is AES key random?

Question

AES key may be generate by this code

KeyGenerator kgen = KeyGenerator.getInstance("AES");
kgen.init(128); 

but

If I have a "very reliable" method of generating random numbers can I use it in such a way

SecureRandom rnd = new SecureRandom();
byte[] key = new byte[16];
rnd.nextBytes(key);

is key obtained by this method reliable ?

or it ONLY must generated by some SPECIAL algorithm

Solution 1

The AES key can be any 128 bits. It should be be practically unguessable, whatever the method of creating it.

For Example:

SecureRandom sr = new SecureRandom()

key = new byte[16];
iv = new byte[16];

sr.nextBytes(key);
sr.nextBytes(iv);

Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
cipher.init(Cipher.ENCRYPT_MODE, new SecretKeySpec(key,"AES"), new IvParameterSpec(IV));

SecretKeySpec, by the way, is just a thin wrapper around a byte[] --- it does not transform the key in any way. No "special algorithm".

Solution 2

To add to the other answers ... I believe that the reason that the basic Random functions aren't secure are two reasons:

1. Slight statistical biases that are acceptable for non-security related situations, but narrow the distributions unacceptably for security applications.
2. They are seeded by the system DATETIME. Even knowing WHEN you generated your key - to a poor accuracy of +/- 6 months - would significantly reduce the brute force search space.

Solution 3

You can add a random algorithm using SecureRandom :

    KeyGenerator keyGen = KeyGenerator.getInstance("AES");
    SecureRandom random = new SecureRandom(); // cryptograph. secure random 
    keyGen.init(random); 
    SecretKey secretKey = keyGen.generateKey();

Solution 4

It sounds like you're trying to generate an AES key based on a password.

If this is the case, you can use javax.crypto.SecretKeyFactory's generateSecret method, passing in a javax.crypto.spec.PBEKeySpec as the parameter. The PBEKeySpec allows to to specify the password as an argument to its constructor.

View more solutions

Comments

[terentev]

Yes but i unlike PBEKeySpec :)

[terentev]

i dont know what PBEKeySpec do (in it) ? :)

[terentev]

this is I wanted to hear, I hope this is true :)

[maybeWeCouldStealAVan]

PBEKeySpecis an implementation of PBKDF2, a standard algorithm which uses many rounds of a hash algorithm and a salt to produce a binary key of a given length based on a passphrase. Essentially, it's a specialized secure pseudorandom generator using the passphrase and salt as a seed. If you're generating secure random numbers as your keys, then you don't need it. If you're using a human-memorizable passphrase to generate your key, PBKDF2 is a good choice to protect against dictionary attacks.

[Paŭlo Ebermann]

@user249654 Yes, it is. And don't use plain java.util.Random like in the question, use at least something like SecureRandom to avoid a fast brute-forcing of your key.

[maybeWeCouldStealAVan]

Oops, sloppy editing on my part. SecretKeyFactory can generate a secret key given the pashphrase, salt, number of rounds, and desired key size specified in a PBEKeySpec object. It does so using any of several standard algorithms. One, for example, is "PBKDF2WITHHMACSHA1", a specific form of the PBKDF2 algorithm.

[Maarten Bodewes]

Note that this answer was provided before the question was changed from Random to SecureRandom - which is cheating in my opinion.

[Maarten Bodewes]

@PaŭloEbermann Please also see this excellent question by Thomas. That it is secure does not necessarily mean that it is the best method of generating the secret key.