Example: Converting a password into a cryptographic key

The following code snippets are actually compiled and run during the BouncyGPG build process. This ensures that all examples are correct.

Usage with Bouncy-GPG

Cryptographic key derivation functions should be used to derive a key from a password (or any other source material). Bouncy-GPG uses SCrypt for key stretching.

The following snippet will derive a 256 bit key from a strong password. The derivation process is configured by SCryptKeyStretchingParameters.forStrongInputKeyMaterial(). This will give a very quick key derivation and is only secure because the password is very long and random. If a shorter password was used the recommendation would be to use forModeratelyStongInputKeyMaterial or forWeakInputKeyMaterial.

import java.nio.charset.StandardCharsets;

import org.bouncycastle.util.encoders.Hex;

import name.neuhalfen.projects.crypto.symmetric.keygeneration.impl.stretching.KeyStretching;
import name.neuhalfen.projects.crypto.symmetric.keygeneration.impl.stretching.SCryptKeyStretching;

// the password
// Rather obviously the password should NOT be stored as part of the source code
String password = "39akldsAFQqwmsx-+#aQcsvcu82cQd,.-ask72=cq8csf";

// The salt value should be application or installation specific.
// It is an absolute MUST that the salt is never changed! A different salt WILL result in a different
// key being generated!
// The salt value SHOULD be roughly as long as the key (e.g. 128/256 bit).
// The salt value SHOULD be random (so this salt value is actually a bad example)
final byte[] SALT = Hex.decode("000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f");

// In this case we are using a 256 bit key, e.g. for AES-256.
final int DESIRED_KEY_LENGTH = 256;

// Use forSensitiveStorage to derive e.g. a masterkey at application startup.
// forSensitiveStorage is really slow. If you are very sure that you have a very
// good (22 characters random or longer) password for 128 bit keys / 44 character 
// passwords for 256 bit keys you can consider using forQuickDerivation.
final SCryptKeyStretching.SCryptKeyStretchingParameters stretchingParameters = SCryptKeyStretching.SCryptKeyStretchingParameters.forStrongInputKeyMaterial();

final KeyStretching streching = new SCryptKeyStretching(stretchingParameters);

byte[] key = streching.strengthenKey(SALT, password.getBytes(StandardCharsets.UTF_8), DESIRED_KEY_LENGTH);
return key;

The key generated by this sequence is 0x89d4a9b23d8a88d09e559aeeaa7cfa4a029673b01d969a2f523a7c1f749c330b.