As in paper documents, it is possible for a digital document signed by more than one person. The samples codes listed here is in the packagetr.gov.tubitak.uekae.esya.api.xades.example.multiple.

esya.api.xmlsignature.SignedDocument.java is created to easily manage multiple signatures. Please first examine this class carefully. It simply creates a signed document in XML format, that can contain one or more signatures.

<?xml version="1.0" encoding="UTF-8" ?> 
<ma3:envelope xmlns:ma3="http://uekae.tubitak.gov.tr/xml/signature#" 
              xmlns:ds="http://www.w3.org/2000/09/xmldsig#"> 
    <ma3:data> 
        <ma3:data-item>...</ma3:data-item>
        <ma3:data-item>...</ma3:data-item> 
        ... 
        <ma3:data> 
    <ma3:signatures> 
        <ds:signature>...</ds:signature> 
        <ds:signature>...</ds:signature> 
        ... 
    </ma3:signatures> 
</ma3:envelope>

They are independent signature in the same document. In the code below, the data to be signed is added to the signed document first, then two parallel signatures are created.See ParallelEnveloped.

Context context = createContext();

SignedDocument signatures = new SignedDocument(context);

Document doc = Resolver.resolve("./sample.txt", context);
String fragment = signatures.addDocument(doc);

XMLSignature signature1 = signatures.createSignature();

// add document as inner reference
signature1.addDocument("#"+fragment, "text/plain", false);

signature1.getSignedInfo().setSignatureMethod(SignatureMethod.RSA_SHA256);

// false-true gets non-qualified certificates while true-false gets qualified ones
X509Certificate cert = JSmartCardManager.getInstance().getSignatureCertificate(true, false);

// add certificate to show who signed the document
signature1.addKeyInfo(new ECertificate(cert.getEncoded()));

// now sign it by using smart card
signature1.sign(JSmartCardManager.getInstance().getSigner(PIN, cert));

XMLSignature signature2 = signatures.createSignature();

// add document as inner reference
signature2.addDocument("#"+fragment, "text/plain", false);

signature2.getSignedInfo().setSignatureMethod(SignatureMethod.RSA_SHA256);

// add certificate to show who signed the document
signature2.addKeyInfo(new ECertificate(cert.getEncoded()));

// now sign it by using smart card
signature2.sign(JSmartCardManager.getInstance().getSigner(PIN, cert));

// write combined document
signatures.write(new FileOutputStream(BASE_DIR + SIGNATURE_FILENAME));

Here is an example for creating parallel detached signatures, signing external data. The codes are in ParallelDetached.

Context context = createContext();

SignedDocument signatures = new SignedDocument(context);

XMLSignature signature1 = signatures.createSignature();

// add document as reference, but do not embed it
// into the signature (embed=false)
signature1.addDocument("./sample.txt", "text/plain", false);

signature1.getSignedInfo().setSignatureMethod(SignatureMethod.RSA_SHA256);

// false-true gets non-qualified certificates while true-false gets qualified ones
X509Certificate cert = JSmartCardManager.getInstance().getSignatureCertificate(true, false);

// add certificate to show who signed the document
signature1.addKeyInfo(new ECertificate(cert.getEncoded()));

// now sign it by using smart card
signature1.sign(JSmartCardManager.getInstance().getSigner(PIN, cert));

XMLSignature signature2 = signatures.createSignature();

// add document as reference, but do not embed it
// into the signature (embed=false)
signature2.addDocument("./sample.txt", "text/plain", false);

signature2.getSignedInfo().setSignatureMethod(SignatureMethod.RSA_SHA256);

// add certificate to show who signed the document
signature2.addKeyInfo(new ECertificate(cert.getEncoded()));

// now sign it by using smart card
signature2.sign(JSmartCardManager.getInstance().getSigner(PIN, cert));

// write combined document
signatures.write(new FileOutputStream(BASE_DIR + SIGNATURE_FILENAME));

Note that the only difference is the use of reference to the data instead of the data itself in XMLSignature.

It is the signature of another signature. In the example below, the creation of a counter signature over an existing signature is listed. The codes are in CounterDetached.

Context context = createContext();

// read previously created signature, you need to run Detached.java first
Document doc = Resolver.resolve(Detached.SIGNATURE_FILENAME, context);
XMLSignature signature = XMLSignature.parse(doc, context);

// create counter signature
XMLSignature counterSignature = signature.createCounterSignature();

counterSignature.getSignedInfo().setSignatureMethod(SignatureMethod.RSA_SHA256);

// false-true gets non-qualified certificates while true-false gets qualified ones
X509Certificate cert = JSmartCardManager.getInstance().getSignatureCertificate(true, false);

// add certificate to show who signed the document
counterSignature.addKeyInfo(new ECertificate(cert.getEncoded()));

// now sign it by using smart card
counterSignature.sign(JSmartCardManager.getInstance().getSigner(PIN, cert));

// signature contains itself and counter signature
signature.write(new FileOutputStream(BASE_DIR + SIGNATURE_FILENAME));

Now take a look at a more complex scenario in which a counter signature is added to a document that has two parallel signatures. The codes are in CounterParallel.

Context context = createContext();

// read previously created signature
Document signatureFile = Resolver.resolve(ParallelDetached.SIGNATURE_FILENAME, context);
SignedDocument signedDocument = new SignedDocument(signatureFile, context);

// get first signature
XMLSignature signature = signedDocument.getSignature(0);

// create counter signature to the first one
XMLSignature counterSignature = signature.createCounterSignature();

counterSignature.getSignedInfo().setSignatureMethod(SignatureMethod.RSA_SHA256);

// false-true gets non-qualified certificates while true-false gets qualified ones
X509Certificate cert = JSmartCardManager.getInstance().getSignatureCertificate(true, false);

// add certificate to show who signed the document
counterSignature.addKeyInfo(new ECertificate(cert.getEncoded()));

// now sign it by using smart card
counterSignature.sign(JSmartCardManager.getInstance().getSigner(PIN, cert));

// signed doc contains both previous signature and now a counter signature
// in first signature
signedDocument.write(new FileOutputStream(BASE_DIR + SIGNATURE_FILENAME));

Similar to the previous example,except the parallel signatures contained in the SignedDocument.