Add documentation for xmlsecurity

* Add basic information
* Add some references

Change-Id: Id036c23f90f53c36052b155f62f68e0000e0266f
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/130289
Tested-by: Jenkins
Reviewed-by: Hossein <hossein@libreoffice.org>

1 files changed, 30 insertions, 2 deletions

diff --git a/xmlsecurity/README.md b/xmlsecurity/README.md
index 796094901112..b983f5dc5eb5 100644
--- a/xmlsecurity/README.md
+++ b/xmlsecurity/README.md
@@ -1,4 +1,32 @@
 # Stuff for Document Signing
 
-This code provides dialogs, and infrastructure wrapping `libxmlsec` and `gpgme` that
-implements document signing.
+This code provides dialogs, and infrastructure wrapping `libxmlsec` and
+`gpgme` that implements document signing.
+
+For signing a document, a personal key pair is used, which consists of a
+private key and a public key, which is added to the document in addition
+to the digital signature of the document, when signing it.
+
+The document signing can be done both for the source ODF/OOXML files and
+the exported PDF files. It is also possible to sign existing PDF files.
+
+To test the signed PDFs, one can use the `pdfverify` in this way:
+
+    ./bin/run pdfverify $PWD/xmlsecurity/qa/unit/pdfsigning/data/2good.pdf
+
+The file parameter should be an absolute path.
+
+This is the output of `pdfverify` for `2good.pdf`:
+
+```
+verifying signatures
+found 2 signatures
+signature #0: digest match? 1
+signature #0: partial? 0
+signature #1: digest match? 1
+signature #1: partial? 0
+```
+
+## References
+* [Adobe: Digital Signatures in a PDF](https://www.adobe.com/devnet-docs/etk_deprecated/tools/DigSig/Acrobat\_DigitalSignatures_in_PDF.pdf)
+* [Adobe: Acrobat DC Digital Signatures - Supported Standards](https://www.adobe.com/devnet-docs/acrobatetk/tools/DigSigDC/standards.html)