Key Exchange

• We have seen already how public-key cryptography may be used for public key distribution;

• Public-key cryptography may be used also for key exchange:

  • Two parties (users) execute some algorithm (protocol) and get a common secret key;
  • The key may be used for subsequent encryption of messages;

• Most known algorithm for key exchange is Diffie-Hellman algorithm (1976);

• The purpose of the algorithm is exchange of a secret key (not encryption);

• DH algorithm is considered as a public-key algorithm because:

  • Users to generate the same secret key rely on publicly known information + some private information;
  • In principle, it is possible to generate a key knowing only public information, but it is computationally expensive;

Discrete logarithms (离散数对)

• Security of DH algorithm relies upon difficulty of computing discrete logarithms;

Diffie-Hellman key exchange

How to break HD key exchange?