Cargo cult signatures

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A lot has been written about Kathryn Johnston, an elderly woman who was killed when an Atlanta SWAT team conducted a raid on her house last week. One of the interesting sub-issues was the observation by Orin Kerr that the judge's signatures on the warrants and affidavits look very similar. See here for a comparison. The explanation seems to be that they use an electronic warrant system where the judge "signs" online. It seems like these systems come in two varieties. In the first, the judge actually signs on some signature capture pad (like you do in a lot of credit card capture applications) and the digitized signature is attached to the relevant documents. In the second, the judge just clicks on some web form and then some pre-scanned signature is cut-and-pasted into the document.

In either case, this sort of manipulation of a digitized signature is kind of weird. In ordinary paper warrants, the signature is an indicator to the relying party (i.e., the person who's supposed to be examining it) that the judge approved the search and that he authorized a warrant with some set of particulars. However, a digitized signature on a document can be easily cut-and-pasted and so attests to no such thing. Certainly, the relying party has no way of knowing that the police (or whoever is presenting the document) didn't just perform this kind of cut-and-pasting. Of course, you can certainly argue that this kind of fraud would be dangerous because there are penalties if you get caught, but the signature doesn't change that one way or the other. Just having the document unsigned would be just as effective.

That said, in settings like credit card processing where a single verifier processes a large number of signatures, digitized signatures do offer some value in detecting replay attacks—since every signature is slightly different. If there was some situation where there was a question about whether a judge signed a given warrant and you could access all the warrants a judge had ever signed, then this might have some value, but that doesn't seem like a very likely case.

More likely the real reason why warrants have these kind of signatures is that old-style paper warrants had signatures and so people expect them despite the fact that they don't have much security value. If you showed up with an unsigned warrant people would probably ask you where the signature was.

3 Comments

It's always — long before the digital age — struck me that signatures on paper are of no value at all on the spot. No random person knows what my signature looks like, nor what Judge Dinkelwaffler's does. With my credit card, I suppose they can look at the back of the card and compare, but those cards are so hard to sign that the signature usually only vaguely resembles what pen puts on paper anyway (and the one on my card is completely rubbed off anyway, and that fact is never challenged). In general, it is just that people look to see that the signature is there, and that's all.


The value only comes later, if something is contested. Then the purported signer can attest that yes, it is his signature, or an expert witness can testify about the handwriting.


Clearly, in the case of facsimile signatures (copied, stamped, digitized, whatever) even that value is gone, except to the extent that it actually makes the purported signer testify that, "Yes, that is my signature, but I didn't put it there."

Of course, with a no-knock warrant, the precise nature of the signature doesn't make much difference.

At least they bothered to get a signature, which is more than the North Carolina Alcohol Law Enforcement agency seems to do. At least in NC, tobacco and firearms are still sacred.

The primary role of signatures nowadays is to indicate an intent to be bound; the house of lords has held that a signature in the Usenet sense of the world can bind, as long as it appears at the end of a message and not the start.

p.s.
My dad called yesterday to tell me that one his rallying mate Irish Paul has an Audi RS4 scheduled for delivery in January, and that he's going over so they can spend a week seeing what actual Irish cops are like on their home turf.

Bastard.

The primary role of signatures has *always* been to indicate an intent to be bound. Modern cryptographers, enthralled as they are with digital signatures, like to think that signatures are all about unforgeability. But signatures have been very forgeable for a very long time--especially those X's that illiterate people used to sign with (and sometimes still do). It's the enormous body of common law precedent that makes them valid--*if* you can demonstrate, by the standards of that body of precedent, that the signature really did indicate the putative signer's intent to be bound.

I've been told repeatedly that the reason that cryptographic digital signatures haven't caught on is that they're sufficiently different from the handwritten kind that nobody knows how the courts will rule on disputes involving them. By comparison, faxed/typed/scanned signatures are much more similar to traditional ones, and their incorporation into the case law has therefore been fairly smooth.

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