There are many ways to address this problem, but the mechanism used in PDF signing is based on X. Here are some of the problems that the validator might have to deal with.
An incremental update is made by appending data to the end of the file. In other words, the onus is on the validator to check whether any incremental updates if present are legitimate or not. To make matters worse, there are currently no standards governing this kind of validation, other than some vague guidelines in the PDF standard.
This makes catering to the lowest common denominator a delicate balancing act. If there are multiple valid chains of trusts, feel free to include those other certificates as well. The size increase is usually not significant, and it helps to account for differences in trust store configurations not everyone validates against the AATL, for example.
I like to include it anyway, if only for documentation purposes. You can provide such a record by having a trusted time-stamping authority TSA issue a time stamp token for your signature. Such a time-stamping token is signed by the TSA. In principle, all certificates that are not self-signed are subject to revocation checking.
However, obtaining accurate revocation information for a certificate at some time in the past is not typically feasible, especially for certificates that have long since expired. There are two revocation checking mechanisms in common use:. These also each have their own advantages and drawbacks.
Just like certificates, revocation information also has an expiry date. Therefore, timestamps are important not just to provide a record of the time of signing, but also to establish the integrity of revocation data. Some document workflows require changes after the first signature was produced. Such documents are by no means rare: digital agreements with multiple parties require multiple signatures.
In PDF-land, that means that the second signature is technically a modification applied to the file signed by the first signer. For this kind of validation, there are much fewer hard and fast rules. Nonetheless, there are a number of general principles to live by; paying heed to those should reduce the possibility of things failing down the road.
In general, whenever you modify a signed document through incremental updates, you should strive to make as few modifications as possible. The fewer changes you apply, the lower the risk. When working with visible signatures i. Making sure the field setup is taken care of beforehand drastically reduces the potential for such mishaps.
Essentially, a certification signature allows you to tell the validator what changes you expect to occur after your signature, which reduces ambiguity. For some of the documents you create, you might want to allow for some kind of interactivity: a contract with multiple parties, a personalized form, etc. You could sign those documents with a certification signature that informs the validator that you want to allow form filling and signing, but nothing else. In other cases e. In that case, you can add a certification signature that disallows all changes, including form filling.
Explicit is better than implicit. XCertificateParser ;. ReadCertificate signature. SignDetached appearance, es, chain, null, null, null, 0, CryptoStandard. I just want to thank you!! I was looking for a clear explanation using the latest version of this librairy! I was quite surprised to see the changes done over the year in this library without clear explanation. I understand that This code was assembled quickly and can certainly be improved.
But I needed something and yes I found it, thanks a lot for this. This is good example that i never seen any where on the internet. Exportable XKeyStorageFlags. PersistKeySet ; Logger. PrivateKey; Logger. GetRsaKeyPair rsa ; pemWriter. WriteObject keyPair. Private ; streamWriter. Flush ;. LogErrorMessage ex. My senerio is so deficult i worked on that from last 1. Please let me know if you can help me.
Open ;. PersistKeySet ;. AppendLine Convert. ToBase64String certificate. Export XContentType. Cert , Base64FormattingOptions.
InsertLineBreaks ;. ToString ;. InvalidOperationException: The signature timestamp could not be verified. What is privateKeyStream? How can i get privateKeyStream? ToCharArray ;. Sorry, i forgot. The signature timestamp could not be verified. How i can read certificates from personal store and sign it? GetKeyPair signature. SignDetached appearance,es, chain, null, null, null, 0, CryptoStandard. ReadCertificate st. Rectangle Convert.
ToInt32 X. Trim , Convert. ToInt32 Y. ToInt32 X1. ToInt32 Y1. Trim , int. Parse TxtPageNo1. Text , null ;. Your code is superb, but as i am a new to asp. But i experience some issues here. When i first sign and then try to lock it finally i am loosing the signature. When i firts lock and then sign then i am loosing the lock SetEncryption. Do you have any idea how i can implement that? One for this one. Otherone while i try to verify CertificateVerification.
I have a pfx file and generated crt from it. Can you please help. I have included namespace using iTextSharp. With the version of itextsharp. IOException; import java.
InputStream; import java. GeneralSecurityException; import java. Principal; import java. XCertificate; import java. Calendar; import java. List; import org. Logger; import org. Miklos Krivan Miklos Krivan 1, 17 17 silver badges 14 14 bronze badges.
You can check a lot of different aspect of the signature and you can make decision suitable for you. It was a simple deal just return with the verify method result and say Yes or No. The op wanted to know if a given PDF file is already digitally signed , not whether an existing signature is valid or whatever. So while it is nice to mention that one can retrieve numerous properties using current iText versions, an answer to this question should in particular be of interest in the context of the actual question.
What I do not understand how can be proved that a document is digitally signed if the signature attached to it is invalid or the document is changed after was signed. I think in both cases we can not say that the document is digitally signed. It was but after changing now it is not signed really. That is my opinion. I have read again the original question and still I feel my understanding was not bad.
I have checked your profile and I see you have much more experience on PDF security than me but I feel my answer would help to lgr. I hope.
0コメント