Android: decrypt RSA text using a Public key stored in a file

Question

I've been several days trying to do it without success.

There are plenty of similar questions here in StackOverflow and even two of them are exactly the same as mine but unanswered and unresolved: 1) Convert PHP RSA PublicKey into Android PublicKey 2) Android: how to decrypt an openssl encrypted file with RSA key?

My scenario: I have some text encrypted using RSA (not encrypted by me). I have a "public.key" file in my res/raw folder with the public key needed to decrypt it (the public key related to the private key used to encrypt the message), with a format like the following example:

I see a lot of examples of how to decrypt a RSA text, like the following one:

public static byte[] decryptRSA( PublicKey key, byte[] text) throws Exception
      { 
          byte[] dectyptedText = null;

          Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
          cipher.init(Cipher.DECRYPT_MODE, key);
          dectyptedText = cipher.doFinal(text);
          return dectyptedText;
      }

But my question is, how to get the proper PublicKey instance from the file? No examples of this.

If I simply try:

    InputStream is = getResources().openRawResource(R.raw.public);
    DataInputStream dis = new DataInputStream(is);
    byte [] keyBytes = new byte [(int) is.available()];
    dis.readFully(keyBytes);
    dis.close();
    X509EncodedKeySpec spec = new X509EncodedKeySpec(keyBytes);
    KeyFactory keyFactory = KeyFactory.getInstance("RSA");
    return keyFactory.generatePublic(spec);

I get an InvalidKeyException in the return sentence. Would I need to decode Hex or Base64? Aren't the first and last lines of the public key file a problem (the ones with "----BEGIN PUBLIC KEY----" and so)?

Maybe we could get the answer of this properly for the first time in StackOverflow:-)

Answer 1

Finally solved!!! Drums, trumpets and a symphony of enchanting sounds!!!

public static byte[] decryptRSA(Context mContext, byte[] message) throws Exception { 

    // reads the public key stored in a file
    InputStream is = mContext.getResources().openRawResource(R.raw.sm_public);
    BufferedReader br = new BufferedReader(new InputStreamReader(is));
    List<String> lines = new ArrayList<String>();
    String line = null;
    while ((line = br.readLine()) != null)
        lines.add(line);

    // removes the first and last lines of the file (comments)
    if (lines.size() > 1 && lines.get(0).startsWith("-----") && lines.get(lines.size()-1).startsWith("-----")) {
        lines.remove(0);
        lines.remove(lines.size()-1);
    }

    // concats the remaining lines to a single String
    StringBuilder sb = new StringBuilder();
    for (String aLine: lines)
        sb.append(aLine);
    String keyString = sb.toString();
    Log.d("log", "keyString:"+keyString);

    // converts the String to a PublicKey instance
    byte[] keyBytes = Base64.decodeBase64(keyString.getBytes("utf-8"));
    X509EncodedKeySpec spec = new X509EncodedKeySpec(keyBytes);
    KeyFactory keyFactory = KeyFactory.getInstance("RSA");
    PublicKey key = keyFactory.generatePublic(spec);

    // decrypts the message
    byte[] dectyptedText = null;
    Cipher cipher = Cipher.getInstance("RSA");
    cipher.init(Cipher.DECRYPT_MODE, key);
    dectyptedText = cipher.doFinal(Base64.decodeBase64(message));
    return dectyptedText;
}

The solution was to Base64 decode not only the public key read from the file, but also the crypted message itself!

By the way, I read the public key from the file the way @Nikolay suggested (tnx again man).

Thank you all very much for your help. StackOverflow rocks!

Answer 2

You are missing a key point -- public and private keys are separate, and you cannot calculate one based on the other. That is kind of the point of public key encryption. Issues with using raw RSA aside, if you have something encrypted with the public key, you need to have the corresponding private key to decrypt it. And vice versa. So if you have public key file, you can only get a public key from it. That would be only useful if your data was encrypted with the corresponding private key.

As for the actual exception: remove the '---' lines at the start and end, the use Base64.decode() to get a byte array, and use this to create your X509EncodedKeySpec. One way to do it -- use something like a BufferedReader to read line by line, ignore the '---' lines and concat the rest into one big String.