Validating a Signature of a SOAP Message

The solution is:

 private boolean validateSignature(Node signatureNode, Node bodyTag, PublicKey publicKey) {
    boolean signatureIsValid = false;
    try {
        // Create a DOM XMLSignatureFactory that will be used to unmarshal the
        // document containing the XMLSignature
        String providerName = System.getProperty
                ("jsr105Provider", "org.jcp.xml.dsig.internal.dom.XMLDSigRI");
        XMLSignatureFactory fac = XMLSignatureFactory.getInstance("DOM",
                (Provider) Class.forName(providerName).newInstance());

        // Create a DOMValidateContext and specify a KeyValue KeySelector
        // and document context
        DOMValidateContext valContext = new DOMValidateContext(new X509KeySelector(publicKey), signatureNode);
        valContext.setIdAttributeNS((Element) bodyTag, "http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd", "Id");

        // Unmarshal the XMLSignature.
        XMLSignature signature = fac.unmarshalXMLSignature(valContext);
        // Validate the XMLSignature.
        signatureIsValid = signature.validate(valContext);

    } catch (Exception ex) {
        logger.error("An Error Raised while Signature Validation");
        logger.error("Cause: " + ex.getCause());
        logger.error("Message: " + ex.getMessage());
    }

    return signatureIsValid;
}

where

public class X509KeySelector extends KeySelector {

PublicKey key;

/**
 * Constructor.
 *
 * @param key a public key of a certificate which need to be validated.
 */
public X509KeySelector(PublicKey key) {
    this.key = key;
}

/**
 * @return a KeySelectorResult with a predefined key.
 */
public KeySelectorResult select(KeyInfo keyInfo,
                                KeySelector.Purpose purpose,
                                AlgorithmMethod method,
                                XMLCryptoContext context) throws KeySelectorException {
    return new KeySelectorResult() {
        @Override
        public Key getKey() {
            return key;
        }
    };
}

}

and give to the X509KeySelector a public key you need to validate the signature.

Well, I've managed to validate the reference digest: canonicalize -> sha1 -> base64. The problem is I can't validate the signature itself. I've tried:

 TransformerFactory transfac = TransformerFactory.newInstance();
 Transformer trans = transfac.newTransformer();
 StringWriter sw = new StringWriter();
 StreamResult result = new StreamResult(sw);
 DOMSource source = new DOMSource(signedInfoTag);
 trans.transform(source, result);
 String xmlString = sw.toString();

 Signature sig = Signature.getInstance("SHA1withRSA", "BC");

 sig.initVerify(cert.getPublicKey());
 sig.update(xmlString.getBytes());

 byte[] signatureValueDecoded = Translator.base64StringToByteArray(signatureValue);
         sig.verify (signatureValueDecoded);

where xmlString is a < SignedInfo > tag. I've read this. Still I don't know, how to format the xml in such a way. Actually it has to be done in-memory somehow