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
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
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
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.