Thanks for the clarification! The part that loses me with the public encryption schemes is: how does the receiver get that private key? Is it dependent on the algorithm?

]]>Your description of the Diffie-Hellman protocol is not quite right. One party chooses exponent x and then computes h=g^x mod p. So it is guaranteed that h lies in Zp* (regardless of whether g is a generator or not).

The fact that g is a generator means that for *any* h in Zp* there exists an exponent x for which g^x = h mod p. However, the *discrete logarithm assumption* is that **finding** such an x (given p, g, and h) is hard.

As for security of public-key encryption, every public key has an associated private key that enables decryption. But security implies that it is hard to compute the private key from the public key. (For example, in the case of RSA that we discussed in lecture 7, given the public key (N, e) it is hard to compute the decryption exponent d.)

]]>Beyond that, I start to get lost at public security. How is it that using a public key can be secure? Assuming the adversary knows the senders public key, what prevents him from decrypting it just the same as the actual recipient? I feel like I’m missing something basic here.

]]>The math *is* hard if you haven’t seen it before, but that’s why I’m especially surprised at the lack of questions. Please ask me in class for more examples, or to make it concrete.

Thanks for the link — I enjoyed it. =)

]]>http://www.moserware.com/2009/09/stick-figure-guide-to-advanced.html

We should do all our lectures in stick figure drawings. ðŸ™‚

]]>Thanks for the comment! I can understand that it would be daunting to question the material, especially since we are not going many of the low-level details the way I would in 456. But I’m surprised that people don’t have questions *about* the material, or about how it’s used.

To sum it up, It’s probably easier to discuss “how they stole the file” than “how they would decrypt the file,” especially after being told that pretty much anyone can encrypt something that would require more energy than is available in the universe to break.

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