# Talk

## Extension work for lesson on substitution ciphers

(1 Post)
nordicwannabe Thu 03-Mar-16 21:30:09

What is your opinion on the following extension work for a lesson on substitution ciphers for Year 6 (part of a WW2 theme)? My daughter is way too young for me to make a sensible judgement, but I'm trying to figure out whether this is good or not.

In the standard task, they assigned a number to each letter in the alphabet and encoded/decoded a message using that key.

The differentiated work was to think of an algebraic expression for each letter in terms of the other letters.

Eg
A = 6 = B-(C+T)
B = 10 = (2×A)-(2×T)
C = 3 = A/2
D = 5 = B/2
...
T = 1 = A-D

So in the basic task CAT is encoded as
3 6 1
In the extended task CAT is encoded as
A/2 C-(B+T) A-T

Am I being ridiculously picky to think that while that gives a bit of practice in manipulating numbers (and since it was part of a maths class, perhaps that was the entire point of the lesson) it's a bit of an unimaginative and artificial way to extend a lesson on CIPHERS - surely one of the most fun possible maths lessons!

Wouldn't the natural way to extend this be to introduce some other ciphers: maybe show a simple shift cipher and show how to crack it either by brute force (trying each shift) or by looking at different incidence of letters in English (giving a suitably 'average' text to work with!); or maybe look at sensible ways of strengthening the substitution cipher.

After all, thinking about ways to crack ciphers - or make them harder to crack - is surely what truly deepens understanding of this subject, and also makes it exciting.

Or would that be too complex, and taking it too far away from the others?

Don't get me wrong, I think it's absolutely brilliant that they looked at cryptography at all in Year 6! But the cynical part of me was left wondering whether the topic was window dressing which just presented basic 'sums' in an attractive way.

Or am I being really pedantic and this is exactly the kind of extension work you would love your clever children to be given in Year 6?

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