DaveNet: Tuesday, October 7, 1997; by Dave Winer.
More on PGP from PGP
From Jon Callas, jon@pgp.com, Chief Scientist, Pretty Good Privacy, Inc.:More on PGP from PGP
I have a number of comments about the New York Times article on PGP 5.5.
If we had built what they said we had, then we'd deserve of all the derision people have directed at us. But we didn't. The New York Times got it flat wrong.
I'll describe what we built, how it works, and its limitations. But first, some background on the problem we're trying to solve in PGP 5.5.
A couple of years ago, the government sugarcoated their surveillance plans by switching from "key escrow" to "key recovery," and trying to sell surveillance to people by pointing out some of the downsides of strong cryptography, and selling key recovery as the way around them.
One of the downsides of cryptography is that if you lose your passphrase (or token, PIN, smart card, or whatever), you've lost your data. My favorite way of expressing this problem is, "if you lose the keys to your car, then you have to get a new car."
This downside is particularly insidious for a number of reasons. First, without fixing that problem, strong cryptography will be in some sort of limbo. You want to use it to protect your valuable information, but you won't want to use it for any information that's *too* valuable, because it's easily lost. Crypto-protected information is fragile, and this fragility could hurt its widespread deployment.
Worse, this gives the government a rationale for regulating cryptography. Like it or not, government has a mandate to protect the people from dangerous technologies, be they in foods, drugs, autos, or information technologies. Many people believe that the government uses this mandate as a rationale for acquiring power, many people would prefer that they let us take our chances, but that's not germane to this discussion.
It *is* germane to note that when you hear some ham-fisted remark about how surveillance is like air bags, they are saying: they have to protect people from dangerous things, crypto is dangerous, therefore they have to protect people from crypto.
When they started mumbling along these lines, the privacy community got their act together and started describing what we believe to be the real solution. This is called "data recovery." The first time I heard the term, I hated it. I still hate it. The reason I hate it is that it's got the word "recovery" in it, which makes it sound to someone who isn't paying a lot of attention that all recovery systems are basically the same thing.
Most of the people in the world don't pay a lot of attention most of the time. When I was at HIP97 this August, I was amused to hear cypherpunks chanting, "Data recovery good, key recovery bad." The sublimely Orwellian tone of this mantra makes me laugh and cringe at the same time. (To explain the reference, in Orwell's "Animal Farm," there's a revolution in the farm and the animals take over, run by the pigs. One of the slogans they have is "four legs good, two legs bad." By the end of the book, the pigs are nigh indistinguishable from the people. But I digress.)
The essence of data recovery is that focusing on the keys is a canard. If you've misplaced your data, you want the data back, not the keys. The only people who want your keys are people who want to spy on you. If you've locked yourself out of your car, you want the use of your car, not the just the key. Thus, the solution to encrypted data being fragile is to let people get to the data. Simple, obvious, but subtle, because the key to getting the data is the key.
If you don't like data recovery, you aren't going to like what we did in PGP 5.5 -- we built a data recovery system. Some people aren't going to like it, and some of those will think this missive is a load of self-serving twaddle. Myself, it gives me the same mildly uncomfortable feeling that fake rocks for spare keys do, or skeleton keys do, financial audits, or any other similar technology. Uncomfortable feelings aside, if the fragility problem is not solved, then many people who should be using crypto won't, and government will continue to view this problem as a question of public safety, and thus in their mandate to regulate.
Data recovery is useful for a number of things. Perhaps you lost your passphrase. Or data might have been encrypted by an employee or co-worker who was in an accident. (As an aside, fifteen years ago, the architect of a product I worked on was in a severe car wreck. He was not killed, but suffered brain damage and has never returned to work.) Your spouse might need access to financial records. Everyone, be they an individual, business, or corporation has a right to having their data protected, and protection not only means being able to put it into a safe, but getting it out of that safe later.
What makes data recovery different from key recovery? In my opinion, data recovery allows you to get encrypted data without compromising the key of the person who encrypted it. Data is property, and keys are property. An ethically built system allows emergency access to data without destroying the property of the key owner.
Ethically built data recovery software has a number of properties:
(1) It is surveillance-surly. It should be impossible or unwieldy for an adversary, be they government (yours or foreign), dirtballs (such as crackers), business competitors (who sometimes count as dirtballs), or others to use this against you. The system should also be aware of how passive surveillance (like traffic analysis) interacts with it.
(2) It is an "opt-in" system. Users must consent to it, and must take some action to start using it. It should be as easy as possible to stop using the system. The system must also allow someone who does not opt in to use all the system's features. Please note that abuses of consent (for example, an employer who says, "consent or we fire you") are something we can't prevent in any system.
(3) It must obey the principle of fair warning. If you send me a message that is subject to data recovery, you should know that before sending the message. This way, if you don't agree with my policy, you can decide not to send me that message. This interacts closely with the opt-in principle above.
(4) The data recovery system should be preferable to an escrow system. A number of corporations who use PGP keep copies of their employees' secret keys. This is both odious and dangerous. Escrowed keys are a target for attackers, subpoena-bait, and potentially ruin the value of digital signatures. It's just bad policy.
(5) The system has to allow someone under a legal threat to respond effectively to that threat. Legal threats include warrants, subpoenas, and discovery processes. You have to be able to respond to the request for information without losing your keys and thus all of your data.
(6) It must also provide a response to those who would regulate crypto in the name of public safety. Fortunately for us, potential regulators have focused on the horsemen of the infocalyse. There are other pseudo-public-welfare threats including the rights of a person to their spouse's records, or the rights of heirs to information property. We, the people who design privacy systems, have to think about what happens when the regulators stop dragging out the pornographers and start dragging out the poor widows and orphans.
Note that these requirements are not completely consistent with each other. For example, an opt-in system is riskier than an opt-out system, yet friendlier to one's own privacy. Balancing these requirements is part of the difficulty of good software design.
If you have been skimming the above, wondering when I'm going to get around to actually telling you what we did in PGP 5.5, this is it.
With PGP 5, there are a number of attributes of your key that are stored in a self-signature. For example, your preferred symmetric algorithms are stored in your self-signature. The data recovery feature -- which we call "Corporate Message Recovery" -- is an attribute in your self-signature that tells anyone who receives your key that you want messages encrypted to you to also be encrypted to that other key. There is also a flag that tells the encryptor, "please" or "I insist." Architecturally, there can even be more than one recovery key. That's it. That's all it is.
Well, that's mostly all it is. There are other bits of the system. For example, if I look up Alice's key on a key server and Alice has a recovery key, I get Alice's recovery key, too. If Alice's recovery key is a "please use" key, then I can encrypt to Alice alone. In any case, the PGP software tells me that Alice has a recovery key, so I can decide to use some other mechanism to talk to her.
Note that design satisfies the opt-in and fair-warning requirements. Also, since Alice's recovery key is an attribute of her self-signature, she can change it. She can even have a second user name (let's call it Bob), that has no recovery key.
Also, we have three encryption products: PGP freeware, PGP for Personal Privacy, and PGP for Business Security. Corporate Message Recovery is included *only* in PGP for Business Security. It is not, and never will be, in either the freeware or the Personal Privacy product. It is an extra cost item that we created for businesses as per their requirements. As I stated above, a number of these businesses keep copies of their employees' secret keys. One of the reasons we created this feature is to satisfy their requirements with some mechanism that is less blunt than key escrow.
When a PGP message is formed, there are a number of "packets" that make up the message. The usual construction is that there is a "session key" packet for each public key that the message is to be read by. Following that is the actual message packet, that is encrypted with a symmetric cypher to session key. The session key packets specify the *key*, by its 64-bit key ID.
This is an important and subtle point. Let's go back to Alice, a.k.a. Bob. The information that specifies a recovery key is in a self-signature of a user name, but the session key specifies a public key by keyID. It is impossible, solely from looking at a message, to know if it is addressed to Alice or Bob because that information is not stored in the message. A message that does not honor recovery is syntactically correct.
I don't know why Bruce Schneier said that this is everything the FBI wants. If it is, then they have changed spots! One of the major ways PGP's system differs from anything else I've seen is that it has no enforcement built into the protocol. This helps make PGP surveillance-surly, with or without Corporate Message Recovery. If this is all the FBI needs, then they've decided the way to get your files is to knock on your door with a warrant, and that's a big, big, big step forward.
Getting back to the system, I'm sure you've noticed a gotcha there. If I mail Alice a message that I encrypted to Bob, she can decrypt it, but the recovery key can't. If you've been paying very close attention, you have wondered something akin to, "hmm, if Alice's key accidentally lost its self-signature, there would be no way to encrypt to her recovery key." You're right. If Alice really wants recovery on messages sent to her, then she has to use our SMTP Policy Management Agent.
The policy management agent is an SMTP proxy. You can configure it to do a number of things. Most relevant to this discussion is that Alice can use the policy agent to require that her recovery key gets used. However, the policy agent does *not* decrypt the message. One of the very good features of Business PGP is that it does not decrypt the message. It does not prevent or even try to prevent multiple-encryption. It's really, really easy to encrypt a message to Alice alone, and then encrypt that message to both Alice and her recovery key. We're not going to change that. Nor does the policy management agent archive messages, make copies, notify your mother, or any of the other things we've been accused of doing with it. It's simply the gatekeeper that enforces Alice's corporate policy.
To sum up, we created the Corporate Mesage Recovery feature to satisfy the requirements of our customers who need emergency access to data. We made careful decisions to make it useful and effective for honest people, while minimizing its potential for abuse. No one has to use it; we do not include it with PGP freeware, nor with PGP for Personal Privacy. We alert all users of all products when they encrypt to someone who has a message recovery key. It is an opt-in system that you can opt out of. It is not a surveillance system. A few weeks ago, we showed it to the FBI and asked their opinion. They told us it doesn't meet any of their needs.