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Cryptography Challenges

RSA Challenge Three: RSA Broadcast Attack

The Problem: Given RSA ciphertexts with different moduli but the same low encryption exponent, recover the message.

Since you thwarted Malland's plots with Horridland, Malland enlisted the help of two more allies - Awfulland and Badland. Horridland, Awfulland and and Badland made the mistake of all having the same low encryption exponent, e = 3. Malland sent Horridland, Awfulland and Badland the same message about a plan of attack. You get your hands on all three copies of the message:

  • Awfulland's message, encrypted as cA = m3 mod nA (using Awfulland's modulus nA),
  • Badland's message, encrypted as cB = m3 mod nB (using Badland's modulus nB), and
  • Horridland's message, encrypted as cH = m3 mod nH (using Horridland's modulus nH).

Use the fact that all three emails are encrypted using the same low encryption exponent to figure out what the evil plot is.

For this exercise, use the following values:

Name Value
Number of ciphertexts:
3
Common public exponent:
3
Ciphertext #1
Modulus #1
Ciphertext #2
Modulus #2
Ciphertext #3
Modulus #3