How do I convert a PEM file to XML RSA key ?

This tutorial guides you on how to convert a PEM file to XML RSA key. Also let’s see how to convert the other way i.e., XML RSA key to PEM file.

Convert a PEM file to XML RSA key

In our previous tutorial I explained how to generate public key and private key with OpenSSL in Windows 10. There might be a situation where you wanted to convert private.pem key file to private.xml format.

You may wanted to generate XML based RSA keys (private and public), so that it can be used in .NET development environment.

For example, private.pem file content or RSA private key will look like below.

The XML structure of the converted XML RSA private key will look like below.

<?xml version="1.0" encoding="UTF-8"?>
<RSAKeyValue>
   <Modulus>n5URTLlhm69L+CdonLd7WYey5MSDp5N1csO58xZbGKTO8A6KuCwHmJ51UW6Q+AxUZtYaIYC2CVeDNmZpfqxvqwz0wq/3T67I5HvSjBpJqUCptu2E5aE/EQs0iGivZnp3Zqh9Qs0EDcWNpT9aM+qwEMDv54zF/+YdSCF5pp+/A7kP5jaaTDYjF8fQoaB84+XWc8B5o16JLiU1YNn4suVYbXQpm/dVlT/UvCA+Aepn79s9mMRxZmNyibGhyQ+NoMipk3jzsB3TfnXShosyWyhFLc5EJ5giEpJclQipC071QZt7ZaAkjjFor93uUAppBvzBCkt0/dv8rkcc7FW2PimMrw==</Modulus>
   <Exponent>AQAB</Exponent>
   <P>0+T91UDfmeSAHpV2tTVXO9yooOggWI/CNUPlCYC+ogT+RLCFxLmtkJVddBfc7LOra/UNmVaSWEGE+urIfJOPjYENPFNWkyLvAx93aFcRmOt9NXiakHuvjaCZGws+Kp4GuEpWuHphmVEmeQ3nujP1J2m07E9HCTYj/hInmvUoiMc=</P>
   <Q>wMyRhKpH2wxrzS0p657a+RLWTaXqky4g78XWMR7QeySy+EFuKWHWrYqzRZObHVt5PDS+acoXolfp3yR2yhw2ewFUogOGY6eS/m0ymvss3HoDtfMYuxAg1Med0i5iFmXpo9s5ONLSpWgs9iMHbWB7re4EIGJDjTH8vzMrAISBhNk=</Q>
   <DP>xojvIEmJeCq1tHI8aGrTEl6cjS8RZ4lC4R99URsVAr+E8h0nKP0EV8zrZCQbtphdjGht1ndb5V92Vpd3Q+oKff4lxZkcK/52TcnE2rmB3nL+vQQEZk3NXfjUm5c8Jzgrea8FdBIOslVro87YiQm3sLcFcsiQ+VuF+79M5o6/7O8=</DP>
   <DQ>UtzZQiKJZYzuHC3oEXuh82D0OEUyD3XI9CScTVpIElUQkoBUn3aMdOPi7ulkxnZVIdotaHxSMhUE0EH0yCLeC3nj7QoEzXntRGqcUF3Zxxyt0Zqcq3FgGvN8UiRbRfMhOqpy6og5vpuKj3M4svq/zV456c0x6VJtpaORPNS2cek=</DQ>
   <InverseQ>PUI0wgLO6DPSnJtO28ia6jtrCjFzQIp4plpReRMPN7hLFH+V6j7eYPt9KFghTLqsaf20JK0761opOPuEUbDsOLzd8X866B8D3K94hK58WUkXHMPyXKld3n6cSgvn0jWBk+CAjD1If6Fb9mjXFNgaLx/7AKp7mzlfdka3GsqXKS4=</InverseQ>
   <D>jOkm7nWfH8aJMWUy9Xb6v1KYf3KawgTuJ1l2v5RU+gzFAaoFiZyvVG9PLomQDsdsW6XgSwcJNnSE0ytZKdZ/gC2fjq2sljrC/yXrA3VO59nGQlW3DMD2HEmlqHGvt7CZ8BcFDFBVrQdtDKZFMySfNRv4eqCOUWjTIFcWwS+7jj+c1iB9M3awBnYSwWpx2ZeHHm51rm43zeE35oWp1w9Eo4/iRDlfTFl9Iu58r4yrInp2txc83+pmYsvE6mljyvrEWH/mhPpGJ8w6Eftp+dfjdn39xWPKnhF4o8zKqNs4ImAQ5xOm/lLNk/o1R7VcdBXjGLo4yHLz+St3ONT/9NST0Q==</D>
</RSAKeyValue>

You need to write some code or use online tools to perform this conversion. For example the following code sneppets shows how to convert a pem file to XML RSA key.

First, you need to get private key from .pem file.

private static PrivateKey getPrivateKey(KeyFactory factory, String pemFileName) throws InvalidKeySpecException, FileNotFoundException, IOException {
   PemFile pemFile = new PemFile(pemFileName);
   byte[] content = pemFile.getPemObject().getContent();
   PKCS8EncodedKeySpec privKeySpec = new PKCS8EncodedKeySpec(content);
   return factory.generatePrivate(privKeySpec);
}

You can use the following maven dependency in your pom.xml file and create PemFile class to handle pem file I/O operations as shown below.

https://mvnrepository.com/artifact/org.bouncycastle/bcprov-ext-jdk16/1.46

public class PemFile {
 
	private PemObject pemObject;
 
	public PemFile(String filename) throws FileNotFoundException, IOException {
		PemReader pemReader = new PemReader(new InputStreamReader(new FileInputStream(filename)));
		try {
			this.pemObject = pemReader.readPemObject();
		} finally {
			pemReader.close();
		}
	}
 
	public PemObject getPemObject() {
		return pemObject;
	}
}

Once you get the private key, then you need to cast the private key using RSAPrivateCrtKey interface and StringBuilder method to convert to XML RSA Key.

RSAPrivateCrtKey privKey = (RSAPrivateCrtKey) getPrivateKey();

BigInteger n = privKey.getModulus();
BigInteger e = privKey.getPublicExponent();
BigInteger d = privKey.getPrivateExponent();
BigInteger p = privKey.getPrimeP();
BigInteger q = privKey.getPrimeQ();
BigInteger dp = privKey.getPrimeExponentP();
BigInteger dq = privKey.getPrimeExponentQ();
BigInteger inverseQ = privKey.getCrtCoefficient(); 

StringBuilder builder = new StringBuilder();
builder.append("<RSAKeyValue>\n");
write(builder, "Modulus", n);
write(builder, "Exponent", e);
write(builder, "P", p);
write(builder, "Q", q);
write(builder, "DP", dp);
write(builder, "DQ", dq);
write(builder, "InverseQ", inverseQ);
write(builder, "D", d);
builder.append("</RSAKeyValue>");

private static void write(StringBuilder builder, String tag, BigInteger bigInt) {
    builder.append("\t<");
    builder.append(tag);
    builder.append(">");
    builder.append(encode(bigInt));
    builder.append("</");
    builder.append(tag);
    builder.append(">\n");
}

Alternatively you can also use any other XML API instead of StringBuilder method.

You can also use the following online rsa key converter tool to quickly perform the conversions below.

RSA Key PEM -> XML RSA Key
XML RSA Key -> PEM RSA Key.

That’s it hope it helped 🙂

Also See:

Convert a PEM file to XML RSA key

References: