RFC 2828 defines a digital signature as "a value computed with a cryptographic algorithm and appended to a data object in such a way that any recipient of the data can use the signature to verify the data's origin and integrity."
Within the XML document, if a signature is the parent element, then it is an
enveloping signature. If it is the child element, it is an
enveloped signature. If it is neither of both, then it is called
detached signature.
We will use the following XML document as subject for XML digital signing:
<?xml version="1.0" ?>
<account>
<number>9876543210</number>
<type>savings</type>
<branch>main</branch>
<name>First M Last</name>
</account>
The resulting signed XML document, which the signature is enveloped, is as follow.
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<account>
<number>9876543210</number>
<type>savings</type>
<branch>main</branch>
<name>First M Last</name>
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo><CanonicalizationMethod Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315#WithComments"/><SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1"/><Reference URI=""><Transforms><Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature"/></Transforms><DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/><DigestValue>pc92d1PtOWfWuYcv8KKzI1VomhM=</DigestValue></Reference></SignedInfo>
<SignatureValue>gQLJpLXZYyay4z3P5so0291bU8hhjlTaKpzOKfKrS87VtuuXwTnATg==</SignatureValue>
<KeyInfo>
<KeyValue>
<DSAKeyValue>
<P>/KaCzo4Syrom78z3EQ5SbbB4sF7ey80etKII864WF64B81uRpH5t9jQTxeEu0ImbzRMqzVDZkVG9
xD7nN1kuFw==</P>
<Q>li7dzDacuo67Jg7mtqEm2TRuOMU=</Q>
<G>Z4Rxsnqc9E7pGknFFH2xqaryRPBaQ01khpMdLRQnG541Awtx/XPaF5Bpsy4pNWMOHCBiNU0Nogps
QW5QvnlMpA==</G>
<Y>x68Jz5Eb/pO6MuNDwhWmSWrgyNN5mvIAYOrvIqf4JzgCll0HvIOXK8Tf1SxrqiYHqPawsqXE0S53
5LW709Dzcw==</Y>
</DSAKeyValue>
</KeyValue>
</KeyInfo>
</Signature></account>
Java code for signing XML documentStep1: instantiate the document to be signed
DocumentBuilderFactory dbf =
DocumentBuilderFactory.newInstance();
dbf.setNamespaceAware(true);
DocumentBuilder builder = dbf.newDocumentBuilder();
Document doc = builder.parse(inputStream);
Step2: generate public-private key pair
KeyPairGenerator kpg = KeyPairGenerator.getInstance("DSA");
kpg.initialize(512);
KeyPair kp = kpg.generateKeyPair();
Step3: create signing context
DOMSignContext dsc = new DOMSignContext(kp.getPrivate(),
doc.getDocumentElement());
Step4: assemble XML signature
XMLSignatureFactory fac =
XMLSignatureFactory.getInstance("DOM");
// Step4a: create reference
Reference ref = fac.newReference("", fac.newDigestMethod(
DigestMethod.SHA1, null),
Collections.singletonList(
fac.newTransform(Transform.ENVELOPED,
(TransformParameterSpec) null)), null, null);
// Step4b: create signed info
SignedInfo si = fac
.newSignedInfo(fac.newCanonicalizationMethod(
CanonicalizationMethod.INCLUSIVE_WITH_COMMENTS,
(C14NMethodParameterSpec) null), fac
.newSignatureMethod(SignatureMethod.DSA_SHA1, null),
Collections.singletonList(ref));
// Step4c: create optional KeyInfo
// - contains information that enables the recipient
// to find the key needed to validate the signature
KeyInfoFactory kif = fac.getKeyInfoFactory();
KeyValue kv = kif.newKeyValue(kp.getPublic());
KeyInfo ki = kif.newKeyInfo(Collections.singletonList(kv));
// Step4d: create XML signature
XMLSignature signature = fac.newXMLSignature(si, ki);
Step5: generate XML signature
signature.sign(dsc); // DOM structure will be updated.
Step6: print signed document
TransformerFactory tf = TransformerFactory.newInstance();
Transformer trans = tf.newTransformer();
trans.transform(new DOMSource(doc), new StreamResult(outputStream));
Validating signed documentStep1: instantiate document that contains signature
DocumentBuilderFactory dbf =
DocumentBuilderFactory.newInstance();
dbf.setNamespaceAware(true);
DocumentBuilder builder = dbf.newDocumentBuilder();
Document doc = builder.parse(inputStream);
Step2: Specify the document to be validated
NodeList nl = doc.getElementsByTagNameNS(XMLSignature.XMLNS, "Signature");
if (nl.getLength() == 0) {
throw new IllegalArgumentException
("Signature element not found");
}
Step3: create validation context
DOMValidateContext valContext = new DOMValidateContext(
new KeyValueKeySelector(), nl.item(0));
Step4: Unmarshal XML signature
XMLSignatureFactory factory =
XMLSignatureFactory.getInstance("DOM");
XMLSignature signature =
factory.unmarshalXMLSignature(valContext);
Step5: Validate
return signature.validate(valContext);
Index: KeySelector class definition
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!");
}
// 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;
}
}
References:
- http://docs.oracle.com/javase/6/docs/technotes/guides/security/xmldsig/XMLDigitalSignature.html
- http://www.xinotes.org/notes/note/753/
- http://docs.oracle.com/javase/6/docs/technotes/guides/security/xmldsig/XMLDigitalSignature.html
