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Cryptography

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  • $\begingroup$ "I defined a protocol using the idea behind D Hellman" - So you're doing a standard diffie-hellman key exchange, just with RSA private keys and a (random) message? $\endgroup$ Commented Dec 12, 2015 at 21:59
  • $\begingroup$ yes, I did that $\endgroup$ Commented Dec 12, 2015 at 22:07
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    $\begingroup$ So, for part c) Should I use digital signatures ? $\endgroup$ Commented Dec 12, 2015 at 22:32
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    $\begingroup$ @Santiago Gonzalez Toral: I'm afraid that you over-engineered your protocol in a) and that sent you on tracks far from what the problem's author is trying to guide you. What you have been asked in a) [be secure against an adversary assumed not to send any message] can be done simply, with RSA encryption alone. Hint: Alice first sends a non-encrypted message to Bob; after the second message, Bob and Alice have their shared secret key. $\;$ When you use this simple protocol, b) and c) becomes easy. Hint: some of the shared secret can be devoted to protection against MiTM. $\endgroup$ Commented Dec 13, 2015 at 11:23
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    $\begingroup$ 1. Alice sends a public key to Bob 2. Bob generates a temporary symmetric key and encrypts it with Alice's public key. Then, he sends the ciphertext to Alice. 3. Alice decrypts it using her private key. Now, both Alice and Bob have a shared key. MiTM ---- Even if, the attacker intercepts communication between Alice and Bob, he is unable to recover the symmetric key because he doesn't have Alice's private key. Right? $\endgroup$ Commented Dec 13, 2015 at 17:49