Modular Inverse Algorithms without Multiplications
for Cryptographic Applications
First version: July 4, 2005
Laszlo Hars, Seagate Research (Laszlo@Hars.US)
Abstract Hardware and algorithmic optimization techniques are presented to the leftshift, rightshift and the traditional Euclidean modular inverse algorithms. Theoretical arguments and extensive simulations determined the resulting expected running time. On many computational platforms these turn out to be the fastest known algorithms for moderate operand lengths. They are based on variants of Euclideantype extended GCD algorithms. On the considered computational platforms for operand lengths used in cryptography, the fastest presented modular inverse algorithms need about twice the time of modular multiplications, or even less. Consequently, in elliptic curve cryptography delaying modular divisions is slower (affine coordinates are best) and the RSA and ElGamal cryptosystems can be accelerated.
Keywords: Computer Arithmetic, Cryptography, RSA cryptosystem, Elliptic curve cryptosystem, ElGamal cryptosystem, Modular multiplication, Extended Greatest Common Divisor algorithm, Modular Inverse, Optimization
Notations

Modular inverse: a^{–1}, the smallest positive integer for integer a, such that a·a^{–1} =^{_} 1 mod m

GCD: Greatest Common Divisor of integers

xGCD or extended GCD: the algorithm calculating g and also two factors c and d: [g,c,d] = xCGD(x,y), such that the greatest common divisor of x and y is g, and g = c·x+d·y

MS/LS bits: the Most/Least Significant bits of binary numbers

m the number of bits in the binary representation of integer m, its binary length

m = {m_{n}_{−1}…m_{1}, m_{0}} = m_{n}_{−1}_{…}_{0 }= _{Σ}_{ }_{i }_{= 0}_{…}_{n}_{−1 }2^{i}^{ }m_{i}, where the bits m_{i} {0, 1} of the integer m form its binary representation
1.Introduction
We present improved algorithms for computing the inverse of large integers modulo a given prime or composite number, without multiplications of any kind. In most computational platforms they are much faster than the commonly used algorithms employing multiplications, therefore, the multiplier engines should be used for other tasks in parallel. The considered algorithms are based on different variants of the Euclidean type Greatest Common Divisor algorithms. They are iterative, gradually decreasing the length of the operands and keeping some factors updated, maintaining a corresponding invariant. There are other algorithmic approaches, too. One can use system of equations or the Little Fermat Theorem (see [21]), but they are only competitive with the Euclidean type algorithms under rare, special circumstances.
Several variants of three extended GCD algorithms are modified for computing modular inverses for operand lengths used in public key cryptography (128…16K bits). We discuss algorithmic improvements and simple hardware enhancements for speedups in digit serial hardware architectures. The main point of the paper is investigating how much improvement can be expected from these optimizations. It helps implementers to choose the fastest or smallest algorithm; allows system designer to estimate accurately the response time of security systems; facilitates the selection of the proper point representation for elliptic curves, etc.
The discussed algorithms run in quadratic time: O(n^{2}) for nbit input. For very long operands more complex algorithms, such as Schönhage's halfGCD algorithm [8] get faster, running in O(n^{ }log^{2}n) time, but for operand lengths used in cryptography they are far too slow (see [9]).
1.1Extended Greatest Common Divisor Algorithms
Given 2 integers x and y the extended GCD algorithms compute their greatest common divisor g, and also two integer factors c and d: [g,c,d] = xCGD(x,y), such that g = c·x+d·y. For example, the greatest common divisor of 6 and 9 is 3; and 3 = (−1)∙6 + 1∙9.
In the sequel we will discuss several xGCD algorithms. (See also [2] or [10].) They are iterative, that is, their input parameters get gradually decreased, while keeping the GCD of the parameters unchanged (or keep track of its change). The following relations are used:

GCD(x,y) = GCD(x±y,y)

GCD(x,y) = 2∙GCD(x/2,y/2) for even x and even y

GCD(x,y) = GCD(x/2,y) for even x and odd y.
1.2Modular Inverse
The positive residues 1, 2, …, p^{ }−1 of integers modulo p (a prime number) form a multiplicative group G, that is, they obey the following 4 group laws:
1. Closure: If x and y are two elements in G, then the product x∙y := x^{ }y mod p is also in G.
2. Associativity: The defined multiplication is associative, i.e., for all x, y, z G: (x∙y)∙z = x∙(y∙z).
3. Identity: There is an identity element i ( = 1) such that i∙x = x∙i = x for every element x G.
4. Inverse: There is an inverse (or reciprocal) x^{−1} of each element x G, such that x∙x^{−1 }= i.
The inverse mentioned in 4. above is called the Modular Inverse, if the group is formed by the positive residues modulo a prime number. For example the inverse of 2 is 3 mod 5, because 2∙3 = 6 =^{_} 1 mod 5.
Positive residues modulo a composite number m do not form a group, as some elements don't have inverse. For example, 2 has no inverse mod 6, because every multiple of 2 is even, never 1 mod 6. Others, like 5 do have inverse, also called modular inverse. In this case the modular inverse of 5, 5^{−1} mod 6, is also 5, because 5∙5 = 25 = 24+1 =^{_} 1 mod 6. In general, if x is relative prime to m (they share no divisors) there is a modular inverse x^{−1} mod m. (See also in [2].)
Modular inverses can be calculated with any of the numerous xGCD algorithms. If we set y = m, by knowing that GCD(x,m) = 1, we get 1 = c·x+d·m from the results of the xGCD algorithm. Taking this equation modulo m we get 1 =^{_} c∙x. The modular inverse is the smallest positive such c, so either x^{−1} = c or x^{−1} = c +^{ }m.
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