Question

Suppose we are given a positive integer \(n,\) along with its prime factorization \(n=p_{1}^{e_{1}} \cdots p_{r}^{e_{r}},\) and that for each \(i=1, \ldots, r,\) we are also given the prime factorization of \(p_{i}-1 .\) Show how to efficiently compute the multiplicative order of any element \(\alpha \in \mathbb{Z}_{n}^{*}\).

Short answer

Answer: Use the given prime factorization of \(n\) and \(p_i-1\) to calculate the Carmichael function \(\lambda(n)\) and Euler's Totient function \(\phi(n)\). Then, find the multiplicative order of \(\alpha\) by searching for the smallest positive integer \(k\) that satisfies \(\alpha^k \equiv 1 \pmod{n}\), with \(k\) dividing \(\lambda(n)\). Follow the steps outlined in the solution to efficiently compute the multiplicative order of \(\alpha\) modulo \(n\).

Step-by-step solution

Calculate Euler's Totient function

Using the prime factorization of \(n\), we can find Euler's Totient function \(\phi(n)\) as follows: $$ \phi(n)=n\prod_{i=1}^{r}(1-\frac{1}{p_i})=n(1-\frac{1}{p_1})(1-\frac{1}{p_2})\cdots(1-\frac{1}{p_r}). $$

Find the Carmichael function λ(n)

The Carmichael function can be defined as the least common multiple of the Carmichael functions for each factor of \(n\), i.e., \(\lambda(n)=\operatorname{lcm}(\lambda(p_1^{e_1}), \lambda(p_2^{e_2}), \ldots, \lambda(p_r^{e_r}))\). For any prime \(p\) and positive integer \(e\), the Carmichael function is given by: $$ \lambda(p^e) = \begin{cases} \phi(p^e) = (p-1)p^{e-1}, & \text{if}\ p > 2,\ e\geq1 \\ \phi(p^e)/2 = (p-1)p^{e-1}/2 , & \text{if}\ p = 2,\ e\geq2 \\ 1, & \text{if}\ p = 2,\ e=1 \\ \end{cases}. $$

Compute the multiplicative order of an element α

To find the multiplicative order of an element \(\alpha \in \mathbb{Z}_{n}^{*}\), we need to find the smallest positive integer \(k\) such that \(\alpha^k \equiv 1 \pmod{n}\). Since \(a^{\lambda(n)}\equiv1\pmod{n}\) for every \(a\in\mathbb{Z}_n^*\), we know that the multiplicative order of \(\alpha\) divides \(\lambda(n)\). Let the prime factorization of \(\lambda(n)\) be \(\lambda(n)=q_1^{f_1}\cdots q_s^{f_s}\). Then, the multiplicative order of \(\alpha\) is given by the smallest \(k\) such that \(\alpha^k\equiv1\pmod{n}\), and \(k\) divides \(\lambda(n)\). The method is to check whether \(\alpha^{\frac{\lambda(n)}{q_j}} \not\equiv 1 \pmod{n}\) for each prime factor \(q_j\). If this condition is satisfied for all prime factors of \(\lambda(n)\), then the multiplicative order of \(\alpha\) is \(\lambda(n)\). Otherwise, we need to iterate through the divisors of the prime factors.

Get the multiplicative order

To get the multiplicative order of \(\alpha\), follow these steps: 1. Compute the prime factorization of \(\lambda(n)\) (using the given prime factorizations of \(p_i-1\) and the algorithm in Step 2). 2. For each prime factor \(q_j\) of \(\lambda(n)\): a. Calculate \(k=\frac{\lambda(n)}{q_j}\). b. Check if \(\alpha^k \not\equiv 1 \pmod{n}\). c. If \(\alpha^k \not\equiv 1 \pmod{n}\) for all \(q_j\), then the multiplicative order of \(\alpha\) is \(\lambda(n)\). d. If \(\alpha^k \equiv 1 \pmod{n}\) for some \(q_j\), then the multiplicative order is a divisor of \(\lambda(n)\). Recursively iterate through the divisors of \(\lambda(n)\) and check if \(\alpha^k \equiv 1 \pmod{n}\), and find the smallest such \(k\). Following these steps, we can efficiently compute the multiplicative order of any element \(\alpha \in \mathbb{Z}_{n}^{*}\) using the prime factorization of \(n\) and \(p_i-1\).

Key concepts

Euler's Totient Function

Understanding the Euler's Totient function is fundamental when studying number theory, particularly in the field of cryptography.
The Euler's Totient function, denoted as \(\phi(n)\), is a measure of the number of positive integers up to \(n\) that are relatively prime to \(n\). This means they share no common factors with \(n\) besides 1. For a prime number \(p\), \(\phi(p)\) is simply \(p-1\) because all the numbers less than a prime are coprime to it.
Calculating \(\phi(n)\) using prime factorization is an efficient method. It involves the formula \[\phi(n)=n\prod_{i=1}^{r}(1-\frac{1}{p_i})=n(1-\frac{1}{p_1})(1-\frac{1}{p_2})\cdots(1-\frac{1}{p_r}).\] Here, \(n\) is the product of its prime factors \(p_{i}\) raised to their respective powers \(e_{i}\).

Why is understanding \(\phi(n)\) important?

• It appears in Euler's theorem, which states that for any integer \(a\), if \(a\) is coprime to \(n\), then \(a^{\phi(n)} \equiv 1 \pmod{n}\).
• It serves as a key component when working with the RSA encryption algorithm, a standard for securing sensitive data transmission.
• Determining \(\phi(n)\) helps establish the foundation for computing multiplicative orders, which are essential in many areas of computational number theory.

By applying the concept of \(\phi(n)\), students can tackle problems involving modular arithmetic and develop a deeper comprehension of number theory's applications.

Carmichael Function

When diving deeper into number theory, one stumbles upon the Carmichael function, denoted as \(\lambda(n)\). This function, closely related to the Euler's Totient function, plays a pivotal role in the context of multiplicative orders.
The Carmichael function of a number \(n\) is the smallest positive integer \(m\) such that \(a^m \equiv 1 \pmod{n}\) for every integer \(a\) which is coprime to \(n\). The clever utilization of the Carmichael function allows us to determine the multiplicative order of an element efficiently.

Finding \(\lambda(n)\):

• For a prime \(p\) and positive integer \(e\), \(\lambda(p^e)\) can vary depending on the values of \(p\) and \(e\).
• The general approach involves combining the Carmichael function values of the individual prime power factors of \(n\) using the least common multiple (LCM).
• The full formula for a prime power is described by the piecewise function given in the provided solution.

The Carmichael function, although less known than \(\phi(n)\), significantly simplifies computations when dealing with large numbers, especially in cryptographic algorithms like RSA. Recognizing and applying \(\lambda(n)\) allows students to calculate the precise cycle at which a certain modular exponentiation starts to repeat, which is vital for decoding and encryption processes.

Prime Factorization

Prime factorization is the process by which a composite number is expressed as the product of its prime factors. This mathematical concept is foundational for understanding the structure of integers and plays a crucial role in various areas of mathematics and computer science.
In the context of the given exercise, prime factorization allows us to efficiently compute different mathematical functions such as Euler's Totient and the Carmichael function, which can then be used to determine the multiplicative order.

How prime factorization is utilized:

• It simplifies the computation of \(\phi(n)\) and \(\lambda(n)\).
• Prime factorization underpins algorithms in number theory, cryptography, and even in error detection and correction techniques.
• It also empowers us to calculate the greatest common divisor (GCD) and the least common multiple (LCM) of numbers, which are necessary for breaking down the Carmichael function into manageable parts.

By mastering prime factorization, students are equipped to tackle more complex problems involving modular arithmetic and cryptography, as well as laying the groundwork for understanding the multiplicative order of elements within a group.