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Ever since I've seen the option to use a key file I was wondering what are good criteria to select or create a key file that will be used for a Truecrypt container?

Can I simply learn the preamble of the UNO carta by heart type it into a text file and have my key file?

Are there any rules to create or select a key file that will be used in encryption?

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Lets start from the beginning, how encryption works.

AES needs a key of a specific length, for AES256 that 256bits. Because your password or file will not be exactly 256bits long, Truecrypt uses a hashing algorithm (eg RIPEMD-160) to generate the correct length.

For example, a header key for the AES-256 cipher is always 256 bits long even if HMAC-RIPEMD-160 is used

Using only a password has the disadvantage that the pool of characters is somewhat limited to the symbols on your keyboard. Using a keyfile, especially one with a high level of randomness will be more secure, just because it has more possible combinations. The disadvantage, of course, is that someone might get ahold of this file.

If the keyfile is somehow destroyed it will be nearly impossible to recreate it, unless it is a file made up of know content (eg the UN-Charta), still you have to ensure that the content is exactly the same, especially for files with meta data (eg creation date, owner name) this will be a hard task. But a plain text file will just be as limited to its number of symbols as your keyboard is, and thus is a very bad idea to start with.

Forgetting a password is as bad as this, writing it down is as bad as having a keyfile on something like an USB-stick. You should always make a backup of your keyfile in case of data-corruption or if your drive dies.

Basically it comes down what to you prefer and what level of security you want to archieve. A very long password that no one can force out of your brain or a keyfile for which you can ensure no one has access to. Next possibility is to combine password and keyfile, this way you might use a shorter password. But for this solution you still have to ensure that a third person cannot aquire the keyfile (eg by storing it inside a container or drive encrypted by a long password).

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See the official documentation on using key files: http://www.truecrypt.org/docs/?s=keyfiles

It seems they recommend using a compressed file, like an MP3 or a JPG. You could also have TrueCrypt generate a random file for you. Using a plain text file with well-known contents seems the equivalent of using "password" or "qwerty" for your password.

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  • So instead of memorizing my extra complex password I will need to make sure that my MP3 file is never changed? A player that stores the play count in the MP3 would change the file after every play. Im just trying to get my head around the key file feature
    – seb
    Apr 20, 2012 at 23:48
  • @seb Yes, you'd need to make sure the file never changes. For MP3 files, you'll also want to make sure that your media player doesn't change the meta data like artist info, album art etc. You could try read-protecting the file, or using a file that's outside your media library altogether.
    – Indrek
    Apr 21, 2012 at 0:24
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A zip file should naturally have pretty good entropy due to the compression process. You can also go to random.org to generate random bitmaps or sound files. Oh yeah and back up your files somewhere.

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