The Chelmsford Code, A way to protect PIN numbers

Pick a 10 letter word or name of a place where all the letters are different. An example is Chelmsford. Allocate numbers to letters as follows:

0 C
1 H
2 E
3 L
4 M
5 S
6 F
7 O
8 R
9 D

Write the number using letters. For example 5160 is SHFC.

This will be very difficult for somebody to decipher unless they know what word you used.

Did you invent this code? There are a limited number of words and names of places with 10 unique letters. Ilfracombe is another example. It will be possible to crack the Chelmsford code given a sufficient number of encoded PIN numbers to obtain all the letters. A more secure code could be constructed using two separate words with a total length of 10 letters.

Yes, I invented it along with a few classmates from school whilst visiting Chelmsford in the early 1990s. It was probably invented long before then with a different name. A google search for Chelmsford code isn't very revealing.