Lecture 8: Pseudorandom Number Generators (PRNGs) and Diffie-Hellman Key Exchange

• Notes
• Sp20 Slides
• Playlist (length: 31:08)

PRNGs: Definition

HMAC-DRBG and PRNG Applications

Stream Ciphers

PRGs

(True/False) A block cipher is a type of PRG

False. A PRGs output is much longer than it’s input. Since a block cipher is a permutation its input and output sizes are the same. However, we can use a block cipher to build PRGs using block cipher modes like we’ve seen.

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)