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Modular Arithmetic & GCD

Greatest Common Divisor (GCD)

(This is extremely important)

Definition

The greatest common divisor (GCD) of two integers $a$ and $b$ is the largest integer that divides both $a$ and $b$.

Notation: $\gcd(a, b)$

Example

$$\gcd(29, 8)$$

Finding factors:

  • Factors of 29: 1, 29
  • Factors of 8: 1, 2, 4, 8
  • Common factors: 1

$$\gcd(29, 8) = 1$$

The Euclidean Algorithm

Goal: Compute $\gcd(a, b)$ fast without factoring!

Why It Works

The "block sizes that work" don't change when we replace the bigger number by the leftover (remainder).

The Algorithm

flowchart TD
    A(["Start: a, b"]) --> B["Divide: a = bq + r"]
    B --> C{r == 0?}
    C -- No --> D["Replace: a ← b, b ← r"]
    D --> B
    C -- Yes --> E(["GCD = b"])

    style A fill:#7c4dff,color:#fff
    style E fill:#00897b,color:#fff
Loading

Repeat:

  1. Divide: $a = bq + r$
  2. Replace: $(a, b) \to (b, r)$
  3. Stop when $r = 0$

Answer: The last non-zero remainder is the GCD.

Example: Find $\gcd(3959, 1177)$

$$3959 = 1177 \times 3 + 428$$ $$1177 = 428 \times 2 + 321$$ $$428 = 321 \times 1 + 107$$ $$321 = 107 \times 3 + 0$$

Last non-zero remainder: $\boxed{\gcd(3959, 1177) = 107}$

Step-by-Step Process

Step Equation $(a, b)$
1 $29 = 8 \times 3 + 5$ $(29, 8) \to (8, 5)$
2 $8 = 5 \times 1 + 3$ $(8, 5) \to (5, 3)$
3 $5 = 3 \times 1 + 2$ $(5, 3) \to (3, 2)$
4 $3 = 2 \times 1 + 1$ $(3, 2) \to (2, 1)$
5 $2 = 1 \times 2 + 0$ $(2, 1) \to (1, 0)$

$$\gcd(29, 8) = 1$$

Modular Arithmetic

Idea: You only care about the remainder after dividing by something.

Definition

Let $a, b, n$ be integers with $n > 0$. We say:

$$a \equiv b \pmod{n}$$

if $n$ divides $(a - b)$.

In other words: $a$ and $b$ have the same remainder when divided by $n$.

Examples

Is $17 \equiv 5 \pmod{6}$?

$$17 - 5 = 12$$ $$12 \div 6 = 2 \quad \text{(exactly)}$$

Since 6 divides 12, yes: $17 \equiv 5 \pmod{6}$ ✓

Is $23 \equiv 7 \pmod{8}$?

$$23 - 7 = 16$$ $$16 \div 8 = 2 \quad \text{(exactly)}$$

Yes: $23 \equiv 7 \pmod{8}$ ✓

Computing $a \bmod n$

Find the remainder when $a$ is divided by $n$.

Examples:

  • $17 \bmod 5 = 2$ (since $17 = 5 \times 3 + 2$)
  • $100 \bmod 7 = 2$ (since $100 = 7 \times 14 + 2$)
  • $-3 \bmod 5 = 2$ (since $-3 = 5 \times (-1) + 2$)

Modular Addition

$$(a + b) \bmod n = ((a \bmod n) + (b \bmod n)) \bmod n$$

Example

Compute $(17 + 23) \bmod 8$:

Method 1: Direct $$17 + 23 = 40$$ $$40 \bmod 8 = 0$$

Method 2: Reduce first $$17 \bmod 8 = 1$$ $$23 \bmod 8 = 7$$ $$(1 + 7) \bmod 8 = 8 \bmod 8 = 0$$

Modular Multiplication

$$(a \times b) \bmod n = ((a \bmod n) \times (b \bmod n)) \bmod n$$

Example

Compute $(17 \times 23) \bmod 8$:

Method 1: Direct $$17 \times 23 = 391$$ $$391 \bmod 8 = 7$$

Method 2: Reduce first $$17 \bmod 8 = 1$$ $$23 \bmod 8 = 7$$ $$(1 \times 7) \bmod 8 = 7$$

Modular Exponentiation

To compute $a^n \bmod m$ efficiently:

Fast Exponentiation (Square and Multiply)

Idea: Break down the exponent using binary representation.

$$a^{13} = a^8 \times a^4 \times a^1$$

(because $13 = 8 + 4 + 1$ in binary: $1101_2$)

Example: Compute $7^{13} \bmod 11$

Binary: $13 = 1101_2$

Power Calculation Result mod 11
$7^1$ $7$ $7$
$7^2$ $7^2 = 49$ $5$
$7^4$ $5^2 = 25$ $3$
$7^8$ $3^2 = 9$ $9$

$$7^{13} = 7^8 \times 7^4 \times 7^1 \equiv 9 \times 3 \times 7 \equiv 189 \equiv 2 \pmod{11}$$

Modular Inverse

Definition

The modular inverse of $a$ modulo $n$ is a number $a^{-1}$ such that:

$$a \times a^{-1} \equiv 1 \pmod{n}$$

When Does It Exist?

$a^{-1} \bmod n$ exists if and only if $\gcd(a, n) = 1$

(i.e., $a$ and $n$ are coprime)

Example

Find $7^{-1} \bmod 11$:

We need: $7x \equiv 1 \pmod{11}$

Testing:

  • $7 \times 1 = 7 \not\equiv 1$
  • $7 \times 2 = 14 \equiv 3 \not\equiv 1$
  • $7 \times 3 = 21 \equiv 10 \not\equiv 1$
  • $7 \times 4 = 28 \equiv 6 \not\equiv 1$
  • $7 \times 5 = 35 \equiv 2 \not\equiv 1$
  • $7 \times 6 = 42 \equiv 9 \not\equiv 1$
  • $7 \times 7 = 49 \equiv 5 \not\equiv 1$
  • $7 \times 8 = 56 \equiv 1$ ✓

$$7^{-1} \equiv 8 \pmod{11}$$

Extended Euclidean Algorithm

Finds integers $x$ and $y$ such that:

$$ax + by = \gcd(a, b)$$

This is used to compute modular inverses efficiently.

Applications to Cryptography

Why Modular Arithmetic Matters

  1. Keeps numbers bounded (prevents overflow)
  2. Makes operations reversible (with modular inverse)
  3. Foundation of RSA (encryption/decryption uses mod $n$)
  4. Diffie-Hellman key exchange
  5. Digital signatures

Key Takeaways

  1. Euclidean algorithm computes GCD fast
  2. Modular arithmetic = clock arithmetic (wrap around)
  3. Modular inverse exists only when $\gcd(a, n) = 1$
  4. Fast exponentiation uses binary representation
  5. All modern crypto uses modular arithmetic