SRP -- SRP-6a Secure Remote Password protocol#

Implementation of the SRP-6a protocol (RFC 5054) for password-authenticated key exchange. Includes support for the 3072-bit group used by HomeKit.

Installation#

Install with opam:

$ opam install srp

If opam cannot find the package, it may not yet be released in the public opam-repository. Add the overlay repository, then install it:

$ opam repo add samoht https://tangled.org/gazagnaire.org/opam-overlay.git
$ opam update
$ opam install srp

Usage#

Server setup (register a user)#

(* Server stores verifier, not the password *)
let salt = ""
let verifier =
  Srp.compute_verifier ~salt ~username:"alice" ~password:"secret"

Client authentication#

let () = Crypto_rng_unix.use_default ()

(* Client side: generate A and ephemeral key; send (username, A) to server. *)
let client = Srp.Client.create ~username:"alice" ~password:"secret"
let big_a = Srp.Client.public_key client

(* Server receives (username, A), looks up salt+verifier; sends (salt, B). *)
let server = Srp.Server.create ~username:"alice" ~salt ~verifier
let big_b = Srp.Server.public_key server

(* Both derive the shared session key. *)
let () =
  match
    ( Srp.Client.compute_session_key client ~salt ~big_b,
      Srp.Server.compute_session_key server ~big_a )
  with
  | Ok client_key, Ok server_key ->
      assert (String.equal client_key server_key)
  | _ -> failwith "session key derivation failed"

API#

• Srp.n / Srp.g -- 3072-bit group parameters
• Srp.compute_verifier -- Compute password verifier for server storage
• Srp.Client.v / Srp.Client.public / Srp.Client.session_key -- Client state
• Srp.Server.v / Srp.Server.public / Srp.Server.session_key -- Server state

Reference#

• RFC 5054 -- Using SRP for TLS Authentication

Licence#

ISC