with the CZipArchive::SetPassword() method.
• Choose the encryption method with the CZipArchive::SetEncryptionMethod() method when compressing. The compression method is set by default to CZipCryptograph::encStandard. The encryption method used to compress a file will be automatically detected while decompressing. You can set different encryption methods for different files.
• You will need both the password and the encryption method set for the file to be encrypted. Check the return value of the CZipArchive::WillEncryptNextFile() method to find out, if the next file you add to the archive will be encrypted.
• To find out, if a file inside an archive is encrypted, call the CZipFileHeader::IsEncrypted() method.
• You can also encrypt data when getting files from other archives. See Compressing Data for information how to get files from other archives.

Encrypting Existing Files in an Archive

To encrypt existing files in an archive, use one of the following methods:

• CZipArchive::EncryptFile()
• CZipArchive::EncryptFiles()
• CZipArchive::EncryptAllFiles()

• These methods will not affect files that are already encrypted.
• The files will not be encrypted, if the archive is segmented or if there is a file opened for compression or extraction.

Callbacks Called

When encrypting existing files, the following callbacks are called:

• CZipActionCallback::cbEncryptPrepare - called when preparing files for encryption.
• CZipActionCallback::cbMultiEncrypt - reports the global progress of encryption (both moving and encrypting data). Registering a callback object for this type of callback, automatically registers it for the CZipActionCallback::cbEncryptMoveData and
  CZipActionCallback::cbEncrypt callbacks as well.
• CZipActionCallback::cbEncryptMoveData - called before the actual encryption takes place to report the progress of making space inside an archive for encrypted data (which is larger than normal data). It counts towards the total progress.
• CZipActionCallback::cbEncrypt - called for every file being encrypted. It counts towards the total progress.

Additional Considerations

• Aborting the encryption process while any of the above callbacks is called will result most probably in a corrupted archive.
• To read more about using callback objects when performing multiple operations, see Progress Notifications: Using Callback Objects.

Standard Zip Encryption

The standard zip encryption method (CZipCryptograph::encStandard) is considered weak and should be only used with low security requirements or for preserving compatibility with applications that do not support the strong encryption.

Strong AES Encryption Conforming to the WinZip Format

The ZipArchive Library supports AES encryption with key sizes 128-bit, 192-bit and 256-bit. The format of AES encrypted data conforms to the WinZip AES Encryption Specification. It is the preferred way to ensure that your data is secure. The ZipArchive Library encrypts files the way the WinZip does:

• If the file size is smaller than 20 bytes, AE-2 encryption format is used. In this case, CRC value is not written in the archive to avoid discovery of the encrypted file contents in some cases.
• For larger files, AE-1 format is used to provide an extra integrity check.

Enabling AES Encryption in the ZipArchive Library

To use the AES encryption, you need to make sure that _ZIP_AES is defined in the _features.h file. Rebuild the ZipArchive Library and your application, if you modify this definition. You may choose to keep the encryption disabled, if you don't use it, to keep the size of the compiled library smaller.

Encrypting Archives with AES

You need to choose the encryption method with the CZipArchive::SetEncryptionMethod() using one of the CZipCryptograph::EncryptionMethod values.

Special Considerations

• If you use AES encryption in the ZipArchive Library in multiple threads, you should define _ZIP_USE_LOCKING in the _features.h file (it is not defined by default). This will ensure the proper locking mechanism when generating AES keys.
• If all the conditions below take place:
  • you are encrypting a file using AES,
  • you are using the CZipArchive::OpenNewFile() method to compress a file,
  • the archive is a segmented archive (see Segmented Archives: Splitting and Spanning),
  • you want the file to be successfully decompressed by WinZip 9.0,
  then you need to set CZipFileHeader::m_uLocalUncomprSize to the size of the data you want to compress (uncompressed data size) before passing the CZipFileHeader object to the CZipArchive::OpenNewFile() method. This will cause the uncompressed size to be written in the local header. Otherwise WinZip 9.0 will complain about the invalid decompressed size when extracting the file. It is not considered to be a valid behavior, because in this case the data descriptor is written and uncompressed data size written in the local header should be discarded. The WinZip 11.0 and newer does not expose this issue.

Checking Password Validity

The password itself is not stored inside of an archive. A preliminary check is performed when opening a file in an archive for extraction (e.g. with the CZipArchive::OpenFile() method). The ZipArchive Library throws CZipException::badPassword, if this preliminary check fails. However, this is not reliable enough to detect all cases of an invalid password.

The final check is performed when a file is completely extracted. A file's data checksum is then verified (but only if the consistency check CZipArchive::checkCRC was not disabled with the CZipArchive::SetIgnoredConsistencyChecks() method). If it fails, an exception with the code CZipException::badCrc is thrown. At this point it is not possible to distinguish whether the bad checksum was caused by an invalid password or a corrupted archive.

See Also API Links

• CZipArchive::WillEncryptNextFile()
• CZipArchive::SetPassword()
• CZipArchive::GetPassword()
• CZipArchive::SetEncryptionMethod()
• CZipArchive::GetEncryptionMethod()
• CZipFileHeader::IsEncrypted()
• CZipArchive::EncryptFile()
• CZipArchive::EncryptFiles()
• CZipArchive::EncryptAllFiles()