Pollard's rho algorithm | EPFL Graph Search

Are you an EPFL student looking for a semester project?

Work with us on data science and visualisation projects, and deploy your project as an app on top of GraphSearch.

Pollard's rho algorithm is an algorithm for integer factorization. It was invented by John Pollard in 1975. It uses only a small amount of space, and its expected running time is proportional to the square root of the smallest prime factor of the composite number being factorized. The algorithm is used to factorize a number , where is a non-trivial factor. A polynomial modulo , called (e.g., ), is used to generate a pseudorandom sequence. It is important to note that must be a polynomial. A starting value, say 2, is chosen, and the sequence continues as , , , etc. The sequence is related to another sequence . Since is not known beforehand, this sequence cannot be explicitly computed in the algorithm. Yet, in it lies the core idea of the algorithm. Because the number of possible values for these sequences is finite, both the sequence, which is mod , and sequence will eventually repeat, even though these values are unknown. If the sequences were to behave like random numbers, the birthday paradox implies that the number of before a repetition occurs would be expected to be , where is the number of possible values. So the sequence will likely repeat much earlier than the sequence . When one has found a such that but , the number is a multiple of , so has been found. Once a sequence has a repeated value, the sequence will cycle, because each value depends only on the one before it. This structure of eventual cycling gives rise to the name "rho algorithm", owing to similarity to the shape of the Greek letter ρ when the values , , etc. are represented as nodes in a directed graph. This is detected by Floyd's cycle-finding algorithm: two nodes and (i.e., and ) are kept. In each step, one moves to the next node in the sequence and the other moves forward by two nodes. After that, it is checked whether . If it is not 1, then this implies that there is a repetition in the sequence (i.e. . This works because if the is the same as , the difference between and is necessarily a multiple of .

About this result

This page is automatically generated and may contain information that is not correct, complete, up-to-date, or relevant to your search query. The same applies to every other page on this website. Please make sure to verify the information with EPFL's official sources.

Related concepts (2)

Related courses (1)

Related lectures (4)

Discrete Log Problem: Pollard's Rho Method MATH-489: Number theory II.c - Cryptography

Introduces Pollard's rho method for the discrete log problem and finding collisions efficiently in a cyclic group.

Shor's Factoring Algorithm PHYS-641: Quantum Computing

Covers Shor's factoring algorithm, aiming to find integer factors efficiently using quantum computation.

Galois Fields and Elliptic Curves COM-401: Cryptography and security

Introduces Galois fields, elliptic curves, factorization algorithms, and the discrete logarithm problem in cryptography.

Pollard's rho algorithm

Integer factorization

In number theory, integer factorization is the decomposition, when possible, of a positive integer into a product of smaller integers. If the factors are further restricted to be prime numbers, the process is called prime factorization, and includes the test whether the given integer is prime (in this case, one has a "product" of a single factor). When the numbers are sufficiently large, no efficient non-quantum integer factorization algorithm is known. However, it has not been proven that such an algorithm does not exist.

MATH-489: Number theory II.c - Cryptography

The goal of the course is to introduce basic notions from public key cryptography (PKC) as well as basic number-theoretic methods and algorithms for cryptanalysis of protocols and schemes based on PKC