(7) (RSA public key cryptosystem) Bob chooses the secret primes p = 11 and q = 23. Bob chooses the encryp- tion exponent e = 7. Bob publishes N = 253 and e = 7. Alice chooses the plaintext m₁ = 5. Compute the ciphertext c₁ that Alice sends to Bob.

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Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
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### RSA Public Key Cryptosystem Example

**Problem:**

Bob chooses the secret primes \( p = 11 \) and \( q = 23 \). Bob chooses the encryption exponent \( e = 7 \). Bob publishes \( N = 253 \) and \( e = 7 \).

Alice chooses the plaintext \( m_1 = 5 \). Compute the ciphertext \( c_1 \) that Alice sends to Bob.

**Solution:**

To encrypt the plaintext \( m_1 = 5 \) using Bob's public key values \( N = 253 \) and \( e = 7 \), Alice uses the RSA encryption formula:

\[ c \equiv m^e \, (\text{mod} \, N) \]

Given:
- \( m = 5 \)
- \( e = 7 \)
- \( N = 253 \)

1. Compute \( 5^7 \):
    \[
    5^7 = 5 \times 5 \times 5 \times 5 \times 5 \times 5 \times 5 = 78125
    \]

2. Compute \( 78125 \mod 253 \):
    \[
    78125 \div 253 \approx 308.8745 \quad \Rightarrow \quad 78125 = 253 \times 308 + 181 \quad \Rightarrow \quad 78125 \mod 253 = 181
    \]

Therefore, the ciphertext \( c_1 \) that Alice sends to Bob is:

\[ c_1 = 181 \]
Transcribed Image Text:### RSA Public Key Cryptosystem Example **Problem:** Bob chooses the secret primes \( p = 11 \) and \( q = 23 \). Bob chooses the encryption exponent \( e = 7 \). Bob publishes \( N = 253 \) and \( e = 7 \). Alice chooses the plaintext \( m_1 = 5 \). Compute the ciphertext \( c_1 \) that Alice sends to Bob. **Solution:** To encrypt the plaintext \( m_1 = 5 \) using Bob's public key values \( N = 253 \) and \( e = 7 \), Alice uses the RSA encryption formula: \[ c \equiv m^e \, (\text{mod} \, N) \] Given: - \( m = 5 \) - \( e = 7 \) - \( N = 253 \) 1. Compute \( 5^7 \): \[ 5^7 = 5 \times 5 \times 5 \times 5 \times 5 \times 5 \times 5 = 78125 \] 2. Compute \( 78125 \mod 253 \): \[ 78125 \div 253 \approx 308.8745 \quad \Rightarrow \quad 78125 = 253 \times 308 + 181 \quad \Rightarrow \quad 78125 \mod 253 = 181 \] Therefore, the ciphertext \( c_1 \) that Alice sends to Bob is: \[ c_1 = 181 \]
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