Question

I am working on a client server based application.

Where I get PublicKey in this format

as I saved it into String.

Now I want to use this key in my Android(Java code), how can I use this ?

Answer 1

First you need to generate the public key from the pem format you provided, here is my method for doing this:

/**
 * 
 * @param PEMString  -A file/string in .pem format with a generated RSA key (with "des3", using "openssl genrsa".)
 * @param isFilePath - If it's a file path or a string
 * @return java.security.PublicKey
 * @throws IOException -No key found
 * @throws NoSuchAlgorithmException 
 * @throws InvalidKeySpecException 
 * 
 * @author hsigmond
 */

private static PublicKey getPublicKeyFromPemFormat(String PEMString,
        boolean isFilePath) throws IOException, NoSuchAlgorithmException,
        InvalidKeySpecException {

    BufferedReader pemReader = null;
    if (isFilePath) {
        pemReader = new BufferedReader(new InputStreamReader(
                new FileInputStream(PEMString)));
    } else {
        pemReader = new BufferedReader(new InputStreamReader(
                new ByteArrayInputStream(PEMString.getBytes("UTF-8"))));
    }
    StringBuffer content = new StringBuffer();
    String line = null;
    while ((line = pemReader.readLine()) != null) {
        if (line.indexOf("-----BEGIN PUBLIC KEY-----") != -1) {
            while ((line = pemReader.readLine()) != null) {
                if (line.indexOf("-----END PUBLIC KEY") != -1) {
                    break;
                }
                content.append(line.trim());
            }
            break;
        }
    }
    if (line == null) {
        throw new IOException("PUBLIC KEY" + " not found");
    }
Log.i("PUBLIC KEY: ", "PEM content = : " + content.toString());

    KeyFactory keyFactory = KeyFactory.getInstance("RSA");
    return keyFactory.generatePublic(new X509EncodedKeySpec(Base64.decode(content.toString(), Base64.DEFAULT)));

}

And here is how I use it to read (decode) the content signed with the public key provided.

/**
 * 
 * @param PEMString  -A file/string in .pem format with a generated RSA key (with "des3", using "openssl genrsa".)
 * @param content
 * @return String value of content Decoded
 * @throws NoSuchAlgorithmException
 * @throws InvalidKeySpecException
 * @throws IOException
 * @throws NoSuchProviderException
 * @throws NoSuchPaddingException
 * @throws InvalidKeyException
 * @throws IllegalBlockSizeException
 * @throws BadPaddingException
 * 
 * @author hsigmond
 */


    public static String getContentWithPublicKeyFromPemFormat(String PEMString,
        String content,boolean isFilePath) throws NoSuchAlgorithmException,
        InvalidKeySpecException, IOException, NoSuchProviderException,
        NoSuchPaddingException, InvalidKeyException,
        IllegalBlockSizeException, BadPaddingException {

    PublicKey publicKey = getPublicKeyFromPemFormat(PEMString,isFilePath);
    if (publicKey != null)
        Log.i("PUBLIC KEY: ", "FORMAT : " + publicKey.getFormat()
                + " \ntoString : " + publicKey.toString());

    byte[] contentBytes = Base64.decode(content, Base64.DEFAULT);
    byte[] decoded = null;

    Cipher cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding", "BC");//BC=BouncyCastle Provider
    cipher.init(Cipher.DECRYPT_MODE, publicKey);
    decoded = cipher.doFinal(contentBytes);
    return new String(decoded, "UTF-8");
}