Q: when is a backdoor not a backdoor?

An encryption backdoor isn’t the same as a house backdoor: the metaphor is faulty.

A: when you’re a politician.

I’m getting pretty bored of having to write about this, to be honest. I’ve blogged twice already on encryption backdoors:

But our politicians keep wanting us to come up with them, as the Register helpfully points out – thanks, both the UK Prime Minister and FBI Director.

I feel sorry for their advisers, because all of the technical folks I’ve ever spoken to within both the UK and US Establishments[1] absolutely understand that what’s being asked for by these senior people really isn’t plausible.

I really do understand the concern that the politicians have. They see a messaging channel which bad people may use to discuss bad things, and they want to stop those bad things. This is a good thing, age part of their job. The problem starts when they think “it’s like a phone: we have people who can tap phones”. Those who are more technologically savvy may even think, “it’s like email, and we can read email.” And in the old days[3], before end-to-end encryption, they weren’t far wrong.

The problem now is that many apps these days set up a confidential (encrypted) link between the two ends of the connection. And they do it in a way which means that nobody except the initiators of the two ends of the connection can read it. And they use strong encryption, which means that there’s no easy for anyone[4] to break it.

This means that it’s difficult for anyone to read the messages. So what can be done about it, then? Well, if you’re a politician, the trend is to tell the providers of these popular apps to provide a backdoor to let you, the “good people” in.

Oh, dear.

I believe that the problem here isn’t really that politicians are stupid, because I honestly don’t think that they are[5]. The problem is with metaphor. Metaphors are dangerous, because humans need them to get a handle on an aspect of something which is unfamiliar, but once they’ve latched on to a particular metaphor, they assume that all the other aspects of the thing to which the metaphor refers are the same.

An encryption backdoor isn’t the same as a house backdoor: the metaphor is faulty[6].

The key[7] similarity is that in order to open up your house backdoor, you need a key. That key gives you entry to the house, and it also allows any other person you give that key access to it, as well. So far, so good.

Here’s where it gets bad, though. I’m going to simplify things a little here, but let’s make some points.

  1. When you give a backdoor key to somebody, it’s not easily copyable if somebody happens to see it. In the electronic world, if you see the key once, you have it.
  2. The cost of copying an electronic key is basically zero once you have it. If one person decides to share the key indiscriminately, then the entire Internet has it.
  3. Access to a house Backdoor let’s you see what’s in the house at that particular moment. Access to an electronic backdoor lets you look at whatever the contents of the house were all the way up to the time the lock was changed, if you’ve taken copies (which is often easy).
  4. And here’s the big one. When you create a backdoor, you’re creating a backdoor for every house, and not just one. Let’s say that I’m a house builder. I’m very, very prolific, and I build thousands of houses a week. And I put the same lock in the backdoor of every house that I build. Does that make sense? No, it doesn’t. But that’s what the politicians are asking for.

So, the metaphor breaks down. Any talk about “skeleton keys” is an attempt to reestablish the metaphor. Which is broken.

What’s the lesson here? We should explain to politicians that backdoors are a metaphor, and that the metaphor only goes so far. Explain that clever people – clever, good people – don’t believe that what they (the politicians) think should be done is actually possible, and the move on to work that can be done. Because they’re right: there are bad people out there, doing bad things, and we need to address that. But not this way.


1 – the capital “E” is probably important here. In the UK, at least, “establishment” can mean pub[2].

2 – and people in pubs, though they may start up clued up, tend to get less clever as the evening goes on, though they may think, for a while, that they’re becoming more clever. This is in my (very) limited experience, obviously.

3 – 10 years ago? Not very long ago, to be honest.

4 – well, who’s owning up, anyway.

5 – mostly.

6 – or Fawlty, for John Cleese fans.

7 – ooh, look what I did there.

Author: Mike Bursell

Long-time Open Source and Linux bod, distributed systems security, etc.. Now employed by Red Hat.

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