El Gamal Challenge: Randomness Reuse AttackThe Problem: Given an encryption key, two ciphertexts which use the same random exponent y and the plaintext corresponding to one of those ciphertexts, recover the second plaintext. You intercept two ciphertexts cM and cA intended for Horridland; one from Malland, and one from Awfulland. Your agent in Awfulland figures out what Awfulland wants to communicate to Horridland; it is "I'm following your lead". Since Malland has always been their plan-maker, in order to determine what Horridland, Malland and Awfulland will do, you need to figure out what Malland is communicating to Horridland. You know that your enemies are pretty bad at using cryptography, and that they sometimes forget to pick fresh randomness. You suspect that they might have used the same random value y to create both ciphertexts. Use this knowledge to figure out what Malland is trying to tell Horridland. For this exercise, use the following values:
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Cryptography Challenges |
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