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Explaining Digital Certificate and Signatures

al4629740
al4629740 asked
I'm trying to understand what a digital certificate is and why I need it to sign a document.  I know in the following instructions I can create my own digital certificate for free, but when and how do I use a digital certificate to sign something?  What is a digital signature in comparison to taking a picture of my signature and pasting it.  

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Sam JacobsCitrix Technology Professional / Director of TechDev Services, IPM
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Commented:
A document signing certificate is used to prove to a user the identity of the creator of a digital resource,
For example, I use my document signing certificate  when I create PowerShell scripts, which adds a digital signature block with a hash to the bottom of my scripts.
This digital signature block does two things:
- It identifies me to the user running the script (if they don't trust me, they should not run the script), and
- It gives the user confirmation that the script has not been modified in any way (or the hash will not match).
al4629740

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Commented:
so how does it relate with an average person signing a pdf or excel sheet?
Sam JacobsCitrix Technology Professional / Director of TechDev Services, IPM
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Commented:
If you send me a signed .PDF or .XLS, I know that it was truly sent by you and that it wasn't altered along the way. The same holds for a signed email.
al4629740

Author

Commented:
I understand that but what am I using to do that?  Is it a third party program?
Citrix Technology Professional / Director of TechDev Services, IPM
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Commented:
While you can create your own digital certificate, it's like creating your own SSL certificate.
It will not be trusted unless it's issued by a known CA, like Entrust, Thawte, DigiCert, etc.

The way you sign a document depends on the document.
I use a PowerShell command to sign my PowerShell scripts.
Adobe PDF has a built-in way to sign .PDF files: https://www.youtube.com/watch?v=7ycTI_A9yhE
Microsoft Office files get signed a different way: https://support.office.com/en-us/article/add-or-remove-a-digital-signature-in-office-files-70d26dc9-be10-46f1-8efa-719c8b3f1a2d

Hope that helps.