On Thu, 23 Aug 2012 15:06:46 -0500, Mark Lloyd
wrote:

: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.
:
ecoding:
:
: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
:
ecoding uses the same process:
:
:9 - 6 = 3
:9 - 2 = 7
:9 - 7 = 2
:9 - 1 = 8

This is really good stuff. Guess I'll squirrel it away in a database. I
do have a combination lock or two but I never use them. Many years ago I
bought a matched set of Master locks that all use the same key. So much
easier to use a key than fiddle with combination locks.

Dan


Email: dmusicant at pacbell dot net