A Password-authenticated key agreement protocol in OCaml
Published: 14 Feb 2023
Spoke, a password-authenticated key agreement protocol in OCaml
The goal of Spoke is to establish an agreement on two strong keys from a shared weak password. This implementation comes from a description of SPAKE2+EE available here.
Let's start with Bob and Alice. They share a weak password and they want to initiate a secure connection. Spoke is able to derive from this weak password 2 keys that can be used to establish a secure connection using symmetric encryption (like AEAD).
Spoke implements a handshake between Alice and Bob and returns the 2 keys for Alice and for Bob. It provides a Mirage_flow.S implementation which uses GCM, CCM or ChaCha20_Poly1305 as a symmetric encryption mechamism between the client to the server and the server to the client (they can be different).
You can simulate this handshake with the
bin/simulate.exe program. It creates a socket and simulates a communication between Alice and Bob and finds an arrangement about 2 keys usable for symmetric encryption. Then, it sends a file to the server which repeats contents to the client. The client check the integrity of the received contents.
.---->----. (via GCM) [ client ] [ server ] (via ChaCha20_Poly1305) '----<----'
You can execute it with:
$ dune exec bin/simulate.exe -- filename 127.0.0.1:9000 hello-world
The goal of this tool is to ensure:
that the handshake is done correctly if Bob & Alice share the same password
the transmission throught a symmetric cipher from the shared keys works
A full explanation of the protocol and the handshake is available on my blog: Spoke, how to implement a little cryptographic protocol. Finally, you should take a look on bob which has a real usage of Spoke.