Find an inverse of a modulo m for the following pairs a=your day of birth, m = 43 I b. a = 29, m = your month of birth a. a=

Database System Concepts
7th Edition
ISBN:9780078022159
Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Chapter1: Introduction
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5- Find an inverse of a modulo m for the following pairs
a. a = your day of birth, m = 43
I
b.
a 29, m = your month of birth
=
Transcribed Image Text:5- Find an inverse of a modulo m for the following pairs a. a = your day of birth, m = 43 I b. a 29, m = your month of birth =
Expert Solution
Step 1: Modular inverse

In modular arithmetic, there is no division operation. Hence, modular inverse is done instead. 

  • The modular inverse of A (mod C) is A^-1
  • (A * A^-1) ≡ 1 (mod C) or equivalently (A * A^-1) mod C = 1
  • Only the numbers coprime to C (numbers that share no prime factors with C) have a modular inverse (mod C)

There are three main methods to find the multiplicative modular inverse.

  • The naive method
  • The extended Euclidean algorithm
  • The Fermat's little theorem

The first step to finding the multiplicative modular inverse exists, need to check if a and m are coprime, i.e., if gcd(a,m) = 1.

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