Locked

Security


Detailed information about the security of KeePass.


Key  Database Encryption

KeePass database files are encrypted. KeePass encrypts the whole database, i.e. not only your passwords. Your user names, notes, even the entry times and UUIDs, etc. are encrypted, too.

The databases are encrypted using one of the following block ciphers:

Cipher Block Size Key Size
Advanced Encryption Standard (AES / Rijndael) 128 bits 256 bits
Twofish 128 bits 256 bits

These algorithms are well-known, analyzed thoroughly and generally considered to be very secure by the cryptographic community (see [1] for comments by the NIST on AES for example).

The block ciphers are used in the CBC (cipher-block chaining) block cipher mode. In CBC mode, plaintext patterns are concealed.

For both algorithms, a 128-bit initialization vector (IV) is generated randomly each time you save the database. This allows multiple databases to be saved using the same key without observable patterns being revealed.


Key  Hashing and Key Derivation

In order to generate the 256-bit key for the block ciphers, the Secure Hash Algorithm SHA-256 is used. This algorithm compresses the user key provided by the user (consisting of password and/or key file) to a fixed-size key of 256 bits. This transformation is one-way, i.e. it is computationally infeasible to invert the hash or find a second message that compresses to the same hash.

Please note that the recently discovered attack against SHA-1 [2] doesn't affect the security of SHA-256. SHA-256 is still considered as being secure [3].

Key Derivation:

If only a password is used (i.e. no key file), the password plus a 128-bit random salt are hashed using SHA-256 to form the final key (but note there is some preprocessing: Protection against Dictionary Attacks). This random salt prevents attacks that are based on pre-computed hashes.

When using both password and key file, the final key is derived as follows: SHA-256(SHA-256(password), key file contents), i.e. the hash of the master password is concatenated with the key file bytes and the resulting byte string is hashed with SHA-256 again. If the key file doesn't contain exactly 32 bytes (256 bits), they are hashed with SHA-256, too, to form a 256-bit key. The formula above then changes to: SHA-256(SHA-256(password), SHA-256(key file contents)).


Binary  Random Number Generation

We need to generate several random bytes (for the IV, the master key salt, etc.). For this, several pseudo-random sources are used: current tick count, performance counter, system date/time, mouse cursor position, memory status (free virtual memory, etc.), active window, clipboard owner, various process and thread IDs, various window focus handles (active window, desktop, ...), window message stack, process heap status, process startup information and several system information structures. Additionally, KeePass uses random bytes provided by the system's default CSP RNG.

This pseudo-random data is collected in a random pool. To generate 16 random bytes, the pool is hashed (SHA-256) with a counter to form the final 16 random bytes. The counter is increased after 16 generated bytes. This way, we can efficiently produce as many secure random bytes as we need.


Key  Protection against Dictionary Attacks

KeePass supports protection against guessing and dictionary attacks.

You can't really prevent dictionary and guessing attacks: nothing prevents an attacker to just try all possible keys and look if the database decrypts. But what we can do (and KeePass does) is to make it harder: by adding a constant time factor to the key initialization, we can make them as hard as we want.

To generate the final 256-bit key that is used for the block cipher, KeePass first hashes the user's password using SHA-256, encrypts the result N times using the Advanced Encryption Standard (AES) algorithm (called key transformation rounds from on now), and then hashes it again using SHA-256. For AES, a random 256-bit key is used, which is stored in the database file. As the AES transformations aren't pre-computable (key is random), an attacker has to perform all the encryptions, too, otherwise he cannot try and see if the current key is correct.

An attacker now needs much more time to try a key. If he can only try a few hundred keys per second, a dictionary attack is not practical any more.

By default, KeePass sets N to 6000 encryption rounds (full encryptions are meant; N has nothing to do with the internal encryption rounds of AES). This has been done in order to provide compatibility with the PocketPC version (PocketPC processors are slower, therefore the key computation takes longer).

If you are using KeePass on PC only, it is highly recommended to increase the number of key transformation rounds. You can change the number in the database options dialog. Right of the field for the rounds, you'll find a button. When clicking this button, KeePass computes the rounds number that leads to a 1-second delay. Waiting 1 second at database opening isn't a problem, but for an attacker of course it is. But, the number can be freely set to a number of your choice. The button only should give you a rough idea how many rounds can be computed in 1 second on your computer.

This protection feature is only useful when using master passwords; key files are random anyway, there's no need to transform the key file contents (guessing the key file contents is equally hard to a brute-force attack on the final key).


Application Protection  In-Memory Passwords Protection

While KeePass is running, your passwords are stored encrypted in process memory. For this, the ARC4 encryption algorithm is used, using a random, 12 bytes long key.

This means that even if you would dump the whole KeePass process memory to disk, you couldn't find the passwords (at least not in plain text). Note that this only applies to the password field, not to the user names, etc. because of performance reasons.

When you are copying a password to the clipboard for example, KeePass first decrypts the password field, copies it to the clipboard and immediately re-encrypts it using the random key.

Additionally, KeePass erases all security-critical memory when it's not needed any more, i.e. it overwrites these memory areas before releasing them (this applies to all security-critical memory, not only the passwords field).


Key  Locking the Workspace

Locking the workspace closes the database file, and remembers the last view settings (i.e. which group and entries you selected, list position, etc.).

This provides maximum security: unlocking the workspace is as hard as opening the database file the normal way. Also, it prevents data-loss (your computer can crash while KeePass is locked, without doing any damage to the database).


Plugins  Plugin Security

A separate page exist about the security of plugins: Plugin Security.


Black Box  Self-Tests

Each time you start KeePass, the program will perform a quick self-test to see whether the block ciphers and the hash are compiled correctly and pass their test-vectors. If one of the algorithms doesn't pass its test vectors, KeePass will show a security exception message box at startup.


References  References and Further Reading

[1] National Institute of Standards and Technology: Report on the Development of the Advanced Encryption Standard (AES) (PDF).

[2] Bruce Schneier's blog: SHA-1 broken.

[3] Bruce Schneier's blog: Cryptanalysis of SHA-1, with comments about the impact of that discovery and what to do now.