## Extended Euclidean algorithm - Wikipedia

Nov 25,  · The extended Euclidean algorithm is an extension to the Euclidean algorithm, which computes, besides the greatest common divisor of integers a and b, the coefficients of Bézout's identity, that is integers x and y such that ax + by = gcd(a,b). The gcd is the only number that can simultaneously satisfy this equation and5/5(3). The extended Euclidean algorithm will give us a method for calculating p efficiently (note that in this application we do not care about the value for s, so we will simply ignore it.) The Extended Euclidean Algorithm for finding the inverse of a number mod n. We will number the steps of the Euclidean algorithm starting with step 0. The extended Euclidean algorithm is an algorithm to compute integers x x x and y y y such that. a x + b y = gcd ⁡ (a, b) ax + by = \gcd(a,b) a x + b y = g cd (a, b) given a a a and b b b. The existence of such integers is guaranteed by Bézout's lemma. The extended Euclidean algorithm can be viewed as the reciprocal of modular exponentiation.

## Extended Euclidean algorithm Implementation in C, C++ and Python - Techie Delight

This is a certifying algorithmbecause the extended euclidean is the only number that can simultaneously satisfy this equation and divide the inputs. It allows one to compute also, with almost no extra cost, the quotients of a and b by their greatest common divisor.

The extended Euclidean algorithm is particularly useful when a and b are coprime. With that provision, x is the modular multiplicative inverse of a modulo bextended euclidean, and y is the modular multiplicative inverse of b modulo a.

Similarly, the polynomial extended Euclidean algorithm allows one to compute the multiplicative inverse in algebraic field extensions and, in particular in finite fields of non prime order. It follows that both extended Euclidean algorithms extended euclidean widely used in cryptography. In particular, the extended euclidean of the modular multiplicative inverse is an essential step in RSA public-key encryption method. The standard Euclidean algorithm proceeds by a succession of Euclidean divisions whose quotients are not used, only the remainders are kept.

For the extended algorithm, the successive quotients are used. Also it means that the algorithm can be done without integer overflow when a and b are representable integers. The following table shows how the extended Euclidean algorithm proceeds extended euclidean input and The greatest common divisor is the last extended euclidean zero entry, 2 in the column "remainder".

The computation stops at row 6, because the remainder in it is 0. Until extended euclidean point, the proof is the same as that of the classical Euclidean algorithm. As they are coprime, they are, up to their sign the quotients of b and extended euclidean by their greatest common divisor. The definitions then show that the aextended euclidean case reduces to the ba case.

If a and b are two nonzero polynomials, then the extended Euclidean algorithm produces the unique pair of polynomials st such that. A third difference is that, extended euclidean, in the polynomial case, the greatest common divisor is defined only up to the multiplication by a non zero constant. There are several ways to define unambiguously a greatest common divisor. In mathematics, it is common to require that the greatest common divisor be a monic polynomial.

To get this, it suffices to divide every element of the output by the leading coefficient of r k, extended euclidean. Otherwise, one may get any non-zero constant. Extended euclidean computer algebrathe polynomials commonly have integer coefficients, extended euclidean, and this way of normalizing the greatest common divisor introduces too many fractions to be convenient. If the input polynomials are coprime, this normalisation also provides a greatest common divisor equal to 1, extended euclidean.

The drawback of this approach is that a lot of fractions should be computed and simplified during the computation, extended euclidean.

A third approach consists in extending the algorithm of subresultant pseudo-remainder sequences in a way that is similar to the extension of the Euclidean algorithm to the extended Euclidean algorithm. This allows that, when starting with polynomials with integer coefficients, all polynomials that are computed have integer coefficients, extended euclidean.

To implement the extended euclidean that is described above, one should first remark that only the two last values of the extended euclidean variables are needed at each step. Thus, for saving memory, each extended euclidean variable must be replaced by just two variables.

For simplicity, the following algorithm and the other algorithms in this article uses parallel assignments. In a programming language which does not have this feature, the parallel assignments need to be simulated with an auxiliary variable. For example, the first one, extended euclidean. The quotients of a and b extended euclidean their greatest common divisor, which is output, may have an incorrect extended euclidean. This is easy to correct at the end of the computation but has not been done here for simplifying the code.

Similarly, if either a or b is zero and the other is negative, the greatest common divisor that is output is negative, and all the signs of the output must be changed. When using integers of unbounded size, the time needed for multiplication and division grows quadratically with the size of the integers. This canonical simplified form can be obtained by replacing the three output lines of the preceding pseudo code by. To get the canonical simplified form, it suffices to move the minus sign for having a positive denominator.

It is the only case where the output is an integer. The extended Euclidean algorithm is the essential tool for computing multiplicative inverses in modular structures, typically the modular integers and the algebraic field extensions.

A notable instance of the latter case are the finite fields of non-prime order. In particular, if n is primea has a multiplicative inverse if it is not zero modulo n. Thus tor, more exactly, the remainder of the division of t by nis the multiplicative inverse of a modulo n.

This results in the pseudocode, in which the input n is an integer larger than 1. The extended Euclidean algorithm is also the main tool for computing multiplicative inverses in simple algebraic field extensions. An important case, widely used in cryptography and coding theory extended euclidean, is that of finite fields of non-prime order. The addition in L is the addition of polynomials. The multiplication in L is the remainder of the Euclidean division by p of the product of polynomials.

Thus, extended euclidean, to complete the arithmetic in Lit remains only to define how to compute multiplicative inverses, extended euclidean. This is done by the extended Euclidean algorithm. The algorithm is very similar to that provided above for computing the modular multiplicative inverse. In the pseudocode which follows, p is a polynomial of degree greater than one, and a is a polynomial. Moreover, div is an auxiliary function that computes the quotient of the Euclidean division.

Note also that 1 being the only nonzero element of GF 2the adjustment in the last line of the pseudocode is not needed. One can handle the case of more than two numbers iteratively. From Wikipedia, the free encyclopedia. Main extended euclidean Modular arithmetic. Number-theoretic algorithms. Binary Euclidean Extended Euclidean Lehmer's. Cipolla Pocklington's Tonelli—Shanks Berlekamp. Categories : Number theoretic algorithms Euclid.

Hidden categories: Articles with example pseudocode, extended euclidean. Namespaces Article Talk. Views Read Edit View history. By using this site, extended euclidean, you agree to the Terms of Use and Privacy Policy. Italics indicate that algorithm is for numbers of special forms.

### The Extended Euclidean Algorithm explained with examples May 29,  · The extended Euclidean algorithm is particularly useful when a and b are coprime (or gcd is 1). Since x is the modular multiplicative inverse of “a modulo b”, and y is the modular multiplicative inverse of “b modulo a”. In particular, the computation of the modular multiplicative inverse is an essential step in RSA public-key encryption /5. The extended Euclidean algorithm will give us a method for calculating p efficiently (note that in this application we do not care about the value for s, so we will simply ignore it.) The Extended Euclidean Algorithm for finding the inverse of a number mod n. We will number the steps of the Euclidean algorithm starting with step 0. Extended Euclidean algorithm also refers to a very similar algorithm for computing the polynomial greatest common divisor and the coefficients of Bézout's identity of two univariate polynomials. The extended Euclidean algorithm is particularly useful when a and b are coprime.