Unable to create public key object from xml in java

Question

I am having trouble converting my public key from an xml file in java. After some research, it appears as though X509 encoding is the most popular format to use.

I have a xml file that looks something like this:

<RSAKeyValue>
<Modulus>hkjhvgkjewrlhfelwrkjvhwerkjlrkwjvbrvkjrbvkwjlvbwekvjbekvbwkbrkvbwrwebrvvbrlkvbklvrbkvlreb</Modulus>
<Exponent>AQAB</Exponent>
</RSAKeyValue>

(this is not my actual key value)

Here is a code sample of what I tried:

 File fXmlFile = new File(".....\\...\\...\\file.xml"); //Path to xml file
 DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();
 DocumentBuilder dBuilder = dbFactory.newDocumentBuilder();
 Document doc = dBuilder.parse(fXmlFile);
 doc.getDocumentElement().normalize();

 NodeList nList = doc.getElementsByTagName("Modulus");
 Node n = nList.item(0);

 byte[] encKey = Base64.getEncoder().encode(n.getTextContent().getBytes());

 X509EncodedKeySpec pubKeySpec = new X509EncodedKeySpec(encKey);
 KeyFactory keyFactory = KeyFactory.getInstance("DSA", "SUN");
 PublicKey pubKey = keyFactory.generatePublic(pubKeySpec);

At the last line I get an exception that looks like this:

java.security.spec.InvalidKeySpecException: Inappropriate key specification: invalid key format
at sun.security.provider.DSAKeyFactory.engineGeneratePublic(DSAKeyFactory.java:119)
at java.security.KeyFactory.generatePublic(KeyFactory.java:328)
at XmlReader.main(XmlReader.java:41)

I'm not to sure what the issue is with the key format. Could it be an issue with the input being converted into a string that is causing it to lose some important data for the key?

Answer 1

You are building a RSA key with a DSA KeyFactory, and using X509EncodedKeySpec while you have modulus and exponent encoded in base64

Try this:

BigInteger modulus = new BigInteger(Base64.getDecoder().decode(modulusAsString));
BigInteger exponent =  new BigInteger(Base64.getDecoder().decode(exponentAsString));

RSAPublicKeySpec keySpec = new RSAPublicKeySpec(modulus, exponent);
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
PublicKey pubKey =keyFactory.generatePublic(keySpec);

I omit the XML part