3.7. Bob's RSA public key has modulus N = 12191 and exponent e = 37. Alice sends Bob the ciphertext c= 587. Unfortunately, Bob has chosen too small a modu- lus. Help Eve by factoring N and decrypting Alice's message. (Hint. N has a factor smaller than 100.)

Elements Of Modern Algebra
8th Edition
ISBN:9781285463230
Author:Gilbert, Linda, Jimmie
Publisher:Gilbert, Linda, Jimmie
Chapter2: The Integers
Section2.7: Introduction To Coding Theory (optional)
Problem 12E: Suppose that the check digit is computed as described in Example . Prove that transposition errors...
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### RSA Encryption Example Problem and Solution

#### Problem Statement:
Bob’s RSA public key has a modulus \( N = 12191 \) and exponent \( e = 37 \). Alice sends Bob the ciphertext \( c = 587 \). Unfortunately, Bob has chosen too small a modulus. Help Eve by factoring \( N \) and decrypting Alice’s message. (Hint: \( N \) has a factor smaller than 100.)

#### Explanation:
In this problem, we are dealing with RSA encryption, where:
- \( N \) is the modulus
- \( e \) is the public exponent
- \( c \) is the ciphertext

To decrypt the message, you would generally need the private key. The private key can be derived if you can factorize \( N \) into its prime factors. Given the hint that \( N \) has a factor smaller than 100, we need to find these factors to proceed with the decryption process.

#### Steps to Solve:
1. **Factorize \( N \)**: Start by finding the prime factors of \( 12191 \). As the hint suggests, look for factors smaller than 100.
2. **Calculate \( \phi(N) \)**: Once you have the factors, compute Euler's totient function \( \phi(N) \).
3. **Find the private exponent \( d \)**: Compute \( d \) using the relation \( d \cdot e \equiv 1 \,(\text{mod}\, \phi(N)) \).
4. **Decrypt the ciphertext \( c \)**: Calculate \( m \), the original message, using the relation \( m = c^d \,(\text{mod}\, N) \).

By following the above steps, you will be able to decrypt the message that Alice sent to Bob.

#### Detailed Steps:

**1. Factorize \( N \)**:
   - Test divisibility by prime numbers less than 100 to find that \( N = 12191 \).
   - Notice that \( 12191 \div 11 = 1109 \), so one of the factors is 11.
   - Continue factoring \( 1109 \) and find that it divides by 101 (as \( 1109 = 11 \times 101 \)).

**2. Calculate \( \phi(N) \)**:
   - Using the prime factors (11 and 101): 
     \[
Transcribed Image Text:### RSA Encryption Example Problem and Solution #### Problem Statement: Bob’s RSA public key has a modulus \( N = 12191 \) and exponent \( e = 37 \). Alice sends Bob the ciphertext \( c = 587 \). Unfortunately, Bob has chosen too small a modulus. Help Eve by factoring \( N \) and decrypting Alice’s message. (Hint: \( N \) has a factor smaller than 100.) #### Explanation: In this problem, we are dealing with RSA encryption, where: - \( N \) is the modulus - \( e \) is the public exponent - \( c \) is the ciphertext To decrypt the message, you would generally need the private key. The private key can be derived if you can factorize \( N \) into its prime factors. Given the hint that \( N \) has a factor smaller than 100, we need to find these factors to proceed with the decryption process. #### Steps to Solve: 1. **Factorize \( N \)**: Start by finding the prime factors of \( 12191 \). As the hint suggests, look for factors smaller than 100. 2. **Calculate \( \phi(N) \)**: Once you have the factors, compute Euler's totient function \( \phi(N) \). 3. **Find the private exponent \( d \)**: Compute \( d \) using the relation \( d \cdot e \equiv 1 \,(\text{mod}\, \phi(N)) \). 4. **Decrypt the ciphertext \( c \)**: Calculate \( m \), the original message, using the relation \( m = c^d \,(\text{mod}\, N) \). By following the above steps, you will be able to decrypt the message that Alice sent to Bob. #### Detailed Steps: **1. Factorize \( N \)**: - Test divisibility by prime numbers less than 100 to find that \( N = 12191 \). - Notice that \( 12191 \div 11 = 1109 \), so one of the factors is 11. - Continue factoring \( 1109 \) and find that it divides by 101 (as \( 1109 = 11 \times 101 \)). **2. Calculate \( \phi(N) \)**: - Using the prime factors (11 and 101): \[
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