gpg --ask-cert-level considered harmful

Occasionally, someone asks me whether we should encourage use of the --ask-cert-level option when certifying OpenPGP keys with gpg. I see no good reason to use this option, and i think we should discourage people from trying to use it. I don't think there is a satisfactory answer to the question "how will specifying the level of identity certification concretely benefit anyone involved?", and i don't see why we should want one.

gpg gets it absolutely right by not asking users this question by default. People should not be enabling this option.

Some background: gpg's --ask-cert-level option allows the user who is making an OpenPGP identity certification to indicate just how sure they are of the identity they are certifying. The user's choice is then mapped into four levels of OpenPGP certification of a User ID and Public-Key packet, which i'll refer to by their signature type identifiers in the OpenPGP spec:

0x10: Generic certification

The issuer of this certification does not make any particular assertion as to how well the certifier has checked that the owner of the key is in fact the person described by the User ID.

0x11: Persona certification

The issuer of this certification has not done any verification of the claim that the owner of this key is the User ID specified.

0x12: Casual certification

The issuer of this certification has done some casual verification of the claim of identity.

0x13: Positive certification

The issuer of this certification has done substantial verification of the claim of identity.

Most OpenPGP implementations make their "key signatures" as 0x10 certifications. Some implementations can issue 0x11-0x13 certifications, but few differentiate between the types.

By default (if --ask-cert-level is not supplied), gpg issues certificates ("signs keys") using 0x10 (generic) certifications, with the exception of self-sigs, which are made as type 0x13 (positive).

When interpreting certifications, gpg does distinguish between different certifications in one particular way: 0x11 (persona) certifications are ignored; other certifications are not. (users can change this cutoff with the --min-cert-level option, but it's not clear why they would want to do so).

So there is no functional gain in declaring the difference between a "normal" certification and a "positive" one, even if there were a well-defined standard by which to assess the difference between the "generic" and "casual" or "positive" levels; and if you're going to make a "persona" certification, you might as well not make one at all.

And it gets worse: the problem is not just that such an indication is functionally useless; encouraging people to make these kind of assertions actively encourages leaks of a more-detailed social graph than just encouraging everyone to use the default blanket 0x13-for-self-sigs, 0x10-for-everyone-else policy.

A richer public social graph means more data that can feed the ravenous and growing appetite of the advertising-and-surveillance regimes. i find these regimes troubling. I admit that people often leak much more information than this indication of "how well do you know X" via tools like Facebook, but that's no excuse to encourage them to leak still more or to acclimatize people to the idea that the details of their personal relationships should by default be public knowledge.

Lastly, the more we keep the OpenPGP network of identity certifications (a.k.a. the "web of trust") simple, the easier it is to make sensible and comprehensible and predictable inferences from the network about whether a key really does belong to a given user. Minimizing the complexity and difficulty of deciding to make a certification helps people streamline their signing processes and reduces the amount of cognitive overhead people spend just building the network in the first place.

Syndicated 2013-05-20 07:21:00 from Weblogs for dkg

OpenPGP User ID Comments considered harmful

Most OpenPGP User IDs look like this:

Jane Q. Public <jane@example.org>

This is clean, clear, and unambiguous.

However, some tools (gpg, enigmail among others) ask the user to provide a "Comment:" field when they are choosing a new User ID (e.g. when making a new key). These UI prompts are evil. The savvy user knows to avoid entering anything in this field, so that they can end up with a User ID like the one above. The user who provides something here (perhaps even something inconsequential like "I like strawberries", due to not being sure what should go in this little box) will instead end up with a User ID like:

Jane Q. Public (I like strawberries) <jane@example.org>

This is bad. This means that Jane is asking the people who certify her key+userid to certify whether she actually likes strawberries (how could they know? what if she changes her mind? should they revoke their certifications?) and anywhere that she is referred to by name will include this mention of strawberries. This is not Jane's identity, and it doesn't belong in an OpenPGP User ID packet.

Furthermore, since User IDs are atomic, if Jane wants to change the comment field (but leave her name and e-mail address the same), she will instead need to create a new User ID, publish it, get everyone who has certified her old key+userid to certify the key+newuserid, and then revoke the old one.

It is difficult already to help people understand and participate in the certification network that forms that backbone of OpenPGP's so-called "web of trust". These bogus comment fields make an already-difficult task harder. And all because of strawberries!

Tools like enigmail and gpg should not expose the "Comment:" field to users who are generating keys or choosing new User IDs. If they feel it absolutely must be present for some weird corner case that 0.1% of their users will have, they could require that the user enters some sort of "expert mode" before prompting the user to do something that is likely to be a mistake.

There is almost no legitimate reason for anyone to use this field. Let's go through some examples of this people use, taken from some examples i have lying around (identifying marks have been changed to protect the innocent who were duped by this bad UI choice, but you can probably find them on the public keyserver network if you want to hunt around):

domain repetition

John Q. Public (Debian) <johnqpublic@debian.org>

We know you're with debian already from the @debian.org address. If this is in contrast to your other address (johnqpublic@example.org) so that people know where to send you debian-related e-mail, this is still not necessary.

Lest you think i'm just calling out debian developers, people with @ubuntu.com addresses and (Ubuntu) comments (as well as @example.edu addresses and (Example University) comments and @example.com addresses and (Example Corp) comments) are out there too.

nicknames already evident

John Q. Public (Johnny) <johnqpublic@example.net>
John Q. Public (wackydude) <wackydude@example.net>

Again, the information these comments are providing offers no clear disambiguation from the info already contained in the name and e-mail address, and just muddies the water about what the people who certify this identity should actually be trying to verify before they make their certification.

"Work"

John Q. Public (Work) <johnqpublic@example.com>

if John's correspondents know that he works for Example Corp, then "Work" isn't helpful to them, because they already know this as the address that they're writing to him with. If they don't know that, then they probably aren't writing to him at work, so they don't need this comment either. The same problem appears (for example) with literal comments of (School) next to their @example.edu address.

This is my nth try at this crazy system!

John Q. Public (This is my second key) <johnqpublic@example.com>
John Q. Public (This is my primary key) <johnqpublic@example.com>
John Q. Public (No wait really use this one) <johnqpublic@example.com>

OpenPGP is confusing, and it can be tricky to get it right. We all know :) This is still not part of John's identity. If you want to designate a key as your preferred key, keep it up-to-date, get people to certify it, and revoke or expire your old keys. People who care can look at the timestamps on your keys and tell which ones are the most recent ones. You do have a revocation certificate for your key handy just in case you lose it, right?

Don't use this key

John Q. Public (Old key, do not use) <johnqpublic@example.com>
John Q. Public (Please only use this through September 2004) <johnqpublic@example.com>

This kind of sentiment is better expressed by revoking the key in question or setting an expiration time on the key or User ID self-sig directly. This sentiment is not part of John's identity, and shouldn't be included as though it were.

"none"

John Q. Public (none) <johnqpublic@example.com>

sigh. This is clearly someone getting mixed up by the UI.

I use strong crypto!

John Q. Public (3092 bits of RSA) <johnqpublic@example.com>

This comment refers to the strength of the key material, or the algorithms preferred by the user. Since the User ID is associated with the key material already, people who care about this information can get it from the key directly. This is also not part of the user's actual identity.

"no comment"

John Q. Public (no comment) <johnqpublic@example.com>

This is actually not uncommon (some keyservers reply "too many matches!"). It shows that the user is witty and can think on their feet (at least once), but it is still not part of the user's identity.

But wait (i hear you say)! I have a special case that actually is a legitimate use of the comment field that cannot be expressed in OpenPGP in any other way!

I'm sure that such cases exist. I've even seen one or two of them. The fact that one or two cases exist does not excuse the fact that that overwhelming majority of these comments in OpenPGP User IDs are a mistake, caused only by bad UI design that prompts people to put something (anything!) in the empty box (or on the command prompt, depending on your preference).

And this mistake is one of the thousand papercuts that inhibits the robust growth of the OpenPGP certification network that some people call the "web of trust". Let's avoid them so we can focus on the other 999 papercuts.

Please don't use comments in your OpenPGP User ID. And if you make a user interface for OpenPGP that prompts the user to decide on a new User ID, please don't include a prompt for "Comment" unless the user has already certified that they are really and truly a special special snowflake.

Thanks!

Tags: openpgp, ui

Syndicated 2013-05-15 02:40:00 from Weblogs for dkg

It's Advertising all the way down

Today i saw a billboard on the side of a bus. It was from a cable TV channel, bragging about how well-connected their viewers are (presumably on the internet, social media, blogs, etc).

It shows a smiling, attractive man, with text next to him saying something like "I told 9000 people what smartphone to buy".

What happened here?

• A TV channel bought an ad on the side of a bus
• trying to demonstrate to other advertisers
• about how good their viewers are at providing advertising-by-proxy
• on services that themselves are mostly advertising platforms
• to sell devices that are themselves often used for advertising delivery.

And almost all of these steps count as positive economic activity when we try to measure whether the US economy is healthy.

I am depressed by this tremendous waste of time and effort.

Syndicated 2013-05-03 17:21:00 from Weblogs for dkg

Make a Woolly Mammoth (thanks, inkscape!)

I feel like i've done a lot of blogging recently about failing to do things with proprietary software. That's annoying.

This post is about something i made successfully with free software (and some non-software crafting): I made a Woolly Mammoth for my nephew!

I documented the pattern (with pictures!) that i came up with using Inkscape (and used markdown, pandoc, emacs, pdftk, and other free software in the process). i've also published the source for the pattern via git if you want to modify it:

git clone git://lair.fifthhorseman.net/~dkg/woolly

Writing up the documentation makes me realize that i don't know of any software tools designed specifically for facilitating fabric/craft construction. Some interesting software ideas:

• Make 3-D models showing the partly assembled pieces, derived from the flat pattern. Maybe something like blender would be good for this?
• Take a 3D-modeled form and produce some candidate patterns for cutting and sewing? This seems like it is an interesting theoretical problem: given a set of (marked?) 3D surfaces and a set of approximation constraints, have the tool come up with a reasonable set of 2D patterns that could be cut and assembled using a set of standard operations into something close to the 3D shape.
• a "pattern lint checker" (maybe an inkscape extension?) that would let you mark certain segments of an SVG as related to other segments (i.e. the two sides of a seam), and could give you warnings when one side was longer than the other (within some level of tolerance)

Anyone have any ideas?

Tags: brainstorming, crafting, inkscape

Syndicated 2013-02-28 00:15:00 from Weblogs for dkg

proprietary software activation fail

i have a colleague who is forced by work situations to use Windows. Somehow, I'm the idiot^W^W^W^W^Wfriendly guy who gets tapped to fix it when things break.

Well, this time, the power supply broke. As in, dead, no lights, no fan, no nothing. No problem, though, the disk is still good, and i've got a spare machine lying around; and the spare is actually superior hardware to the old machine so it'll be an upgrade in addition to a fix. Nice! So i transplant the disk and fire up the new chassis.

But WinXP fails to boot with a lovely "0x0000007b" BSOD. The internet tells me that this might mean it can't find its own disk. OK, pop into the new chassis' BIOS, tell it to run the SATA ports in "legacy IDE" mode, and try again.

Now we get a "0x0000007e" BSOD. Some digging on the 'net makes me think it's now complaining now about the graphics driver. Hmm. Well, i figure i can probably work around that by installing new drivers from Safe Mode. So i reboot into Safe Mode.

Success! It boots to the login screen in Safe Mode. And, mirabile dictu, i happen to know the Administrator password. I put it in, and get a message that this Windows installation isn't "activated" yet -- presumably because the hardware has changed out from under it. And by the way, i'm not allowed to log in via safe mode until it's activated. So please reboot to "normal" Windows and activate it first.

Except, of course, the whole reason i'm booting into safe mode was because normal Windows gives a BSOD. Grrrr. Who thought up this particular lovely catch-22?

OK, change tactics. Scavenging the scrap bin turns up a machine with a failed mainboard, but a power supply with all the right leads. It's rated for about 80W less than the old machine's failed supply, but i figure if i rip out the DVD-burner and the floppy drive maybe it will hold. Oh, and the replacement power supply doesn't physically fit the old chassis, but it hangs halfway out the back and sort of rattles around a bit. I sacrifice the rest of the scrap machine, rip out its power supply, stuff the power supply into the old chassis, swap the original disk back in, and ... it boots successfully, finally.

That was the shorter version of the story :P

So now my colleague has a horrible mess of a frankencomputer which is more likely to fail again in the future, instead of a nice shiny upgrade. Why? Because Microsoft's need to control the flow of software takes priority over the needs of their users.

This is what you get when you let Marketing and BizDev drive your technical decisions.

Do i still need to explain why i prefer free software?

Syndicated 2013-02-01 16:25:00 from Weblogs for dkg