Elliptic curve cryptography has several uses including public key encryption, digital signatures, and secure protocols, but it is not the right answer. Quantum cryptography also has several uses including key exchange and secure communications, but it too is not the right answer either. Key stretching is used to strengthen encryption keys and passwords. So, that is not the correct answer here. The Tor network depends upon perfect forward secrecy to enable true anonymity. In this approach, each link in the Tor chain only knows the identity of the immediately adjacent links in the chain.