## Friday, 17 November 2017

### Russian peasant cryptography

What is the relationship between Russian peasant multiplication and modern cryptography? Unlike the multiplication procedures commonly taught, it can easily be adapted to compute powers rather than products; and we need to calculate powers for some cryptosystems.
I should probably admit right away that this is not going to be at all informative about how Russian peasants managed keep their communications secret from their aristocratic overlords.

# Russian Peasant Multiplication

If you haven't seen this before, it's a rather neat approach to multiplication which doesn't require the memorization of multiplication tables, but only the ability to double or half integers. It seems to be rather unclear just what it has do do with Russian peasants: it's certainly less than Danish pastries have to do with Denmark. The method was known to the ancient Egyptians, who considerably predate Russian peasants.
The general description is simple, but not very informative. Nevertheless, I will start by presenting it.
If you want to multiply together two integers, $$m$$ and $$n$$, form two columns, one headed by $$m$$ and the other by $$n$$. Then repeatedly add a new row obtained by halving $$m$$ (discarding any remainder) and doubling $$n$$, stopping when you end up with a $$1$$ in the $$m$$ column. Finally, add up all the entries in the $$n$$ column where the $$m$$ value is odd.
It's much more informative to see an example. So let's work out $$13 \times 7$$. This gives us the table $\begin{array}{|c|c|} \hline m & n \\ \hline 13 & 7\\ 6 & 14\\ 3 & 28\\ 1 & 56\\ \hline \end{array}$ after which we add up the right hand entries when the left hand entry is odd, giving $7+28+56 = 91$ which we can quickly check is $$13 \times 7$$.
The interesting question is, of course, why does this mysterious looking procedure work?
If we look at the left hand column, and work our way up, recording a $$1$$ when we see an odd number, and a $$0$$ when we see an even one, we obtain $$1101$$, which is (by no coincidence whatever) the binary representation of $$13$$. In fact, this is exactly the standard algorithm for converting numbers to their binary representation.
Then we can see that the repeated doubling of the $$m$$ column is calculating the product of $$m$$ with the corresponding power of $$2$$, so that adding those corresponding to an odd value in the $$n$$ column is adding up $$m$$ multiplied by those powers of $$2$$ which sum to $$m$$.
So by using this we can multiply any integers together, and what's more the algorithm is reasonably efficient. I admit that it's not as efficient as the column or grid methods usually taught now, but at least you don't need to know your times tables to do it.
This is rather nice. At least, I like it. But rather remarkably, it can be adapted slightly to work out something apparently much more complicated, and very useful in modern (public key) cryptography.

# Russian peasant exponentiation

We make two changes to the above algorithm above, both only involving the $$m$$ column.
• Instead of doubling, we square

Then in each row of the table, instead of having $$m$$ multiplied by the corresponding power of $$2$$, we have it raised to the corresponding power of $$2$$. Multiplying all these together then gives us $$n^m$$.
Let's see how it works with the previous $$m$$ and $$n$$. $\begin{array}{|c|c|} \hline m & n \\ \hline 13 & 7\\ 6 & 49\\ 3 & 2401\\ 1 & 5764801\\ \hline \end{array}$ and multiplying the relevant terms together gives $5764801 \times 2401 \times 7 = 96889010407$ which is, indeed, $$7^{13}$$.
But unlike the multiplication, this isn't just reasonably efficient: this is remarkably efficient.
If we want to find $$n$$ to the power of $$m$$, we could do it by multiplying together $$m$$ lots of $$n$$. This is like finding $$m \times n$$ by adding together $$m$$ lots of $$n$$: you can do it, but if $$m$$ is bigger than about $$3$$ you really have better things to do with your time.
But with this algorithm, the number of squarings that you have to do is at most about $$3$$ times the number of digits of $$n$$ (since the binary representation of a decimal number has about $$3$$ times as many bits as the decimal one has digits), followed by that many multiplications.
Of course, we need to be able to multiply to do this, but that's OK: the multiplication algorithm lets us multiply by doubling and adding, and now the adapted version lets us exponentiate by squaring and multiplying. It's still a good deal.

# A quick outline of RSA

The point of this is that in the RSA (Rivest-Shamir-Adleman) approach to public key cryptography (and also in some others), we need to compute powers in order to encrypt and decrypt messages.
The way that the procedure works is that you choose a private pair of primes, $$p$$ and $$q$$, and tell the world the value of $$N=pq$$. (You keep $$p$$ and $$q$$ secret: the algorithm relies on the fact that factorizing $$N$$ is hard.)
Then you pick a number $$e$$ and find $$d$$ with the property that $$ed$$ is $$1$$ more than a multiple of $$(p-1)(q-1)$$. You tell the whole world $$e$$. (You keep $$d$$ secret. The algorithm relies on the fact that finding $$d$$ without knowing $$p$$ and $$q$$ is hard.)
Then you represent your message as an integer $$M \lt N$$. The encrypted form of your message, $$E$$ is the remainder when you divide $$M^e$$ by $$N$$.
Then by the magic of number theory (actually, it isn't magic, it's Fermat's Little Theorem) the original message is the remainder when you divide $$E^d$$ by $$N$$.
In principle, this is all there is to it, and there are many descriptions available, often illustrated using values of $$N$$ which are small enough to make all the arithmetic tractable.
But there's an issue.
In practice, the values of $$M$$ and at least one of $$e$$ or $$d$$ is very large. If you had to do repeated multiplication, the universe would die of old age before you were finished. This algorithm reduces the amount of work to manageable proportions. $$e$$ (or $$d$$) multiplications are reduced to a number of multiplications that is a small multiple of the number of digits of $$e$$ (or $$d$$).
That wasn't the only issue.
The value of $$M^N$$ is almost inconceivably large. You couldn't write it down if you used every elementary particle in the known universe to hold one digit. So how does that work?
The answer is more number theory. One of the really nice properties of working out remainders is that it doesn't matter where in a calculation you do it. If you want to know the remainder of some quantity when you divide by $$N$$, then you can calculate the whole thing then divide by $$N$$; or, rather more sensibly, you can work it out in stages and take the remainder whenever the working value exceeds $$N$$. You are guaranteed to obtain the same result in the end.

# Russian Peasant RSA

So now we can see how to compute an encrypted message. We can use the adapted Russian peasant multiplication, but now we just add the final detail that whenever the squared number exceeds $$N$$, we take the remainder on division by $$N$$.
Let's see this work with finding the remainder on dividing $$7^{13}$$ by $$59$$.
$\begin{array}{|c|c|} \hline m & n \\ \hline 13 & 7\\ 6 & 49\\ 3 & 2401 \rightarrow 41\\ 1 & 1681 \rightarrow 29\\ \hline \end{array}$ giving $29 \times 41 \times 7 = 8323 \rightarrow 4$ which is (phew!) the remainder on dividing $$96889010407$$ by $$59$$.
But here's the punchline. This algorithm for calculating the exponential is what is usually referred to as exponentiation by (repeated) squaring. And this (or some variation on it) is in fact the algorithm used in implementations of public key cryptography (such as RSA) which rely on computing exponentials.
So there we have it. What is essentially an algorithm for multiplication which can be used by people who don't know their times tables turns into an algorithm for exponentiation which underlies contemporary cryptographic methods.

## Acknowledgements

At the 2017 MathsJam Gathering, Elaine Smith and Lynda Goldenberg gave a talk about techniques for multiplication including the Russian peasant method. That was when the penny dropped for me that the repeated squaring algorithm was an adaptation of the Russian peasant multiplication, and I decide it was worth sharing the realisation. About twenty minutes lates, Oliver Masters gave a talk about the Fibonacci matrix where he mentions the repeated squaring algorithm for matrix exponentiation. Fortunately he didn't relate it to the multiplication algorithm. About five minutes after that, Colin Wright (@ColinTheMathmo) was wrapping up the talks and he mentioned the relationship. But by that time I had the bit between my teeth and had decided to do this anyway, so I have.
If you enjoyed this, then you'll probably enjoy @ColinTheMathmo's series of articles about Perrin numbers.