Lecture 7: Public Key Exchange

• Sp20 Slides
• Playlist (length: 59:11)

Asymmetric Encryption

(True/False) Asymmetric encryption is the same as symmetric, except a user has two private keys instead of one

False. One key is private but the other is publically known

One-Way Functions

Is the one-time pad (with an unknown-key) a one-way function?

No. Given some y=OTP(k, x), an attacker can learn k in a single query and subsequently learn x

Discrete Logarithm

(True/False) Given g^x (mod p) and p, it is difficult to find x even when given additional knowledge of g

True.

Diffie-Hellman Key Exchange

Why can't Alice and Bob simply send their keys a/b to eachother in order to compute the shared g^(ab)?

Then an eavesdropper could also derive the key so it wouldn’t be secret.

Diffie-Hellman Key Exchange Security

When Malice performs the MiTM attack, she must pick the same g^m (mod p) that she sends to both parties

False. She can pick any exponent as long as she knows it

Detecting Pairing MiTM attacks

Why don't we always use this method of entering a digest of the derived key to check that the same key was derived?

Because it relies on an alternate secure channel (in this case physical access) which we oftentimes don’t have. In fact, the whole point of asymmetric cryptography was to create a secure channel from nothing, so using a different secure channel is kind of cheating (you could just share the key through the alternate channel)

Public Key Encryption and Semantic Security

What is the main difference between the IND-CPA and Semantic Security games we've seen in class?

Instead of a query phase, the adversary is simply given the public key

El-Gamal Encryption Scheme

(True/False) In El-Gamal encryption, the exponent r must be random

True. It must be random because if an attacker could predict it, they could decrypt a ciphertext