I really don't know enough about your use case to advise from a security perspective. However, to directly solve the issue of generating a reproducible key from user-supplied input, you can make use of password-based key derivation, in which we treat the user supplied input as a password (edited to be a full example):
import java.security.NoSuchAlgorithmException;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.KeySpec;
import javax.crypto.Cipher;
import javax.crypto.SecretKey;
import javax.crypto.SecretKeyFactory;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.PBEKeySpec;
import javax.crypto.spec.SecretKeySpec;
import sun.security.provider.SecureRandom;
public class DerivationExample {
private static SecretKey makeKeyFromUserInput(String userInput, byte[] salt)
throws NoSuchAlgorithmException, InvalidKeySpecException {
SecretKeyFactory factory = SecretKeyFactory
.getInstance("PBKDF2WithHmacSHA1");
KeySpec keySpec = new PBEKeySpec(userInput.toCharArray(), salt, 1024, 256);
byte[] derivedKey = factory.generateSecret(keySpec).getEncoded();
return new SecretKeySpec(derivedKey, "AES");
}
public static void main(String[] args) throws Exception {
String userInput = "foo";
// PBKDF2 standard recommends at least 64-bit salt
// Note: you want to randomly generate this elsewhere and keep it constant
byte[] salt = new byte[8];
new SecureRandom().engineNextBytes(salt);
SecretKey derivedKey = makeKeyFromUserInput(userInput, salt);
Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
cipher.init(Cipher.ENCRYPT_MODE, derivedKey, new IvParameterSpec(
new byte[16]));
String plaintext = "Hello, World!";
byte[] cipherText = cipher.doFinal(plaintext.getBytes());
// Derive key again to demonstrate it is the same
SecretKey derivedKey2 = makeKeyFromUserInput(userInput, salt);
cipher.init(Cipher.DECRYPT_MODE, derivedKey2, new IvParameterSpec(
new byte[16]));
byte[] plainText = cipher.doFinal(cipherText);
// Prints "Hello, World!"
System.out.println(new String(plainText));
}
}
Provided the salt is kept constant, the resulting key will be reproducible. Share the salt between any other devices needing to produce the same key.
Note: You will need to install the unrestricted policy files (see bottom of this page) for this example to work.
Please remember that a security system composed of "secure" bits and pieces is not guaranteed to be secure when viewed as a whole. We've only seen a snippet of your requirements and as such our advice should be taken with a pinch of salt (no pun intended). For a more complete answer, we'd need to understand the end-to-end process you are trying to secure.
However, as StackOverflow is not the place to design a security system, you may need to seek this help elsewhere.
