Chapter 12 - Practical Problems with DH

Question 1

What is the Logjam attack?

Answer 1

• Exploits the fact, that safe primes have to be chosen in DH
• Uses TLS-DHE-Export 512 bit primes instead 1024 bit primes
• Attacker actively downgrades user capabilities
• Then cracks key in realtime using a pre-computed database with the logarithms for the most common primes
• Use key to sign finish message at the end of handshake

Question 2

What are ways to prevent a logjam attack?

Answer 2

• Use EC-DH
• Increase prime size to 2048 bit
• Use new 1024 primes