Java XML 署名 API の使い方 -公式サンプル-

署名編の評判が良いようなので、次は検証（Validation）編。

と思ったのだが、オラクル公式サンプルが興味深かったので再掲。
レビューしてみる。

そのコードはこうだ。

import javax.xml.crypto.*;
import javax.xml.crypto.dsig.*;
import javax.xml.crypto.dom.*;
import javax.xml.crypto.dsig.dom.DOMValidateContext;
import javax.xml.crypto.dsig.keyinfo.*;
import java.io.FileInputStream;
import java.security.*;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import javax.xml.parsers.DocumentBuilderFactory;
import org.w3c.dom.Document;
import org.w3c.dom.NodeList;

/**
 * This is a simple example of validating an XML 
 * Signature using the JSR 105 API. It assumes the key needed to
 * validate the signature is contained in a KeyValue KeyInfo. 
 */
public class Validate {

    //
    // Synopsis: java Validate [document]
    //
    //	  where "document" is the name of a file containing the XML document
    //	  to be validated.
    //
    public static void main(String[] args) throws Exception {

	// Instantiate the document to be validated
	DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
	dbf.setNamespaceAware(true);
	Document doc =
            dbf.newDocumentBuilder().parse(new FileInputStream(args[0]));

	// Find Signature element
	NodeList nl = 
	    doc.getElementsByTagNameNS(XMLSignature.XMLNS, "Signature");
	if (nl.getLength() == 0) {
	    throw new Exception("Cannot find Signature element");
	}

	// Create a DOM XMLSignatureFactory that will be used to unmarshal the 
	// document containing the XMLSignature 
	XMLSignatureFactory fac = XMLSignatureFactory.getInstance("DOM");

	// Create a DOMValidateContext and specify a KeyValue KeySelector
        // and document context
	DOMValidateContext valContext = new DOMValidateContext
	    (new KeyValueKeySelector(), nl.item(0));
	
	// unmarshal the XMLSignature
	XMLSignature signature = fac.unmarshalXMLSignature(valContext);

	// Validate the XMLSignature (generated above)
	boolean coreValidity = signature.validate(valContext); 

	// Check core validation status
	if (coreValidity == false) {
    	    System.err.println("Signature failed core validation"); 
	    boolean sv = signature.getSignatureValue().validate(valContext);
	    System.out.println("signature validation status: " + sv);
	    // check the validation status of each Reference
	    Iterator i = signature.getSignedInfo().getReferences().iterator();
	    for (int j=0; i.hasNext(); j++) {
		boolean refValid = 
		    ((Reference) i.next()).validate(valContext);
		System.out.println("ref["+j+"] validity status: " + refValid);
	    }
	} else {
    	    System.out.println("Signature passed core validation");
	}
    }

    /**
     * KeySelector which retrieves the public key out of the
     * KeyValue element and returns it.
     * NOTE: If the key algorithm doesn't match signature algorithm,
     * then the public key will be ignored.
     */
    private static class KeyValueKeySelector extends KeySelector {
	public KeySelectorResult select(KeyInfo keyInfo,
                                        KeySelector.Purpose purpose,
                                        AlgorithmMethod method,
                                        XMLCryptoContext context)
            throws KeySelectorException {
            if (keyInfo == null) {
		throw new KeySelectorException("Null KeyInfo object!");
            }
            SignatureMethod sm = (SignatureMethod) method;
            List list = keyInfo.getContent();

            for (int i = 0; i < list.size(); i++) {
		XMLStructure xmlStructure = (XMLStructure) list.get(i);
            	if (xmlStructure instanceof KeyValue) {
                    PublicKey pk = null;
                    try {
                        pk = ((KeyValue)xmlStructure).getPublicKey();
                    } catch (KeyException ke) {
                        throw new KeySelectorException(ke);
                    }
                    // make sure algorithm is compatible with method
                    if (algEquals(sm.getAlgorithm(), pk.getAlgorithm())) {
                        return new SimpleKeySelectorResult(pk);
                    }
		}
            }
            throw new KeySelectorException("No KeyValue element found!");
	}

        //@@@FIXME: this should also work for key types other than DSA/RSA
	static boolean algEquals(String algURI, String algName) {
            if (algName.equalsIgnoreCase("DSA") &&
		algURI.equalsIgnoreCase(SignatureMethod.DSA_SHA1)) {
		return true;
            } else if (algName.equalsIgnoreCase("RSA") &&
                       algURI.equalsIgnoreCase(SignatureMethod.RSA_SHA1)) {
		return true;
            } else {
		return false;
            }
	}
    }

    private static class SimpleKeySelectorResult implements KeySelectorResult {
	private PublicKey pk;
	SimpleKeySelectorResult(PublicKey pk) {
	    this.pk = pk;
	}

	public Key getKey() { return pk; }
    }
}

このコード自体はそんなにおかしくないと思うかもしれないが、対象となる xml ファイルも掲示されていてそれはこういうものだ。

<Envelope xmlns="urn:envelope">
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
<CanonicalizationMethod xmlns="http://www.w3.org/2000/09/xmldsig#" Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315#WithComments"/>
<SignatureMethod xmlns="http://www.w3.org/2000/09/xmldsig#" Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/>
<Reference xmlns="http://www.w3.org/2000/09/xmldsig#" URI="">
<Transforms xmlns="http://www.w3.org/2000/09/xmldsig#">
<Transform xmlns="http://www.w3.org/2000/09/xmldsig#" Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/>
</Transforms>
<DigestMethod xmlns="http://www.w3.org/2000/09/xmldsig#" Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
<DigestValue xmlns="http://www.w3.org/2000/09/xmldsig#">uooqbWYa5VCqcJCbuymBKqm17vY=</DigestValue>
</Reference>
</SignedInfo>
<SignatureValue xmlns="http://www.w3.org/2000/09/xmldsig#">eO7K1BdC0kzNvr1HpMf4hKoWsvl+oI04nMw55GO+Z5hyI6By3Oihow==</SignatureValue>
<KeyInfo xmlns="http://www.w3.org/2000/09/xmldsig#">
<KeyValue xmlns="http://www.w3.org/2000/09/xmldsig#">
<DSAKeyValue xmlns="http://www.w3.org/2000/09/xmldsig#">
<P xmlns="http://www.w3.org/2000/09/xmldsig#">/KaCzo4Syrom78z3EQ5SbbB4sF7ey80etKII864WF64B81uRpH5t9jQTxeEu0ImbzRMqzVDZkVG9 xD7nN1kuFw==</P>
<Q xmlns="http://www.w3.org/2000/09/xmldsig#">li7dzDacuo67Jg7mtqEm2TRuOMU=</Q>
<G xmlns="http://www.w3.org/2000/09/xmldsig#">Z4Rxsnqc9E7pGknFFH2xqaryRPBaQ01khpMdLRQnG541Awtx/XPaF5Bpsy4pNWMOHCBiNU0Nogps QW5QvnlMpA==</G>
<Y xmlns="http://www.w3.org/2000/09/xmldsig#">OqFi0sGpvroi6Ut3m154QNWc6gavH3j2ZoRPDW7qVBbgk7XompuKvZe1owz0yvxq+1K+mWbL7ST+ t5nr6UFBCg==</Y>
</DSAKeyValue>
</KeyValue>
</KeyInfo>
</Signature>
</Envelope>

これを NetBeans あたりで実行すると（ちなみに Java17 あたりでもほとんど手直しなしで動く）エラーが出てきて、そのメッセージはなかなか興味深い。

Exception in thread "main" javax.xml.crypto.MarshalException: It is forbidden to use algorithm http://www.w3.org/2000/09/xmldsig#dsa-sha1 when secure validation is enabled

要するに dsa-sha1 が現在ではセキュリティ的に問題あるので使えませんよ、と警告してくれるわけだ。

また、Signature 要素を探すとき doc.getElementsByTagNameNS メソッドを使っていて、なんでだ？と訝しんでいたのだが、対象となる xml を眺めてなんか納得。