CODEWITHSUNDEEP
androidencryptionencryption-asymmetric
Dean Blakely
Dean Blakely

Reputation: 3585

Trying to get asymmetric RSA encryption to work

The following method does not work. decodedMessage ends up with garbage in it instead of the expected results.

I'm following an example here that supposedly works.

public static void POCSimple()
{
    String secretMessage = "short message";
    PublicKey publicKey = null;
    PrivateKey privateKey = null;
    String encodedMessage = "";
    byte[] encodedBytes = null;
    String decodedMessage ="";
    byte[] decodedBytes = null;


    try
    {
        KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
        kpg.initialize(1024);
        KeyPair kp = kpg.genKeyPair();
        publicKey = kp.getPublic();
        privateKey = kp.getPrivate();

        Cipher c1 = Cipher.getInstance("RSA");
        c1.init(Cipher.ENCRYPT_MODE, publicKey);
        encodedBytes = c1.doFinal(secretMessage.getBytes());
        encodedMessage = Base64.encodeToString(encodedBytes, Base64.DEFAULT);

        Cipher c2 = Cipher.getInstance("RSA");
        c2.init(Cipher.DECRYPT_MODE, privateKey);
        decodedBytes = c2.doFinal(encodedBytes);
        decodedMessage = Base64.encodeToString(decodedBytes, Base64.DEFAULT);

        String mystring = "look at results";

    }
    catch (Exception e)
    {
        String status = e.toString();
    }


}

Any help would be most appreciated. Thanks, Dean

Upvotes: 0

Views: 125

Answers (2)

Dean Blakely
Dean Blakely

Reputation: 3585

It turns out that in my original code, decodedBytes contained the properly decrypted bytes. The following command was turning decodedBytes into junk characters ...

decodedMessage = Base64.encodeToString(decodedBytes, Base64.DEFAULT);

I replaced that code with ...

String str = new String(decodedBytes, "UTF-8");

And this solved the problem probably because decodedBytes had never been Base64 encoded in the first place.

I also found that using straight RSA I can only encrypt a maximum of 245 bytes if I use a 2048 bit key. less if I use a 1024 bit key.

If larger strings need to be encrypted using asymmetric Public/Private keys then I need to first encrypt a string using symmetric AES and then encrypt the AES key with the public RSA key and send both the encrypted AES key and the encrypted message over the wire where the receiver can decrypt the AES key using their private RSA key. The AES key can be randomly generated in the sending code.

Upvotes: 1

Steve
Steve

Reputation: 826

Of course you are getting garbage, this is your sequence:

Plaintext message -> Encrypt -> Encode -> Encoded message

Encoded message -> Decrypt -> Decode -> GARBAGE

You need to undo the Base64 encoding before you can decrypt the message, you are doing the reverse process in the incorrect order!

Edit

Actually its worse, this is your sequence:

Plaintext message -> Encrypt -> Encode -> Encoded message

Encrypted message -> Decrypt -> Encode -> GARBAGE

Try this:

Cipher c1 = Cipher.getInstance("RSA");
c1.init(Cipher.ENCRYPT_MODE, publicKey);
encodedBytes = c1.doFinal(secretMessage.getBytes());
encodedMessage = Base64.encodeToString(encodedBytes, Base64.DEFAULT);

Cipher c2 = Cipher.getInstance("RSA");    
c2.init(Cipher.DECRYPT_MODE, privateKey)      
decodedBytes = Base64.decode(encodedMessage.toByteArray(), Base64.DEFAULT);
decryptedMessage = c2.doFinal(decodedBytes);

Upvotes: 1

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