How to lock pdf with digital signature

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   Hello. 

We are implementing embedded digital signatures in PDF files. One of the requirements is to be able to provide a locking functionality, preferably after a signature has been placed. 

What we found so far is to use the Encrypt function to prevent any changes to the file but when there is a signature in the file it is no longer valid after encrypting. 

The other thing that we tried is to encrypt the file first and add the signatures later. That works on Windows 10 but on Windows 7 the file requires a password to be provided. The Encrypt function is being called with an empty string for parameter user password. 

Is there any other way to lock the pdf file or a fix for Windows 7? 

*Edit * 

After further testing was discovered that the OS is not the factor in the password problem, but the version of the pdf file. The problematic version is 1.4. 

I have tried to change the version to 1.5 (since files with that version do not preset the problem) but even after that password is required. 

Thanks. 

Edited by mimkawee - 10 May 21 at 11:23AM

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mimkawee Members Profile Find Members Posts Beginner Joined: 10 May 21 Status: Offline Points: 4	Post Options Post Reply Quote mimkawee Report Post Thanks(0) Quote Reply Topic: How to lock pdf with digital signature Posted: 10 May 21 at 10:02AM
	Hello. We are implementing embedded digital signatures in PDF files. One of the requirements is to be able to provide a locking functionality, preferably after a signature has been placed. What we found so far is to use the Encrypt function to prevent any changes to the file but when there is a signature in the file it is no longer valid after encrypting. The other thing that we tried is to encrypt the file first and add the signatures later. That works on Windows 10 but on Windows 7 the file requires a password to be provided. The Encrypt function is being called with an empty string for parameter user password. Is there any other way to lock the pdf file or a fix for Windows 7? Edit After further testing was discovered that the OS is not the factor in the password problem, but the version of the pdf file. The problematic version is 1.4. I have tried to change the version to 1.5 (since files with that version do not preset the problem) but even after that password is required. Thanks. Edited by mimkawee - 10 May 21 at 11:23AM

Ingo Members Profile Find Members Posts Moderator Group Joined: 29 Oct 05 Status: Offline Points: 3529	Post Options Post Reply Quote Ingo Report Post Thanks(0) Quote Reply Posted: 10 May 21 at 9:14PM
	Hi Mariya, from my understanding signing is the last process on a pdf. It's almost like a seal. You can't change it the other way round. This is what's Google telling: The digital signature and the PDF document are linked cryptographically and given a tamper-proof seal. Clearly. The personal data and the identity of the signatory are legitimized using a unique digital certificate and a PIN. This means to you entering an encryption afterwards will broke the signature/the seal. Cheers and welcome here, Ingo
	Cheers, Ingo

mLipok Members Profile Find Members Posts Senior Member Joined: 23 Apr 14 Location: Poland, Zabrze Status: Offline Points: 453	Post Options Post Reply Quote mLipok Report Post Thanks(0) Quote Reply Posted: 17 May 21 at 3:32PM
	There is no need to lock pdf. There is a need to check if was or not modified.
	Here you can find description how to test my examples: http://www.quickpdf.org/forum/forum_posts.asp?TID=2932&PID=12600&title=drawcapturedpagematrix-matrix-howto#12600