About Encryption

About encryption and encryption methods

Encryption gives you a way to protect sensitive documents contained in your archives. The contents of the files that you want to protect are encrypted by WinZip® based on a password that you specify. In order for WinZip to later extract the original contents of the encrypted files, the correct password must again be supplied.

WinZip provides two encryption methods for Zip files:

AES encryption: AES is the Advanced Encryption Standard, which is the result of a three-year competition sponsored by the U.S. Government's National Institute of Standards (NIST). This encryption method, also known as Rijndael, has been adopted by NIST as a Federal Information Processing Standard.
WinZip supports AES encryption in two different strengths: 128-bit AES and 256-bit AES. These numbers refer to the size of the encryption keys that are used to encrypt the data. 256-bit AES is stronger than 128-bit AES, but both of them can provide significantly greater security than the standard Zip 2.0 method described below. An advantage of 128-bit AES is that it is slightly faster than 256-bit AES, that is, it takes less time to encrypt or decrypt a file.
The security of your data depends not only on the strength of the encryption method but also on the strength of your password, including factors such as length and composition of the password, and the measures you take to ensure that your password is not disclosed to unauthorized third parties. Please read about encryption passwords.
Note that the Zip file format extension used by WinZip to store AES-encrypted files is not supported by earlier versions of WinZip and is not yet supported by most other Zip file utilities. Because the technical specification for WinZip's AES format extension is available on the WinZip web site, we anticipate that other Zip file utilities will add support for this format extension.
Standard Zip 2.0 encryption: this older encryption technique provides a measure of protection against casual users who do not have the password and are trying to determine the contents of the files. However, the Zip 2.0 encryption format is known to be relatively weak, and cannot be expected to provide protection from individuals with access to specialized password recovery tools.
You should not rely on Zip 2.0 encryption to provide strong security for your data. If you have important security requirements for your data, you should use WinZip's AES encryption, described above.
The only advantage of Zip 2.0 encryption over the more secure AES encryption is that it is supported by most Zip file utilities, including earlier versions of WinZip. Files that you encrypt using this technique can be extracted by anyone who knows the correct password and has access to almost any Zip file utility. Additionally, Zip 2.0 encryption is supported by WinZip Self-Extractor 2.2 and by WinZip Self-Extractor Personal Edition (included in WinZip 9.0); the AES encryption method described above is not supported by either self-extractor program.

Some points to be aware of:

Encryption applies only to the contents of files stored within a Zip file. Information about an encrypted file, such as its name, date, size, attributes, and compression ratio, is stored in unencrypted form in the Zip file's directory and can be viewed, without a password, by anyone who has access to the Zip file.
WinZip requires that the correct password be specified before the contents of an encrypted file within a Zip file can be viewed or accessed in unencrypted form. The password is not required, however, for actions that do not require access to the unencrypted contents of the file. In particular, encrypted files can be deleted from a Zip file, or can be replaced within the Zip file, without a password.
WinZip's encryption support is only available for Zip files and not for the other archive formats supported by WinZip. (Encryption can also be used with ARJ files, but this encryption is handled by the external ARJ program, not by WinZip.)

Please read about using encryption.

Notes on encryption safety

Encryption provides a measure of safety for your sensitive documents, but even encrypted documents can be compromised (regardless of whether they were encrypted by WinZip or by other encryption software). Here are some of the ways this can occur. This is by no means an exhaustive list of potential risks; it is intended only to give you an idea of some of the safety issues involved with sensitive documents.

If a keystroke monitor or other malicious code (such as a virus) is running on your computer, your password may be recorded when you type it. Be sure to check frequently for viruses and follow other recommended computer safety procedures.
If you extract an encrypted file and then delete the file, it may be possible for someone to later "undelete" the file using file recovery software or the Recycle Bin.
When you open or view a file from an archive (e.g., by double clicking it), WinZip must extract the file to a temporary location so that the associated program can open it. If you subsequently close WinZip without first closing the program that is using the file, WinZip may not be able to delete the temporary copy of the file, thereby leaving it on disk in unencrypted form. The associated program may also make one or more backup copies of the decrypted file, and WinZip will not be able to delete these. In addition, as described above, it may be possible for someone to later recover deleted files using file recovery software or the Recycle Bin.
When you "move" files to a Zip file by choosing the Move action in the Add dialog, WinZip moves the files into the Zip file by compressing them and then deleting the original files from the disk. It may be possible to recover the original, unencrypted files from the disk.
After adding or extracting encrypted files, some or all of the unencrypted file contents may remain in your computer's memory or the page swap files on disk. A malicious user may be able to retrieve this unencrypted information.
WinZip does not encrypt Zip file comments or, as described above, information about encrypted files such as their names, dates, etc. Any user with access to the Zip file can view this information without a password.

You may be able to eliminate some of these exposures using specialized software such as virus scanners, disk erasers, etc.

Technical information on AES key generation

When you use AES encryption with WinZip, the passwords that you enter are converted into keys of the appropriate length (128 bits or 256 bits, depending on the AES key length that you specify). This is done through the PBKDF2 algorithm defined in RFC 2898 (also available as Public Key Cryptography Standard #5) with an iteration count of 1000. 8-byte salt values are used with 128-bit AES encryption and 16-byte salt values are used with 256-bit encryption.

As part of the process outlined in RFC 2898 a pseudorandom function must be called; WinZip uses the HMAC-SHA-1 function for this purpose, since it is a well-respected algorithm that has been in wide use for this purpose for several years. The PBKDF2 function repeatedly calls HMAC-SHA-1, which produces a 160-bit hash value as a result, mixing the outputs in a fairly complicated way, eventually yielding a 128- or 256-bit encryption key as a result.

Note that, if you are using 256-bit AES encryption, the fact that HMAC-SHA-1 produces a 160-bit result means that regardless of the password that you specify, the search space for the encryption key is unlikely to reach the theoretical 256-bit maximum, and cannot be guaranteed to exceed 160 bits. This is discussed in section B.1.1 of the RFC 2898 document.

Information for software developers

Zip file utility developers who wish to provide WinZip-compatible AES encryption support in their own products can find complete technical information at http://www.winzip.com/aes_info.htm.