encrypt files with AES , with shared password

Question

I want to encrypt/ decrypt with AES, with shared passwod, my code same as here.

the linked code works fine, but there is no shared passworn in it.

how can I add a shared password to the following implementation?

I need something like

String shared="xxx..";//some password with 16 digits length

Is it possible?

and adding this shared password to the encryption.

Answer 1

It is very important that the key used for AES encryption is not easy to guess so in a lot of implementations the keys are generated randomly. The key itself is a byte array of 16 (128 bit), 24 (192 bit) or 32 (256 bit) byte length and a byte array is not usuable as source for a shared password.

The solution is to encode the byte array into a Base64-encoded string and pass this string to the recepient on a secure way. The recepient decodes the string back to a byte array and further via the SecretKeySpec to a secret key.

The small example shows the way to securly generate a random password with different lengths (the example uses only the 128 bit keylength, encode it and decode it back to a secret key - the orginal SecretKey k is compared to the regenerated SecretKex kReceived.

Just a last notice but it is a security warning: Your encryption method is using the AES ECB mode that is unsecure - please do not use this mode in production (mode is defined here: AES/ECB/PKCS5Padding).

Result:

https://stackoverflow.com/questions/62782129/encrypt-files-with-aes-with-shared-password
sharedKey: UT7PPJwX2fnYTazSOZAhxg==
keySpecReceived equals secretKey: true

Code:

import javax.crypto.KeyGenerator;
import javax.crypto.SecretKey;
import javax.crypto.spec.SecretKeySpec;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
import java.util.Base64;

public class Main {
    public static void main(String[] args) throws NoSuchAlgorithmException {
        System.out.println("https://stackoverflow.com/questions/62782129/encrypt-files-with-aes-with-shared-password");
        // random key creation taken from https://stackoverflow.com/a/41414233/9114020
        KeyGenerator keyGenerator = KeyGenerator.getInstance("AES");
        SecureRandom secureRandom = new SecureRandom();
        int keyBitSize = 128; // aes keylength can be 128, 192 or 256 bit
        keyGenerator.init(keyBitSize, secureRandom);
        SecretKey k = keyGenerator.generateKey();
        // encode the key and then base64-encoding
        String sharedKey = Base64.getEncoder().encodeToString(k.getEncoded());
        System.out.println("sharedKey: " + sharedKey);

        // share this key with another party on a secure way
        String sharedKeyReceived = sharedKey; // simulates the receiving
        byte[] sharedKeyByteReceived = Base64.getDecoder().decode(sharedKeyReceived);
        SecretKeySpec kReceived = new SecretKeySpec(sharedKeyByteReceived, "AES");
        System.out.println("keySpecReceived equals secretKey: " + kReceived.equals(k));
    }
}