# Master Key

Details about components of a master key.

- Master Password
- Key File
- Windows User Account
- For Administrators: Specifying Minimum Properties of Master Keys

Your KeePass database file is encrypted using a master key. This master key can consist of multiple components: a master password, a key file and/or a key that is protected using the current Windows user account.

For opening a database file, all components of the master key are required.

If you forget/lose any of the master key components (or forget the composition), all data stored in the database is lost. There is no backdoor and no universal key that can open your database.

## Master Password

If you use a master password, you only have to remember one password or passphrase (which should be good!) to open your database.

KeePass features a protection against brute-force and dictionary attacks; see the security help page for details.

## Key File

A key file is a file that contains a key (and possibly additional data, e.g. a hash that allows to verify the integrity of the key). The file extension typically is 'keyx' or 'key'.

A key file must not be modified, otherwise you cannot open your database anymore. If you want to use a different key file, open the dialog for changing the master key (via 'File' → 'Change Master Key') and create/select the new key file.

Two-factor protection. A key file is something that you must have in order to be able to open the database (in contrast to a master password, which you must know). If you use both a key file and a master password, you have a two-factor protection: possession and knowledge.

Location. As mentioned above, the idea of a key file is that you have something. If an attacker obtains both your database file and your key file, then the key file provides no protection. Therefore, the two files must be stored in different locations. For example, you could store the key file on a separate USB stick.

Hiding the location. The key file content must be kept secret, not its location (file path/name). Trying to hide the key file (e.g. by storing it among a thousand other files, in the hope that an attacker does not know which file is the correct one) typically does not increase the security, because it is easy to find out the correct file (e.g. by inspecting the last access times of files, lists of recently used files of the operating system, file system auditing logs, anti-virus software logs, etc.).

KeePass has an option for remembering the paths of key files, which is turned on by default; turning it off typically just decreases the usability without increasing the security. This option only affects KeePass itself (i.e. turning it off does not prevent the operating system or other software from remembering the paths). If you only want to prevent a key file from appearing in the recently used files list of Windows (which does not really affect the security) after selecting it in KeePass, consider turning on the option for entering the master key on a secure desktop (KeePass will then show a simpler key file selection dialog that does not add the file to the recently used files list of Windows).

Backup. You should create a backup of your key file (onto an independent data storage device). If your key file is an XML file (which is the default), you can also create a backup on paper (KeePass 2.x provides a command for printing a key file backup in the menu 'File' → 'Print'). In any case, the backup should be stored in a secure location, where only you and possibly a few other people that you trust have access to. More details about backing up a key file can be found in the ABP FAQ.

Formats. KeePass supports the following key file formats:

- XML (recommended, default). There is an XML format for key files. KeePass 2.x uses this format by default, i.e. when creating a key file in the master key dialog, an XML key file is created. The syntax and the semantics of the XML format allow to detect certain corruptions (especially such caused by faulty hardware or transfer problems), and a hash (in XML key files version 2.0 or higher) allows to verify the integrity of the key. This format is resistant to most encoding and new-line character changes (which is useful for instance when the user is opening and saving the key file or when transferring it from/to a server). Such a key file can be printed (as a backup on paper), and comments can be added in the file (with the usual XML syntax: `<!-- ... -->`). It is the most flexible format; new features can be added easily in the future.
- 32 bytes. If the key file contains exactly 32 bytes, these are used as a 256-bit cryptographic key. This format requires the least disk space.
- Hexadecimal. If the key file contains exactly 64 hexadecimal characters (0-9 and A-F, in UTF-8/ASCII encoding, one line, no spaces), these are decoded to a 256-bit cryptographic key.
- Hashed. If a key file does not match any of the formats above, its content is hashed using a cryptographic hash function in order to build a key (typically a 256-bit key with SHA-256). This allows to use arbitrary files as key files.

Reuse. You can use one key file for multiple database files. This can be convenient, but please keep in mind that when an attacker obtains your key file, you have to change the master keys of all database files protected with this key file.

In order to reuse an existing key file, click on the 'Browse' button in the master key creation dialog.

## Windows User Account

KeePass can make the database dependent on the current Windows user account. If you enable this option, you can only open the database when you are logged in as the same Windows user when creating the database.
Be very careful with using this option. If your Windows user account gets deleted, you won't be able to open your KeePass database anymore. Also, when using this option at home and your computer breaks (hard disk damaged), it is not enough to just create a new Windows account on the new installation with the same name and password; you need to copy the complete account (i.e. SID, ...). This is not a simple task, so if you don't know how to do this, it is highly recommended that you don't enable this option. Detailed instructions how to recover a Windows user account can be found here: 'Recover Windows User Account Credentials' (a short technical tutorial can be found in a Microsoft TechNet article: 'How to recover a Vault corrupted by lost DPAPI keys').
You can change the password of the Windows user account freely; this does not affect the KeePass database. Note that changing the password (e.g. a user using the Control Panel or pressing ``Ctrl`+`Alt`+`Delete`` and selecting 'Change Password') and resetting it to a new one (e.g. an administrator using a `NET USER <User> <NewPassword>` command) are two different things. After changing your password, you can still open your KeePass database. When resetting the password to a new one, access usually is not possible anymore (because the user's DPAPI keys are lost), but there are exceptions (for example when the user is in a domain, Windows can retrieve the user's DPAPI keys from a domain controller, or a home user can use a previously created Password Reset Disk). Details can be found in the MSDN article 'Windows Data Protection' and in the support article 'How to troubleshoot the Data Protection API (DPAPI)'.
If you decide to use this option, it is highly recommended not to rely on it exclusively, but to additionally use one of the other two options (password or key file).
Instead of backing up the Windows user account, you can alternatively create an unencrypted backup of the key using the 'Windows User Account Backup and Restore Utility'. As such a backup is not encrypted, it must be stored in a secure location.
Protection using user accounts is unsupported on Windows 98 / ME.

## For Administrators: Specifying Minimum Properties of Master Keys

Administrators can specify a minimum length and/or the minimum estimated quality that master passwords must have in order to be accepted. You can tell KeePass to check these two minimum requirements by adding/editing appropriate definitions in the INI/XML configuration file.

The value of the `Security/MasterPassword/MinimumLength` node specifies the minimum master password length (in characters). For example, by setting it to `10`, KeePass will only accept master passwords that consist of at least 10 characters.
The value of the `Security/MasterPassword/MinimumQuality` node specifies the minimum estimated quality (in bits) that master passwords must have. For example, by setting it to `80`, only master passwords with an estimated quality of at least 80 bits will be accepted.
The `Security/MasterKeyExpiryRec` and `Security/MasterKeyExpiryForce` nodes can be set to an XSD date or an XSD duration (see XSD Date and Time Data Types). If the master key has not been changed since the specified date or if the time span between now and the last master key change exceeds the specified duration, KeePass recommends/forces to change it. These settings apply to all databases that are opened with this KeePass instance; a master key expiry can also be configured for each database individually (in 'File' → 'Database Settings' → tab 'Advanced').
By specifying `KeyCreationFlags` and/or `KeyPromptFlags` (in the `UI` node), you can force states (enabled, disabled, checked, unchecked) of key source controls in the master key creation and prompt dialogs. These values can be bitwise combinations of one or more of the following flags:

Flag (Hex) | Flag (Dec) | Description
--- | --- | ---
0x0 | 0 | Don't force any states (default).
0x1 | 1 | Enable password.
0x2 | 2 | Enable key file.
0x4 | 4 | Enable user account.
0x8 | 8 | Enable 'hide password' button.
0x100 | 256 | Disable password.
0x200 | 512 | Disable key file.
0x400 | 1024 | Disable user account.
0x800 | 2048 | Disable 'hide password' button.
0x10000 | 65536 | Check password.
0x20000 | 131072 | Check key file.
0x40000 | 262144 | Check user account.
0x80000 | 524288 | Check 'hide password' option/button.
0x1000000 | 16777216 | Uncheck password.
0x2000000 | 33554432 | Uncheck key file.
0x4000000 | 67108864 | Uncheck user account.
0x8000000 | 134217728 | Uncheck 'hide password' option/button.

The values of `KeyCreationFlags` and `KeyPromptFlags` must be specified in decimal notation.
For example, if you want to enforce using the user account option, you could check and disable the control (such that the user can't uncheck it anymore) by specifying 263168 as value (0x40000 + 0x400 = 0x40400 = 263168).