5 February 2012

Hashing Scheme for UK Civil and Police ANPR Cameras

Date: Sun, 5 Feb 2012 14:00:46 +0000
Subject: Re: Buckinghamshire CC ANPR cameras
From: John Wilson <tugwilson[at]gmail.com>
To: UK Cryptography Policy Discussion Group <ukcrypto[at]chiark.greenend.org.uk>

Rather to my surprise Bucks CC have given me the details of the hashing scheme used by ANPR cameras which implement the UTMC protocol (which is, I think, all of the civil and police ANPR cameras). This was the result of an FoI request.

D 0 Q are replaced with O (Q isn't used in the current numbering scheme)

1 is replaced with I            (I isn't used in the current numbering scheme)

5 is replaced with S

Y is replaced with V

8 and B are replaced with 3  (this may cause problems after 2030)

Z is replaced with 2

F is replaced with E

C is replaced with G

M N W are replaced with H

In the scheme used since 2002 replacing a number by a letter or a letter by a number will not cause extra collisions.

The transformed plate number is then hashed with the one-at-a-time hash function described here

http://www.burtleburtle.net/bob/hash/doobs.html

The 32 bit result is reduced to 24 or 18 bits simply by masking.

This is described in the UTMC Technical Guide TR007.001b which, as far as I can tell is not published on the UTMC site.

If anybody would like a copy of the document please contact me off list.

It would appear that the Highways Agency's statement that a large prime number is used is untrue.

I'm going to be doing some experiments to see how well the function does with some generated numberplate data.

John Wilson