can you see any problem with this?

[Adrian Thurston]

My application is here:

http://www.cs.queensu.ca/~thurston/fif/

I'm encrypting messages and making them publicly available over static
HTTP. Anyone who knows the right URL can grab a message and I don't want
recipients to be identifiable.

Another issue is that the number of recipients and the size of messages
may both get very large. A single encrypted message is therefore very
attractive.

Thanks,
 Adrian

David Shaw wrote:
> On Tue, Feb 05, 2008 at 12:02:48PM -0500, Adrian Thurston wrote:
>> Hi,
>>
>> I'd like to serve messages that have been encrypted to a large number of
>> people, however I don't want to reveal the list of recipients so I'm
>> going to use --throw-keys.
>>
>> But speed at the decryption end is a concern, so I thought I would break
>> up an encrypted message into packets and when a client requests it serve
>> up only the packet that corresponds to the session key encrypted to
>> them, then the content packet. I haven't tried it yet, but it seems as
>> though it should work. I'd like to know if there is any non-obvious
>> reason why it is a bad idea.
> 
> It's hard to really answer this as there isn't enough information to
> say one way or another.
> 
> Speaking strictly to your question, and not the "is this wise"
> question, if I understand it, you are proposing encrypting to a large
> number of people, breaking the resultant encrypted message into many
> PKESK packets (one per recipient) and one encrypted packet.  Then,
> send each recipient their own PKESK plus the encrypted packet.
> 
> So, yes, that would work.  GPG even ships with the tools to make such
> a message.  And it's safe to do so with the caveat that every user
> will have the same encrypted message and be able to decrypt it.  On
> the one hand, no big deal, becuase you sent everyone the same message,
> so you clearly wanted them to have it.  On the other hand, it gives
> Alice the ability to know that Baker got the same message that Alice
> did.  Whether that is important or not depends on what you are doing.
> 
> Also, given that you are only sending each recipient their own PKESK,
> why bother to use --throw-keyid ?  It might be easier to just encrypt
> the whole message to each recipient individually rather than do all
> the packet surgery.
> 
> David
> 
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