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PDFBox - Define visual signature template after signing

I've been reading the official PDFBox examples to visually sign a PDF document, especifically the example from CreateVisualSignature2.java, which generates an empty document as a template to define the signature appearance and then sets it to the real document by calling SignatureOptions.setVisibleSignature().
In my case, I've been using an HSM service to do the signing for me, so I don't have direct access to private keys or certificates. I send the document hash to this service and it returns a PKCS7 byte array which I add to my document using ExternalSigningSupport.setSignature().
The code, which is based on the PDFBox example linked above, looks like this:
// Read the document and prepare a signature.
PDDocument document = PDDocument.load( "path/to/file.pdf" );
PDSignature signature = new PDSignature();
signature.setFilter( PDSignature.FILTER_ADOBE_PPKLITE );
signature.setSubFilter( PDSignature.SUBFILTER_ADBE_PKCS7_DETACHED );
signature.setReason( "Test" );
InputStream template = createVisualSignatureTemplate( document ); // Implementation defined below.
SignatureOptions options = new SignatureOptions();
options.setVisibleSignature( template );
options.setPage( 0 );
document.addSignature( signature, options );
// Get the content to sign using PDFBox external signing support.
FileOutputStream outputStream = new FileOutputStream();
ExternalSigningSupport externalSigning = document.saveIncrementalForExternalSigning( outputStream );
byte[] content = IOUtils.toByteArray( externalSigning.getContent() );
// Send the content to the HSM service and get the response.
byte[] hash = MessageDigest.getInstance( "SHA-256" ).digest( content );
byte[] pkcs7 = MyHSMService.getSignedHash( hash );
// Add the signature to the PDF.
externalSigning.setSignature( pkcs7 );
document.close();
And my template method, based on the method of the same name in the linked PDFBox example, simplified:
PDDocument emptyDocument = new PDDocument();
emptyDocument.addPage( new PDFPage( document.getPage(0).getMediaBox() ) );
// Create the PDAcroForm, PDSignatureField, PDAppearanceDictionary, etc,
// just like in the official example.
(...)
// Define the content stream of the visual signature.
PDPageContentStream content = new PDPageContentStream( emptyDocument, appearanceStream );
content.beginText();
content.showText( "Signed by: ... " ); // The signer name should be here.
content.newLine();
content.showText( "Date: ..." ); // The signature datetime should be here.
content.endText();
content.close();
// Convert the empty document as an input stream and return it, just like the example.
(...)
This works fine and I can add valid visual signatures without issues. My problem is that I need to add the signer name and sign date in the signature appearance, but since I create the template BEFORE calling my HSM service to sign, I don't have access to that data yet, which is why I need to define the content AFTER signing the document.
Is there a way to achieve this? I'm new to PDF signing, so my understanding of the fundamentals is pretty poor.
Thanks in advance!

The in-document visualization of a pdf signature is a pdf widget annotation and as such it is part of the signed content. Thus, it must be added before signing and only information can be used in it which is known before signing.
There is one option, though, most signature types allow changing annotations in a revision after the signed revision. Read here about allowed and disallowed changes to signed pdf documents.
But a pdf viewer then still may warn that changes have been applied to the document after signing. Such a warning would look like this:
(Here the signature appearance has been changed to a red diagonal cross.)
If you sign this changed document again, you can even get rid of the warning in the signature ribbon at the top:
(Images are from this answer where changes to signature appearances using itext have been tested.)
When it comes to the datetime, I could show the date of the PDSignature object itself, but in this case the time shown in the visualization won't be the same as the time shown in the signatures panel in the PDF viewer, since the actual signature will happen a few seconds later when I call the HSM service. In this case I have no other choice than to make a new revision just to correct the time, right?
Well, first of all there is no need for a signing time attribute in the CMS signature container embedded in a pdf signature because the signing time can also be given in the M entry if the signature dictionary.
Thus, one choice is to try and retrieve a signature container without signing time attribute. In that case the PDSignature signing time will be shown in the signature panel.
If you use digital time stamps, though, to document the signing time, it is part of the CMS signature container or even a separate document time stamp, so this's option wouldn't work.
Furthermore, you mention the signature panel. As the exact information can be found there, do you really need the signing time in the signature visualization? The visualization is only cosmetic in nature, it may contain arbitrary information, so nobody should trust it anyways. Adobe had warned about this already many years ago, even before pdf became an ISO standard...
Thus, another choice is simply to put a nice looking logo there and that's it!

Is there a way to decrypt S/MIME public key data?

I would like to get the email address and expire date to a S/MIME certificate based on it's public key. Is this aproach even possible? Or am I totally wrong? Can I decrypt the public key to get these kind of data via java?
I searched in google, read the wiki pages and read about an oracle s/mime project. But it doesn't seam like its possible. Are those data only availabe in the csr??
Thanks in advance

I'm amazed this isn't a dupe, but I couldn't find a good one.
Although Bouncy is fine and has many features if you want to use it, core Java can handle X.509 certificates since forever. For a cert in a file (or anything that can be accessed as a Stream) in either PEM or DER format (although the javadoc isn't clear on that) all you need is CertificateFactory:
CertificateFactory fact = CertificateFactory.getInstance("X.509");
// from a real file
InputStream is = new FileInputStream ("filename");
Certificate cert = fact.generateCertificate(is);
is.close(); // or use try-resources to do automatically
// from an alternate/custom filesystem, such as a ZIP
Path p = Paths.get("somespecification"); // or any other creation of a Path
InputStream is = Files.newInputStream(p); // add open options if needed
// same as before
// from the classpath (usually a JAR)
InputStream is = ClassLoader /*or any Class<?> object*/ .getResourceAsStream("name");
// same as before
// from a byte[] in memory
InputStream is = new ByteArrayInputStream (bytearray);
// same as before, except don't really need to close
// you get the idea
Although JCA APIs like this one are defined to allow a lot of extension, reading an X.509 cert will actually give you not just Certificate but subclass X509Certificate from which .getNotAfter() gives the expiration date-time directly. The email address if present (which isn't required by X.509 certs in general, but should always be the case in a cert used for S/MIME) will usually be an attribute in the subject name, which actually has internal structure that Java doesn't let you get at directly so you need to:
String x500name = ((X509Certificate)cert).getSubjectX500Principal()) .toString();
// simple case: no multivalue RDN, no reserved chars ,+="<>\;# or extra spaces
for( String attr : x500name.split(", ") )
if( attr.startsWith("EMAILADDRESS=") )
... use attr.substring(13) ...
// other cases require slightly more complicated parsing
Note there is no encryption at all in X.509, and thus no actual decryption, although many people use 'decrypt' to describe anything unfamiliar not an actual cipher.

File file = new File(fileName);
FileReader fileReader = new FileReader(file);
PEMParser pemParser = new PEMParser(fileReader);
X509CertificateHolder caCertificate = (X509CertificateHolder) pemParser.readObject();

How to associate certificate information to already signed data?

I am trying to add digital signature to pdf document using pdf-box library (v2.0.8). I am receiving already signed content from a webservice (signed with only private key). Now I would need to associate certificate information to this signed data so that it can be added to PDF document. How can we add certificate to already signed content, preferably using bouncy castle api ?
// here content is data which has to be signed
public byte[] sign(InputStream content) throws IOException {
try {
CMSSignedDataGenerator gen = new CMSSignedDataGenerator();
List<Certificate> certList = new ArrayList<Certificate>();
certList.add(certificate);
Store certs = new JcaCertStore(certList);
gen.addCertificates(certs);
CMSProcessableInputStream msg = new CMSProcessableInputStream(signPrivate(content));
CMSSignedData signedData = gen.generate(msg, false);
ByteArrayOutputStream baos = new ByteArrayOutputStream();
DEROutputStream dos = new DEROutputStream(baos);
dos.writeObject(signedData.toASN1Structure());
return baos.toByteArray();
} catch (Exception e) {
throw new IOException(e);
}
}
Here, I am able to generate digital signature, but it does not contain any certificate information. I already checked this and this question but they donot take the case where content is already signed using private key seperatly and only certificate needs to be associated.

(The code you posted refers to CMS signature containers, so I assume we are talking about adbe.pkcs7.detached or ETSI.CAdES.detached PDF signatures.)
When creating a signature in a CMS signature container, one has the choice whether the signature value really only signs the (hash of the) document data or whether it signs a collection of so-called signed attributes (signedAttrs in the SignerInfo specification) and the hash of the document data is but a value of one of those attributes.
SignerInfo ::= SEQUENCE {
version CMSVersion,
sid SignerIdentifier,
digestAlgorithm DigestAlgorithmIdentifier,
signedAttrs [0] IMPLICIT SignedAttributes OPTIONAL,
signatureAlgorithm SignatureAlgorithmIdentifier,
signature SignatureValue,
unsignedAttrs [1] IMPLICIT UnsignedAttributes OPTIONAL }
(RFC 5652 section 5.3. SignerInfo Type)
All profiles hereof to be taken seriously, though, require that you use signed attributes, in particular they require you to use an ESS signing-certificate (RFC 2634 section 5.4) or ESS signing-certificate-v2 (RFC 5035 section 3) signed attribute to reference the signer certificate.
In these attributes, therefore, the association of the signature with its signing certificate is fixed before the signature value is generated.
Thus, you cannot freely associate a signing certificate to an already generated signature.

Signing a message digest using BouncyCastle

At the moment in C# I'm signing a challenge like this:
RSACryptoServiceProvider rsa;
RSAPKCS1SignatureFormatter RSAFormatter = new RSAPKCS1SignatureFormatter(rsa);
RSAFormatter.SetHashAlgorithm("SHA1");
byte[] SignedHash = RSAFormatter.CreateSignature(paramDataToSign);
Then I give the SignedHash to Windows, it accepts it and everything is OK. But I need to move this part to Android and there's the problem, that I just can't get the same signed hash value.
In Android I tried to make the signed hash but they differ from the one generated in C#.
Signature signer = Signature.getInstance("SHA1withRSA", "BC");
signer.initSign(privateKey);
signer.update(paramDataToSign);
signer.sign();
In C# - using the following piece of code - I get the same result as in Android, but it is not an option cause then Windows does not accept the signed hash.
ISigner signer = SignerUtilities.GetSigner("SHA1withRSA");
signer.Init(true, privateKey);
signer.BlockUpdate(paramDataToSign, 0, paramDataToSign.Length);
signer.GenerateSignature();
Here's written that C# PKCS1SignatureFormatter and Java Signature should give the same result, but they do not. http://www.jensign.com/JavaScience/dotnet/VerifySig/
What could be the problem?
Here are the base 64 (WebSafe) values that I get:
Challenge = zHyz12Tk4m151nssYIBWqBCAxhQ
RSAPKCS1SignatureFormatter SignedHash = kmu39keplCAV4Qnu22wdprLz4nGSsrVtHbxQ5YMUG7p-0YwReCG4ROIlFvYs4CGfjCiAGFPw4PLrLx7mrlAA6iuhJMkgm_PMTW9alQYTH612hLEUP4EmK0M2kw8CveLcjI3HA08z8bByllIzRyAlM8bcR438vw2uhx_CbgvOOHn8vwBPnvWbFqpi2doYoq2xEuFBRe7eBPrxbMRqEd3ExdQ9c9rYT4ivOJ4pbioyi6D5i5_1crvGwM6nQanMZCmooRYJO65NP3B4wWnvQZpJLRD0U08wWcvyGBFWp188ZovDjnkTQZku6lzmwGXfqQwtBz9uNvLcTbp7cVyt5EyQxw
Signature and ISigner SignedHash = Vt-b5QfGPnSPpZuIB8-H4N1K5hQXpImS4e8k56_HruDSqy3DLsz96QKUrccshjr1z9nTK3Mwvd5yPdyTJOqSUcDQqxV46LPhWQNsubqKxAz97ePpeslIH1gHdnzkh46ixsWqgDrhR7egQtDkU8PPsph1qahCxaVkRYspQBV0jPZ-LK4EjoGGnuWTCihVKjruXJZ2VY8yZ9QRAsHVptr0Nv-mldO2MFK-oEVbtVbHqUPf5So8im3oRSm68OqY4g56bCdFNSbhcFBjrZ1QPjnxiIk43-_5tevafqoOB2D_E_mQHCJwmRg3MrNij6IdAdloCejnhCWzgMHdcG1Ug_Qmig
EDIT:
So the simplest solution is using Bouncy Castle API:
AsymmetricBlockCipher rsaEngine = new PKCS1Encoding(new RSABlindedEngine());
rsaEngine.init(true, privateKey);
DigestInfo dInfo = new DigestInfo(new AlgorithmIdentifier(X509ObjectIdentifiers.id_SHA1, DERNull.INSTANCE), paramDataToSign);
byte[] digestInfo = dInfo.getEncoded(ASN1Encoding.DER);
rsaEngine.processBlock(digestInfo, 0, digestInfo.length);

The problem is that RSAFormatter.CreateSignature(paramDataToSign); passes the hash value, while signer.update(paramDataToSign); passes the data before it is hashed. So it is likely that you have to remove a MessageDigest calculation for your Java code for this to work.
Alternatively, if you only have the hash value, you may have a look into the Bouncy Castle lightweight API to find a method that accepts a value that is pre-hashed. This can probably be performed using new RSADigestSigner(new StaticDigest(paramDataToSign, "SHA-1")).generateSignature().
Problem is that StaticDigest does not exist, so you'll have to comment here if you really require it. Alternative, mirror the implementation of RSADigestSigner but substitute a pre-calculated hash.

View details of a certificate in Android

How to display details of the subject in a certificate (DER format ie .cer file) in java ? eg: email, country, Name or seperate OIDs etc.
Reading a certificate could be done from the above code. Will work in Android as well. Thank you GreyBeardedGeek. But to more elaborate on it (As curious requested), if you display the subject details by using methods getSubjectDN() or cert.getSubjectX500Principal().getname() it will display the whole details. Some may be encoded. I believe it ASN1.(not sure). What I need is to extract only the information I need. For an example please refer the output of the code for a sample certificate I had created. using method - cert.getSubjectX500Principal().getname() Subject Name 2.5.4.5=#130d4e2d61626c652f49542f303130,2.5.4.72=#1308456e67696e656572,2.5.4.41=#13104e7577616e20446520416c6d65696461,1.2.840.113549.1.9.1=#16106e7577616e406e2d61626c652e62697a,OU=Information Technology,O=N-able Pvt Ltd\ ,ST=Western,C=LK
2.5.4.5 is an OID (Object Identifier) which is encoded.
Using method - getSubjectDN()
OID.2.5.4.5=N-able/IT/010, OID.2.5.4.72=Engineer, OID.2.5.4.41=Nuwan De Almeida, OID.1.2.840.113549.1.9.1=#16106E7577616E406E2D61626C652E62697A, OU=Information Technology, O="N-able Pvt Ltd ", ST=Western, C=LK
Here also some information is encoded eg: email address.
So coming back to my question , how can we extract information(not encoded) separately based on the OID. Further if you install the certificate in windows OS you could view the subject information correctly.What I need is a code to get the OID value information passing the OID in java, to extract subject details separately.
Thank you again in advance.

Look into the Bouncy Castle ASN.1 parsing libraries and especially X500Name. It can parse a distinguished name (DN) into its parts (CN, O, etc.).
http://www.bouncycastle.org/docs/docs1.5on/index.html

The following code (no error handling included) will produce an instance of X509Certificate from the .cer file. You can then use that object's methods to inspect the properties of the certificate. The code is generic java, but should work in Android.
X509Certificate cert = null;
FileInputStream fis = null;
ByteArrayInputStream bais = null;
String source = "certificate.cer";
String certType = "X.509"
fis = new FileInputStream(source);
byte[] value = new byte[fis.available()];
fis.read(value);
bais = new ByteArrayInputStream(value);
java.security.cert.CertificateFactory cf = java.security.cert.CertificateFactory.getInstance(certType);
cert = (X509Certificate)cf.generateCertificate(bais);