RSA encryption

Database System Concepts
7th Edition
ISBN:9780078022159
Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Chapter1: Introduction
Section: Chapter Questions
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### Problem 22: Efficient Computation of Large Powers Modulo

You are tasked to compute \(123456789^{65537} \mod 581859289607\). A friend offers their assistance with a pricing model of 1 cent per multiplication mod \(581859289607\). Your friend aims to earn more than $650. However, your challenge is to strategize so the computation costs below 25 cents.

#### Solution Strategy:

To minimize the number of multiplications, you can utilize the method of **"Exponentiation by Squaring"**. This is an efficient algorithm that reduces the number of multiplication operations typically from \(n\) to \(O(\log n)\), significantly lowering your costs.

- **Exponentiation by Squaring**: This method takes advantage of binary decomposition of the exponent to repeatedly square and multiply:
  - Break down the exponent (65537) into powers of 2.
  - Compute successive squares starting from the base number \(123456789\).
  - Combine the computed powers using the properties of exponents.

#### Key Observation:

- **RSA Exponent**: The exponent \(65537\) mentioned is commonly used in RSA encryption due to its balance of security and computational efficiency, being \(2^{16} + 1\).

By following this approach, you can achieve the required computation with significantly fewer multiplications, ensuring you remain well within the budget of 25 cents.
Transcribed Image Text:### Problem 22: Efficient Computation of Large Powers Modulo You are tasked to compute \(123456789^{65537} \mod 581859289607\). A friend offers their assistance with a pricing model of 1 cent per multiplication mod \(581859289607\). Your friend aims to earn more than $650. However, your challenge is to strategize so the computation costs below 25 cents. #### Solution Strategy: To minimize the number of multiplications, you can utilize the method of **"Exponentiation by Squaring"**. This is an efficient algorithm that reduces the number of multiplication operations typically from \(n\) to \(O(\log n)\), significantly lowering your costs. - **Exponentiation by Squaring**: This method takes advantage of binary decomposition of the exponent to repeatedly square and multiply: - Break down the exponent (65537) into powers of 2. - Compute successive squares starting from the base number \(123456789\). - Combine the computed powers using the properties of exponents. #### Key Observation: - **RSA Exponent**: The exponent \(65537\) mentioned is commonly used in RSA encryption due to its balance of security and computational efficiency, being \(2^{16} + 1\). By following this approach, you can achieve the required computation with significantly fewer multiplications, ensuring you remain well within the budget of 25 cents.
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