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.

We support 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. 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.

Note that the compressed file format extension used to store AES-encrypted files is not supported by most other file compression utilities.

Limitations to be aware of

The AES encryption facility represents a significant advance on previous encryption methods, and it can help meet the need that many users have for preventing their confidential information from being viewed by unauthorized individuals. There are, however, some limitations that you should be aware of:

Encryption applies only to the contents of files stored within a compressed file. Information about an encrypted file, such as its name, date, size, attributes, and compression ratio, is stored in unencrypted form in the file's directory and can be viewed, without a password, by anyone who has access to the file.

The encryption method is not the same thing as an authentication method for the compressed file. Encryption is intended to prevent someone who doesn't know the correct password from finding out the contents of your encrypted data. The password is not needed for actions that do not involve decryption of the encrypted contents of data stored within a file. In particular, encrypted files can be deleted from a compressed file, or can be renamed within a compressed file, and new, unencrypted, files can be added to a compressed file, without a password.

We use password-based encryption, and even a strong encryption algorithm like AES is of little or no benefit if the passwords you use are weak, or you do not keep track of them in a secure manner.

We recommend that if you are going to be using the same password to encrypt very large numbers of files with the AES encryption (that is, files totaling in the millions, for example 2000 compressed files, each containing 1000 encrypted files) you use 256-bit AES keys rather than 128-bit AES-keys.

Notes on encryption safety

Encryption provides a measure of safety for your sensitive documents, but even encrypted documents can be compromised (regardless of what software they were encrypted by). 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), the uncompress program must extract the file to a temporary location so that the associated program can open it. If you subsequently close the uncompress program without first closing the program that is using the file, the uncompress program 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 the uncompress program 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.

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.

We do not encrypt compressed file comments or, as described above, information about encrypted files such as their names, dates, etc. Any user with access to the compressed 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, 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. We use 8-byte salt values with 128-bit AES encryption and 16-byte salt values with 256-bit encryption.

One purpose for the "salt" values used with AES encryption is to yield different encryption keys for each file, even if multiple files are encrypted with the same password. With the 8-byte salt values used with our 128-bit encryption it is likely that, if approximately 4 billion files are encrypted with the same password, two of the files will be encrypted with the same key. Someone who obtained copies of two files encrypted with the same key could learn information about their contents, so it is advisable to stay well below this limit. This is why we recommend that if you are going to be using the same password to encrypt very large numbers of files with AES encryption (that is, files totaling in the millions, for example 2000 compressed files, each containing 1000 encrypted files), you use 256-bit AES keys, which use 16-byte salt values, rather than 128-bit AES-keys, with their 8-byte salt values.

As part of the process outlined in RFC 2898 a pseudorandom function must be called; we use 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.

You should keep the following considerations in mind when choosing passwords for your files:

In general, longer passwords are more secure than shorter passwords. In fact, taking maximum advantage of the full strength of AES encryption requires a password of approximately 32 characters for 128-bit encryption and 64 characters for 256-bit encryption.

Passwords that contain a mixture of letters (upper and lower case), digits, and punctuation are more secure than passwords containing only letters.

Because you can use spaces and punctuation, you can create "pass phrases" that are long enough but still easy to remember and type.

Avoid using easily guessed passwords such as names, birthdays, Social Security numbers, addresses, telephone numbers, etc.

Be sure to keep a record of the passwords you use and to keep this record in a secure place. We have no way to access the contents of an encrypted file unless you supply the correct password. Before storing your only copies of critical information in encrypted form, you should carefully consider the risks associated with losing or forgetting the passwords involved.