23 Full Station-to-Station Protocol

$$\begin{array}{rl}1.& A\to B:g^x\\ 2.& B\to A:g^y,{\text{Cert}}_B,\{S_B(g^y,g^x){\}}_{g^{xy}}\\ 3.& A\to B:{\text{Cert}}_A,\{S_A(g^y,g^x){\}}_{g^{xy}}\end{array}$$

Why add certificates ?

• The “full” STS protocol includes certificates for A and B.
• Certificates contains public keys signed by a Trusted Third Party.
• Alice and Bob don’t need to know each other’s public key beforehand.