qr code generator java class Protecting your PDF in Java

Generation QR-Code in Java Protecting your PDF

Protecting your PDF
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wouldn t be legal. Surely there must be a way for company Y to revoke the certificate for employee X so that he no longer can act on behalf of his former company.
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CERTIFICATE REVOCATION LIST
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Every certificate authority keeps lists of certificates that are no longer valid, whether because the owner thinks the private key was compromised, or the token containing the private key was lost or stolen, or the original owner of the key is no longer entitled to use it. Such a list is called a certificate revocation list (CRL), and they are made public at one or more URLs provided by the CA who signed the certificate. You can create a CRL object like this:
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InputStream is = new URL(url_of_crl).openStream(); CertificateFactory cf = CertificateFactory.getInstance("X.509"); CRL crl = (CRL)cf.generateCRL(is);
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An array of CRL objects can be passed as a parameter to the setCrypto() method. However, CRLs are generally large, and this technique is considered to be old technology. It might be a better idea to use the Online Certificate Status Protocol (OCSP).
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ONLINE CERTIFICATE STATUS PROTOCOL
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OCSP is an internet protocol for obtaining the revocation status of a certificate online. You can post a request to check the status of a certificate over HTTP, and the CA s OCSP server will send you a response. You no longer need to parse and embed long CRLs. An OCSP response is small and constant in size, and can easily be included in the PKCS#7 object.
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Revocation information in a PDF document is a signed attribute, which means that the signing software must capture the revocation information before signing. A similar requirement in this use case applies to the chain of certificates. The signing software must capture and validate the certificate s chain before signing. CRLs will lead to bigger PDF documents, and using OCSP will not take as much space. But the OCSP connection to check the status can take time, whereas CRLs can easily be cached on the filesystem. It s always a tradeoff.
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Now let s look at another problem that might arise. Suppose somebody sends you a signed contract. He has used a private key that is still valid, and you re sure that the document you ve received is genuine. However, at some point the author of the document regrets what he s written. By resetting the clock on his computer, he could create a new document with a new digital signature that is as valid as the first one. This way, you could end up with two documents signed with the same private key at almost the same time, but with slightly different content. How can anybody know which document is more genuine
TIMESTAMPING
This problem can be solved by involving a third party: a timestamping authority (TSA). The TSA will take the hash of the document and concatenate a timestamp to it. This is
Digital signatures, OCSP, and timestamping
A signed PDF with a timestamp
done on a timestamp server that is contacted during the signing process. The timestamp server will return a hash that is signed using the private key of the TSA. Figure 12.9 shows a PDF with a timestamped signature. In previous examples and screen shots, the signature panel informed you that the Signature date/time are from the clock on the signer s computer. Now it says: Signature is timestamped. You can also check the certificate of the TSA in the signature properties. That solves the potential problem of antedated documents. The next listing can be used to add a timestamp (if withTS is true) and to check the revocation status of the certificate with OCSP (if withOCSP is true).
Listing 12.21 Signatures.java
PdfReader reader = new PdfReader(src); FileOutputStream fout = new FileOutputStream(dest); PdfStamper stp = PdfStamper.createSignature(reader, fout, '\0'); PdfSignatureAppearance sap = stp.getSignatureAppearance(); sap.setVisibleSignature( new Rectangle(72, 732, 144, 780), 1, "Signature"); sap.setCrypto(null, chain, null, PdfSignatureAppearance.SELF_SIGNED); PdfSignature dic = new PdfSignature( PdfName.ADOBE_PPKLITE, Creates new PdfName("adbe.pkcs7.detached")); signature dic.setReason(sap.getReason()); dictionary dic.setLocation(sap.getLocation()); dic.setContact(sap.getContact()); dic.setDate(new PdfDate(sap.getSignDate())); sap.setCryptoDictionary(dic); int contentEstimated = 15000; HashMap<PdfName,Integer> exc Reserves space = new HashMap<PdfName,Integer>(); for signature exc.put(PdfName.CONTENTS, content new Integer(contentEstimated * 2 + 2));
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