PGP 5.5 CMR/GAK: a possible solution

[mark@unicorn.com]

[Posted to cypherpunks, cc:-ed to Jon Callas]

You know, in the last few days I've read a lot of messages on both sides
of this debate, and I've umm-ed and ahh-ed and slowly gone from thinking
that what PGP are doing is evil to thinking that it's just bad. While I
believe that Adam Back's suggestions for alternate systems are good and
neccesary ideas for the long-term, I'm coming to the conclusion that PGP
are heading in the right direction for the short-term, but have things
backwards. 

The current system sends out a user's personal key, with a tag to say that
if I don't encrypt to the company as well, my mail will bounce. But think
about this: how often do I want to send email to a particular person in a
company, and ensure that only they see it? And how often do I want to send
mail to a particular group inside a company? All I want is to ensure that
I get a response from the company, I usually don't care who I talk to in the
process.

So PGP's "everything private unless you choose to make it public" system
seems backwards. Surely what we really need to meet these customer demands
is an "everything public (within the company) unless you choose to make it
private" system? That is, all mail to my department inside the company
should be encrypted to a department key, shared by all members, *unless*
it is confidential, in which case it should *only* be encrypted to me. 

Here's how I see this working: when Joe Blow joins Foo-Bah Cryptosystems,
he creates his own personal PGP key. He also gets a copy of the department
key, which he can use to decrypt any mail which is encrypted to his
department; or this decryption could be handled automatically by a
department email server to ensure that individuals never have access to the
department's private key. PGP then creates a public key for 'Joe Blow
<joe.blow@foo-bah.com>', which would be the department key with a signed tag
linking it to his personal public key. This is the tagged corporate public
key which he would give out to any customers. 

When a customer wishes to send email to Joe, he would use this public key.
When encrypting, PGP would detect the tag and put up a dialog box pointing
out that this is a corporate key and if they click on the 'confidential'
button it will be encrypted to the user's personal key prior to encrypting
to the corporate key (by which I mean superencryption, to avoid traffic
analysis). The default would be not to superencrypt; and as a side effect
this system would be compatible with any version of PGP for
non-confidential mail (assuming that version understands the encryption
algorithms in use). 

The effect of this is that if someone wants to send email about an urgent
bug and I'm out at lunch, any of my co-workers can read that mail. But if
they want to send *me* mail about confidential inter-company negotiations,
the co-workers could decrypt the outer layer of the message, but would be
blocked by the inner layer encryption to my personal key. 

As I see it, this system is simple, solves the problems which PGP claim
they need to solve without creating the snooping problems Tim and others
have discussed, cannot easily be adapted to GAK ('This message is to be
encrypted to the FBI public key. If it is confidential, click here to
superencrypt to the recipient's personal key'), and won't require a
massive change to the PGP source code. 

There are some obvious security issues with having the department key
shared amongst the members of the department, but I don't see that they
are any worse than PGP's current CMR implementation, which has already
discussed the use of department keys; it's certainly better than using
plaintext. There are also problems with encrypting confidential mail to
multiple recipients, but they're surmountable; an easy solution, if you
don't care about traffic analysis, is to only encrypt confidential mail
to the personal key rather than superencrypt with the corporate key. In most 
cases such mail wouldn't be sent to multiple recipients anyway. 

So here's how I'd see the simple system working:

A PGP CMR key would consist of

1. A corporate key; this might be company-wide, department-wide, or
   an individual escrowed key; this choice is a seperate key-management
   issue for the corporation.
2. Optionally a personal key, which could only be decrypted by the
   individual.
3. A signature from the corporate key linking the personal key to it
   and the specified User Id.
4. Optional flag to indicate which key to encrypt to by default.
5. User Id, signatures, etc

When PGP was asked to encrypt to such a key, it would check for the
optional personal key. If it wasn't there, it would put up a warning
box to tell the user that the message can be read by people other than
the recipient. If it is, then it would put up a dialog box allowing the
user to choose whether to encrypt to the corporate key or the individual
key, normally defaulting to the corporate key. This system could not easily
become GAK because it will only encrypt to one of the keys and not both;
the FBI could create FBI CMR keys from all our public keys, but then PGP
would either encrypt to the FBI and I wouldn't be able to read it, or
encrypt to me and the FBI wouldn't be able to read it.

Anyone care to pick any holes?

        Mark

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|Mark Grant M.A., U.L.C.                       EMAIL: mark@unicorn.com  |
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