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Modular Exponentiation

Tool to compute modular power. Modular Exponentiation (or power modulo) is the result of the calculus a^b mod n. It is often used in informatics and cryptography.

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Modular Exponentiation -

Tag(s) : Arithmetics

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# Modular Exponentiation

## a^b mod n Solver

Solver limited to integer solutions < 10000

### Find unknown n

Tool to compute modular power. Modular Exponentiation (or power modulo) is the result of the calculus a^b mod n. It is often used in informatics and cryptography.

### How to calculate a raised to power b modulo n?

It consists in an exponentiation followed by a modulus, but it exists optimized algorithms with big numbers to return a fast result without having to actually perform the calculation (called fast, thanks to mathematical simplifications).

Example: $$12^{34} \equiv 16 \mod 56$$

The word power indicates the name of the operation, and exponent to indicate the operand.

### How to calculate the last digits of an exponentiation?

Calculating the last $x$ digits of $a ^ b$ is equivalent to calculating $a ^ b \mod n$ with $n = 10 ^ x$ (the number $1$ followed by $x$ zeros)

Example: $3 ^ 9 = 19683$ and $3 ^ 9 \mod 100 = 83$ (the last 2 digits)

### What is the algorithm of powmod?

There are several algorithms, here is the shortest one in pseudocode:// pseudocode function powmod(base b, exponent e, modulus m) if m = 1 then return 0 var c := 1 for var a from 1 to e c := (c * b) mod m end for return c

### How to solve for exponent with base and modulo?

This calculation is known as the discrete logarithm problem. Some solutions can be found by brute force but there is no trivial general solution.

### Why modular exponentiation is limited to integers?

Calculus uses exponent and modulos that are generally defined over the natural number domain set N. It is possible to use rational numbers but it is not handled here.

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