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1
The Trivial Solution to the High Degree Congruence x
n
≡ a
(mod p) in
(p)
Shenghui Su
School of Information Engineering
University of Science & Technology Beijing
Beijing 100083
P. R. China
sheenway@126.com
Abstract
This paper gives the definition of the trivial solution to the high degree congruence x
n
≡ a
(mod p), presents one sufficient and necessary condition for the congruence to have
solutions, derives and proves one new judgment condition complementarily. Elaborates
two methods for computing the trivial solution to the congruence in determinate
polynomial time, argues non-trivial solutions to the congruence can not be obtained
cyclically from the trivial solution, infers and demonstrates the two new methods for
seeking the trivial solution. In summary, resolves the root existing problem and the trivial
root computing problem for x
n
≡ a (mod p) with an arbitrary n. At last, points out that
even through a probabilistic polynomial time algorithm, seeking a specific non-trivial
solution is still infeasible when the number of the solutions is large enough.
Keywords: High degree congruence, Cryptosystem, Trivial solution, Polynomial time
algorithm.
1 Introduction
The discrete logarithm problem and the root finding problem are two closely related problems in
computational number theory.
The discrete logarithm problem, shortly the DLP, is to seek an exponent n such that x
n
≡ a (mod u), given
the triple <x, a, u>. This problem is intractable because no polynomial time algorithm for it has been found
yet
[1]
. The famous public-key cryptosystem ElGamal is built on this hardness
[2]
.
The root finding problem is to seek a root x such that x
n
≡ a (mod u), given the triple <n, a, u>. This
problem is slightly easier than the DLP since there are polynomial time algorithms for it, provided u is a
prime power. However, when
φ
(u) is unknown, even seeking square roots is equivalent to the integer
factorization hardness on which the security of RSA is based
[3]
, where
φ
(u) is the Euler’s phi function
which represents the number of those positive integers coprime to and less than u.
When u is equal to a prime p (if p is a prime power, the discussion is similar), the polynomial time
algorithms for solving x
n
≡ a (mod p) may be partitioned into the probabilistic type
[4]
and the determinate
type. The former is employed for any random solution to x
n
≡ a (mod p), and the latter is employed only for
the trivial solution to x
n
≡ a (mod p) if it exists.
[Definition 1] Under some special conditions, a solution to the high degree congruence x
n
≡ a (mod p) can
be found in determinate polynomial time. This solution may be written as a to a certain power modulo p,
and hence, is called the trivial solution or the trivial root.
Through this paper, unless otherwise specified, n ≥ 3, p is an odd prime, and a ≤ p or p
ł
a, that is, a is not