Re: anti-GAK design principles: worked example #1

[Jon Callas <jon@pgp.com>]

At 11:45 PM 10/15/97 +0100, Adam Back wrote:
   

Okay, Adam, I'll be civil here, but here's something I want to note:

You've ranted, raved, politicized, propagandized, given ad hominem attacks,
and stated the opinion that anyone who disagrees with you is evil. You've
sent flames to our internal development lists, which is at least impolite.
Yet you say, "constructive criticism only." Sure. I'd like an apology from
you, though. Deal?


   
>   Also I have been accused of using "lots of anti GAK rhetoric, but
>   giving no proposals" by Kent.  I reject that claim.  (I did use lots
>   of rhetoric, but this was to try to impress upon those arguing for CMR
>   of it's dangers.  They do not seem to acknowledge them.) I'll try in
>   this post to steer clear of anti-GAK rhetoric.  We'll instead take it
>   as a given that pgp5.5 and pgp5.0 are GAK compliant because of CMR and
>   that this is a bad thing.

Uh huh. Steer clear of rhetoric, but we'll take it as a given that you're
right and everyone else is wrong. At least this is a de-escalation.
   
>   Design 1.
>   
>   Instructions:
>   
>   - scrap the CMR key extension
>   
>   - store a copy of the private half of the users PGP encryption key
>     encrypted to the company data recovery key on the users disk.

Okay -- constructive criticism only. I sincerely hope I'm reading this
correctly. You're saying that someone's private key should be encrypted to
the corporation's key. This sounds like key escrow to me. How does this
differ from the overly strict, nit-picking, freedom-threatening definition
that I gave? 

This is better than the throw-the-floppy-in-the-safe model in that the
company-readable version of your key is sitting on your machine. That's good. 

I see a threat here that if the corporation backs up my disk, they they
have my secret key and thus can read all files that key has ever encrypted.
This is bad. Normally, if they back up my system, they have my secret key,
but they have to hack my secret key. Most people's passphrases are easier
to crack than a public key, but I think this is worse.

With this system, the corporation can read everything I encrypt with that
key, because they effectively own it. Encrypting my secret key to them
essentially gives it to them. With CMR, I have the option of making some
files readable, and some not. This isn't necessarily a good thing -- some
companies want access to all data, and your proposal helps them.

I'm actually very surprised by this design of yours. On the scale of
property-balanced-with-privacy, you've come down hard on the side of
property. Your system makes it so that an employee of a company can *never*
use this key for a purpose the company can't snoop on. This isn't
necessarily bad, I think that people *should* have separate keys for work
and personal use. This just makes the work key definitely the work key. A
number of our customers will like that.
   
>   - (optional) design the software to make it hard to copy the data
>     recovery packet from the disk, hide the data, bury it in keyrings,
>     stego encode it, whatever, use your imagination.  This is to attempt
>     to restrict the third parties ability to by pass the principle of
>     non communication of recovery information
   
This is security through obscurity. We publish our source code, so this
won't work.
   
>   Recovery method:
>   
>   Custodian of recovery key inserts recovery floppy disk in machine,
>   decrypts copy of users private key, hands control back to user to
>   choose new passphrase.

Choosing a new passphrase is not sufficent. If the custodian ever uses that
key, it *must* be revoked, a new encryption key issued, and all data
encrypted with it re-encrypted. There is also the problem of
re-distributing the revocation and new encryption key to all the people who
have your old one. This is no worse than any other revocation problem, but
CMR does not require revoking the user's key.   
   
>   Possible objections:
>   
>   objection #1. what if disk burns?
>   counter #1:   backup your disk
>   
>   objection #2: users don't back up disks
>   counter #2:   that is a good way to loose data :-) if they don't have
>                 the data the key protecting the data won't help them

This is no different with CMR. One of the design goals of CMR is to avoid
the myriad logistic and security problems associated with data archival.
   
>   GAK-hostility rating:
>   
>   Harder to pervert for GAK than pgp5.5 / pgp5.0 CMR design.

Why? With your mechanism, if the G manages to A the K, then they can
decrypt every message that key has ever encrypted. I think this is a design
flaw. 
   
   
>   I'd be interested to see Will, or Hal, or other PGPer's criticisms of this
>   simple modification, perhaps criticisms could most constructively answer:
>   
>   - what is stopping you implementing this
>   - are there any plug ins which can't cope with this
>   - are there user requirements which it can't meet
>   - is there some fundamental flaw you think I have missed
>   - can you see ways that this could be perverted to implement GAK
>     (yes I can too, btw, but...)
>   - are those ways logisitically harder for GAKkers to acheive than for CMR
>   
>   Please be specific, no general waffle about understanding the
>   complexities of balancing user ergonomics, user requirements etc.
>   That is a no-brainer, you need to do this analysis, the cost function
>   for evaluating such design issus is now expressed explicitly in design
>   principle 4 rather than being assumed.  List problems and explain the
>   significance of the all important deployability criteria.
>   
>   Cryptographic protocol designs are very flexible; most design goals can
>   be met, or worked around I claim within the positive GAK-hostility
>   side of the cryptographic protocol and product design solution space.
>   
>   Lastly, I would encourage readers to be critical of the GAK-hostile design
>   principles themselves:
>   
>   - can you see any aspects which inaccurately reflect trade-offs
>   - can you see methods to bypass inadvertently or deliberately the design 
>     that might require another corollary to correct.
>   
>   In anticipation of constructive criticism,

Okay, general observations:

I'm really surprised at this. In the continuum between privacy and
property, you've come down hard on the side of property. You've said that a
key owned by a corporation is *fully* owned by the corporation, and any
employee who uses it for personal purposes is daft. This is not what I
expected you to be arguing.

Enforcement. Most corporations want some level of enforcement on their
policies. The enforcement we put in isn't fool-proof, but it's far easier
to comply than resist. This is a design goal. I have a concern that the
only enforcement that the corporation has is to take your private key. If
this is their only way to make you follow their rules, they'll do it. Many
of them will play nice if possible, but hardball if they have to.

Fair-warning. In my first missive, I talked about my own principles, and
one of them is the "fair-warning" principle. It states that users should
know what is going on. If you have a key that is used in this system, there
is nothing in it that tells me that your company can read a message I send
you. I see this as a flaw, and one that I consider to be a *very* big deal.
Full disclosure is one of my hot buttons.



I think this is breaks a number of your principles.

Principle 1: The end-user's keys *are* escrowed with the company. If my
disk is ever backed up, then the corporation has my secret key. In order to
keep it from being implicitly escrowed, I have to put it someplace like
off-line media that can be gotten to if I'm hit by a bus. If you disagree,
please tell me how this is different from escrow.

Principle 2: The corporation is always a tacit crypto-recipient. It's no
different than CMR, and has the additional disadvantage that senders don't
know that the implicit receivers are there.

Principle 3: Again, the corporation is a tacit recipient in *all* uses of
the key. With CMR, they are an explicit recipient, and it's possible to
exclude them. There's no way to exclude the corporation here.

Principle 4: I don't see how this differs between your proposal and CMR.


Lastly, I here's a summation of what I think.

I think it's an interesting proposal. You're much more of a
corporate-control proponent than I am. I think control and privacy have to
be balanced, whereas you obviously think corporate control is trump. We
disagree there.

I am uncomfortable at the ease with which the end user can lose their key.
The end user must somehow prevent the employer from even so much as backing
up their computer, or it's just plain escrow.

I am uncomfortable not only with your siding with the corporation against
the employee's privacy, but also with your siding against the privacy of
someone who sends a message to the employee. Furthermore, I think that the
absence of a disclosure mechanism in your protocol is for us, a fatal flaw.
We'd never implement a system that does not have disclosure.

I do not see how your system is GAK-hostile. I think it is no more
GAK-hostile than CMR, and potentially more GAK-friendly, because it is
based around manipulating the actual secret key material. The failure mode
of CMR is that an adversary can decrypt messages, whereas the failure mode
of your proposal is that the adversary gets the key.

	Jon

   
   Adam
   
   [1]
   ==============================8<==============================
   GAK-hostile design principles
   
   If we take the design goal of designing systems including
   confidentiality which are not GAK compliant, we can most succinctly
   state this design goal as the task of ensuring that:
   
   - at no point will any data transferred over communications links be
     accessible to anyone other than the sender and recipient with out
     also obtaining data on the recipient and/or senders disks
   
   
   We can then derive the design principles required to meet the design
   goal of a non-GAK compliant system with confidentiality services down
   to ensuring that:
   
   principle 1:
      no keys used to secure communications in any part of the system are
      a-priori escrowed with third parties
   
   principle 2:
      second crypto recipients on encrypted communications are not
      used to allow access to third parties who are not messaging
      recipients manually selected by the sender
   
   principle 3:
      communications should be encrypted to the minimum number of
      recipients (typically one), and those keys should have as short a
      life time as is practically possible
   
   principle 4:
      deployment wins.  violating any of principles 1 to 3 whilst
      still retaining some GAK-hostility can be justified where
      deployment is thereby increased to the extent that the violations
      increase the degree of GAK hostility in the target jurisdictions
      overall
   
   Corrollary 1: Included in design principle 2) is the principle of not
   re-transmitting keys or data after decryption over communication
   channels, re-encrypted to third parties -- that is just structuring --
   and violates design principle 2.
   
   Corrollary 2: where communications are transmitted which violate
   principles 1, 2 or 3 it is in general more GAK hostile to enforce as
   far as possible that the recovery or escrow information remains in as
   close proximity to the data as possible.
   
   Corrollary 3: where communications are transmitted which violate
   principles 1, 2 or 3 it is in general more GAK hostile to make these
   communications as difficult to automate as possible.  For example no
   scripting support is given to enforce that GUI user interaction is
   required, and/or that the process is made artificially time consuming,
   and/or that the communication must not use electronic communication
   channels
   
   ==============================8<==============================
   
   

-----
Jon Callas                                  jon@pgp.com
Chief Scientist                             555 Twin Dolphin Drive
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