On 08/22/2012 09:39 PM, Metspitzer wrote:
On Wed, 22 Aug 2012 18:52:30 -0700 (PDT), Harry K
wrote:

[snip]

Engrave the combination on the back of the lock. Add 4 to each
number.
Or subtract 3
or subtract 5

I often use a XS3 (excess 3) code. It's very easy to encode. Just add 3
to each digit (IGNORE CARRIES). For example, to encode 3728:

3 + 3 = 6
7 + 3 = 0 (remember to ignore carries)
2 + 3 = 5
8 + 3 = 1 (remember you're ignoring carries)

the encoded number is then 6051.

Decoding:

6 - 3 = 3
0 - 3 = 7 (ignore carries again)
5 - 3 = 2
1 - 3 = 8 (still ignore the carry)

it's 3728.

With a little practice I could do this without writing down ANYTHING but
the encoded number. The arithmetic is on individual digits only, so it's
a lot simpler than ordinary addition / subtraction.

You could use hexadecimal, and then there's be no carries to deal with:

3 + 3 = 6
7 + 3 = A
2 + 3 = 5
8 + 3 = B

6 - 3 = 3
A - 3 = 7
5 - 3 = 2
B - 3 = 8

That could help confuse things, since 6A5B looks less like a number than
6051. In case you don't know hex A = 10, B = 11, C = 12.

BTW, thinking of hex, the TV series "Doctor Who" began in the year 7AB.

I've also done nines compliment, which is even easier. Subtract each
digit from 9. For example, starting with 3728:

9 - 3 = 6
9 - 7 = 2
9 - 2 = 7
9 - 8 = 1

Decoding uses the same process:

9 - 6 = 3
9 - 2 = 7
9 - 7 = 2
9 - 1 = 8

--
Mark Lloyd
http://notstupid.us

"Asking about a time before the beginning of our spherical spacetime is
like asking what lies north of the North Pole. There is no such thing."
Taner Edis, Is Anybody Out There?