# Introduction

## Congruence

Congruence modulo n is a congruence relation, meaning that it is an equivalence relation that is compatible with the operations of addition, subtraction, and multiplication. Congruence modulo n is denoted :

$$a \equiv b [n]$$

The congruence relation may be rewritten as (k ∈ ℤ):

$$a = b + k \times n$$

### Practical example

$$12 \equiv 3 $$

$$21 \equiv 3 $$

$$-6 \equiv 3 $$

Because :

$$12 = 3 + 1 \times 9$$

$$21 = 3 + 2 \times 9$$

$$-6 = 3 - 1 \times 9$$

In python, you can use the % symbol.

>>> 12 % 9 == 21 % 9 == -6 % 9 == 3
True


$$12 \equiv 3 $$

You can add or substract any number. Example with +3 and -5 :

$$15 \equiv 6 $$

$$7 \equiv -2 $$

The last equivalence is equals too (sometimes it's easier to deal with positive numbers):

$$7 \equiv 7 $$

## Multiplication

$$12 \equiv 3 $$

You can multiply with any number € Z (you cannot make a division, ex: multiply by 1/2).

1. Example with * 3 :

$$36 \equiv 9 $$

Which is equals to :

$$0 \equiv 0 $$

$$36 - 9 * 4 = 9 - 9 * 1$$

2. Example with * -5 :

$$-60 \equiv -15 $$

Which is equals to :

$$3 \equiv 3 $$

$$-60 + 9 * 7 = -15 + 2 * 9$$

## Divsion (warning)

You can't divise a equivalence relation.

$$12 \equiv 2 $$

Divide by 2 :

$$6 \not\equiv 1 $$

## Modular inverse

The multiplicative inverse :

$$x * a \equiv 1 [n]$$

$$x * a + n * k = 1$$

$$x \equiv a^{-1} [n]$$

It may be efficiently computed by solving Bézout's equation a * x + n * k = 1 using the Extended Euclidean algorithm (used to compute the GCD - Greatest common divisor).

### Practical example

$$5 \times x \equiv 1 $$

$$5 \times x = 1 + 34 \times k$$

$$1 = 5 \times 7 - 34$$

You can use the modular inverse in Python :

$$5 \equiv x^{-1} $$

>>> pow(5, -1, 34)
7
>>> from Crypto.Util.number import inverse
>>> inverse(5, 34)
7
>>> from gmpy2 import invert
>>> invert(5, 34)
mpz(7)