PicoCTF Challenge: EvenRsaCanBeBroken

August 02, 2025 PicoCTF Easy

Challenge Overview:

  • The following was the description for the challenge: stepic challenge

    Intro

  • RSA, which is a public-key cryptography, is an asymmetric encryption that relies on the difficulty of factorizing the modulus N.
  • Modulus N is the product of two large prime numbers, p and q.
  • so because of limited computing power in the old days, computers were slow, making factoring large numbers (n = p * q) computationally infeasible.
    • Hence, it made sense for it be regarded as a strong encryption.
  • But, the modern technology and considering “quantum computing” in the future makes these encryption methods easy to break.

Approach

  • First We Have To Understand How RSA works:
    1. The plaintext(m) is converted into an integer
    2. Encryption is performed using the public exponent(e)-regarded as public key, and Modulus N: Ciphertext(C) = m^e * mod N)
    3. The ciphertext C that is generated is sent to the receiver.
    4. Decryption uses a private key (d) and Modulus(N): Plaintext = C^d * mod N