Two snippets that I rewrite every three or four months… April 23rd, 2013
Patrick Stein

A friend of mine is taking a cryptography course this semester. One of his current homework problems involves creating n, e, and d values for RSA Encryption. Always one to play the home game when math is involved, I found myself rewriting two snippets of code that I always end up writing any time I do any number-theory related coding.

One of them is simple EXPONENT-MODULO that takes a base b, an exponent e, and a modulus m and returns b^e \pmod m. There are more efficient ways to do this, but this one is much faster than the naïve method and quite easy to rewrite every time that I need it. The idea is that when e is odd, you recursively calculate b^{e-1} \pmod m and return b \cdot b^{e-1} \pmod m. When e is even, you recursively calculate a = b^{e/2} \pmod m and return a^2 \pmod m.

(defun expt-mod (b e m)
    ((zerop e) 1)
    ((evenp e) (let ((a (expt-mod b (/ e 2) m)))
                 (mod (* a a) m)))
    (t (mod (* b (expt-mod b (1- e) m)) m))))

The other snippet of code is a good deal trickier. Every time I need it, I spend way more time than I want to futzing with examples, getting lost in subscripts, etc. So, while I could rewrite the above in a moment, what’s below, I hope I have the search-fu to find this post next time I need it.

If you have a number n and some number a which is relatively prime to (shares no factors except for one with) n, then there exists some number b such that a \cdot b \equiv 1 \pmod n. And, in fact, all such numbers b for a given a differ by a multiple of n, so there is a unique such b \in [1,n).

Another way of saying a and n are relatively prime is to say that their greatest common divisor \gcd(a,n) is one. Besides being the biggest number that divides into both n and a, the \gcd(a,n) is also the smallest positive number expressible as x \cdot a + y \cdot n for integers x and y. If you keep track of the steps you use when calculating the greatest common divisor (which in this case is 1) while using the Euclidean algorithm you can (with enough care) backtrack through the steps and determine x and y for your given a and n.

For example, suppose you had n = 40 and a = 31. You want to know what b you’d need for 31b \equiv 1 \pmod{40}.
The steps in the Euclidean algorithm show:

\begin{array}{rcl}40 & = & 31 \cdot 1 + 9 \\31 & = & 9 \cdot 3 + 4 \\9 & = & 4 \cdot 2 + 1\end{array}

Working backwards, you can express 1 as a combination of x_0 \cdot 4 + y_0 \cdot 9. Then, you can express 4 as a combination of x_1 \cdot 9 + y_1 \cdot 31 and so on:

\begin{array}{rcl}1 & = & (-2)\cdot4 + 9\cdot{1} \\  & = & (-2)\cdot\left((-3)\cdot9 + 31\right) + 9\cdot{1} \\  & = & (-2)\cdot31 + 7\cdot9 \\  & = & (-2)\cdot31 + 7\cdot(40 - 31\cdot1) \\  & = & (-9)\cdot31 + 7\cdot(40) \equiv 1 \pmod{40}\end{array}

Wheee… in my example chosen to be easy to backtrack, the answer comes out to be -9 which is equal to 31 modulo 40. So, 31^2 \equiv 1 \pmod{40}. I picked a lucky number that is its own inverse modulo 40. It’s not usually like that. For example, 3 \cdot 27 \equiv 7 \cdot 23 \equiv 1 \pmod{40}.

So, anyhow, here’s a function that takes a and n and returns two values x and y so that x\cdot{}a + y\cdot{}n \equiv \gcd(a,n) \pmod n.

(defun gcd-as-linear-combination (a n)
  (multiple-value-bind (q r) (truncate n a)
    (if (zerop r)
        (values 1 0)
        (multiple-value-bind (x y) (gcd-as-linear-combination r a)
          (values (- y (* q x)) x)))))

Assuming that a and n are relatively prime with a < n, the first value returned is a‘s inverse modulo n.

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