How to edit PDF?

[Derek Broughton]

Colin Watson wrote:

> On Sun, Nov 27, 2005 at 11:18:02AM -0500, David Teague(T-bird acct) wrote:
>> Tristan Wibberley wrote:
>> >David Teague(T-bird acct) wrote:
>> >>That is one use for MD5 check sums. It doesn't make the file
>> >>inviolable, but it gives the recipient an almost unbreakable check
>> >>against tampering.
>> >
>> >MD5 based signatures probably don't do that anymore.
>> >[...]
>> 
>> Well Dang! Is there a mechanism that is better
>> than MD5 (i.e. that is 'almost' unbreakable) ??
>> 
>> A slightly different way might be to encrypt. There is
>> 128 bit encryption .... is it any good for this purpose?
> 
> Almost certainly not.
> 
> Since anyone can encrypt a document using a public key, the only
> protection that public-key encryption can possibly provide against
> tampering is that it makes it harder to get at the original document to
> figure out what might plausibly be substituted. That's very weak,
> though. Don't use public-key encryption if you want tamper-resistance.

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.
-- 
derek