Extended Euclidean Algorithm

Objective

The Extended Euclidean Algorithm not only computes $\gcd(A, B)$, but also finds integers $x$ and $y$ that satisfy:

$$Ax + By = \gcd(A, B)$$

The pair $(x, y)$ is also known as the Bézout coefficients.

The Extended Euclidean Algorithm is used to compute the modular inverse of an integer, which is essential in many cryptographic algorithms like RSA, ECC, and digital signatures.

Code (Recursive Version)

def extended_gcd(a, b):
    if b == 0:
        return (a, 1, 0)
    else:
        d, x1, y1 = extended_gcd(b, a % b)
        x = y1
        y = x1 - (a // b) * y1
        return (d, x, y)

Proof

Base Case: $B = 0$

If $B = 0$, then:

$$\gcd(A, 0) = A \Rightarrow x = 1,\ y = 0$$

which satisfies the identity:

$$A \cdot 1 + B \cdot 0$$

---

Inductive Hypothesis: Assume the recursive call returns $(d, x_1, y_1)$ such that:

$$d = b \cdot x_1 + (A \bmod B) \cdot y_1$$

Then:

$$\begin{align*} d &= B \cdot x_1+ \left(A - \left\lfloor\frac{A}{B}\right\rfloor \cdot B\right) \cdot y1 \\ &= A \cdot y_1 + B\left(x_1 - \left\lfloor\frac{A}{B}\right\rfloor \cdot y1 \right) \end{align*}$$

Now define:

$$x = y_1, \quad y = x_1 - \left\lfloor\frac{A}{B}\right\rfloor \cdot y1$$

So:

$$Ax + By = d$$

which proves that the identity holds at each step.

Code (Non-Recursive Version)

def extended_gcd(a, b):
    # Initialize: (r, s, t) ← (a, 1, 0), (r', s', t') ← (b, 0, 1)
    r_prev, r = a, b
    s_prev, s = 1, 0
    t_prev, t = 0, 1

    while r != 0:
        q = r_prev // r  # quotient

        # Update (r, s, t)
        r_prev, r = r, r_prev - q * r
        s_prev, s = s, s_prev - q * s
        t_prev, t = t, t_prev - q * t

    # At termination: r_prev = gcd(a, b), s_prev = x, t_prev = y
    return r_prev, s_prev, t_prev

Written by ryan Kim from A41