How to edit PDF?

[Tony Arnold]

Derek,

Derek Broughton wrote:
> It seems that one of us doesn't understand public-key encryption very
> well :-)  Surely, the idea is that I encrypt the PDF with my _private_ key,
> and readers decrypt it with my public key.  They can't, however, change the
> document because they would then need to re-encrypt it with _my_ private
> key, or any recipient will know it isn't the version I published.  afaik,
> this is a very good way to distribute documents you don't want changed.

This is not right! If I encrypt a document to send to you, then I
encrypt it with your public key and only you can decrypt it with your
private key.

If you want documents to stay unchanged, but you are not too worried
about who reads them, then signing them is the answer. I would sign a
document with my pivate key and anyone can verify that signature using
my public key.

For top security, one can do both, i.e., encrypt for a given reader(s)
and sign by the sender. That way secrecy can be preserved as well as
integrity.

Regards,
Tony.
-- 
Tony Arnold, IT Security Coordinator, University of Manchester,
IT Services Division, Kilburn Building, Oxford Road, Manchester M13 9PL.
T: +44 (0)161 275 6093, F: +44 (0)870 136 1004, M: +44 (0)773 330 0039
E: tony.arnold at manchester.ac.uk, H: http://www.man.ac.uk/Tony.Arnold