How to create Digital XML Signatures using keypair algorithm “RSASSA-PSS” with Java?

Question

I need to sign XML in Java using Digital Signatures.

I have a key pair using signature algorithm RSASSA-PSS:

SunRsaSign RSASSA-PSS private CRT key, 2048 bits
Sun RSASSA-PSS public key, 2048 bits

See this code here from the OpenJDK test framework: https://github.com/openjdk/jdk/blob/master/test/lib/jdk/test/lib/security/XMLUtils.java

I’m using this utility to load the value:

    Document doc = XMLUtils.string2doc("<a><b>Text</b>Raw</a>");
    KeyPairGenerator instance = KeyPairGenerator.getInstance("RSASSA-PSS");
    instance.initialize(2048);
    KeyPair keyPair = instance.generateKeyPair();

    PSSParameterSpec pspec = new PSSParameterSpec("SHA-384", "MGF1",
            MGF1ParameterSpec.SHA512, 48, TRAILER_FIELD_BC);

    Document signed = XMLUtils.signer(keyPair.getPrivate(), keyPair.getPublic())
            .dm(DigestMethod.SHA384)
            .sm(SignatureMethod.RSA_PSS, new RSAPSSParameterSpec(pspec))
            .sign(doc);
    System.out.println(XMLUtils.doc2string(signed));

    System.out.println("Good? " + XMLUtils.validator().validate(signed, keyPair.getPublic()));

Unfortunately, this is not working

Exception in thread "main" java.security.KeyException: unsupported key algorithm: RSASSA-PSS
    at java.xml.crypto/org.jcp.xml.dsig.internal.dom.DOMKeyInfoFactory.newKeyValue(DOMKeyInfoFactory.java:85)

because:

https://github.com/openjdk/jdk/blob/master/src/java.xml.crypto/share/classes/org/jcp/xml/dsig/internal/dom/DOMKeyInfoFactory.java#L82

@Override
public KeyValue newKeyValue(PublicKey key)  throws KeyException {
    String algorithm = key.getAlgorithm();
    if ("DSA".equals(algorithm)) {
        return new DOMKeyValue.DSA((DSAPublicKey) key);
    } else if ("RSA".equals(algorithm)) {
        return new DOMKeyValue.RSA((RSAPublicKey) key);
    } else if ("EC".equals(algorithm)) {
        return new DOMKeyValue.EC((ECPublicKey) key);
    } else {
        throw new KeyException("unsupported key algorithm: " + algorithm);
    }
}

It does not allow “RSASSA-PSS”.

It appears that if it were to be changed to:

        } else if ("RSASSA-PSS".equals(algorithm) || "RSA".equals(algorithm)) {
            return new DOMKeyValue.RSA((RSAPublicKey) key);

That appears to work? Or maybe it just looks like it’s working.

If this were changed KeyPairGenerator.getInstance("RSA") that also works but then it wouldn’t be the algorithm that I need.

Here is a recent change to the JDK related to this:
https://github.com/openjdk/jdk/commit/8dbf7aa1#diff-fa025c90d09b130fe3a8da89b4ebc5fc3ec157ded54b858dabe8241d383880abR66

I notice that they convert the public key into a “self certificate” before using it in the XML signature.

X509Certificate cert = g.getSelfCertificate(new X500Name("CN=Me"), 100);

Those classes aren’t public to Java so I can’t use them. What is this doing exactly?

How do I properly sign an XML Signature using the RSASSA-PSS algorithm?

Leave a Comment Cancel reply