Support in Erlang's SSL library for GRiSP's Secure Element without using OpenSSL crypto engines.

NOT TESTED YET!

Usage:

    Key = #{algorithm => ecdsa, sign_fun => {my_module, my_sign_fun}}.

Then add {key, Key} to the SSL options (server and/or client possible). The sign_fun has arity one
and will get the to be signed Message (TLS 1.3) or {digest, Digest} (TLS 1.2), analogous to ssl:sign/3.
The intended use here is providing a callback for grisp_cryptoauth:sign/2. While this patch makes
heavy use of the existing infrastructure for crypto engines it is completely backwards compatible, e.g.
crypto engines still work.

diff --git a/lib/ssl/src/ssl_config.erl b/lib/ssl/src/ssl_config.erl
index 89198eaa2b..c6e181f9b0 100644
--- a/lib/ssl/src/ssl_config.erl
+++ b/lib/ssl/src/ssl_config.erl
@@ -163,6 +163,10 @@ init_certificates(undefined, #{pem_cache := PemCache} = Config, CertFile, server
     end;
 init_certificates(OwnCerts, Config, _, _) ->
     {ok, Config#{own_certificates => OwnCerts}}.
+
+%% HACK: allow callbacks for signing using the GRiSP Secure Element
+init_private_key(_, #{algorithm := ecdsa, sign_fun := _SignFun} = Key, _, _, _) ->
+    Key;
 init_private_key(_, #{algorithm := Alg} = Key, _, _Password, _Client) when Alg == ecdsa;
                                                                            Alg == rsa;
                                                                            Alg == dss ->
diff --git a/lib/ssl/src/ssl_handshake.erl b/lib/ssl/src/ssl_handshake.erl
index 6ee0aa8f7b..76a5aa6501 100644
--- a/lib/ssl/src/ssl_handshake.erl
+++ b/lib/ssl/src/ssl_handshake.erl
@@ -2043,6 +2043,12 @@ digitally_signed(Version, Msg, HashAlgo, PrivateKey, SignAlgo) ->
            throw(?ALERT_REC(?FATAL, ?HANDSHAKE_FAILURE, bad_key(PrivateKey)))
     end.

+%% HACK: allow callbacks for signing using the GRiSP Secure Element
+%% We only care here for sha256 + ecdsa for TLS 1.2 and 1.3 (SSL 3.3 and 3.4)
+do_digitally_signed({3, 4}, Msg, sha256, #{algorithm := ecdsa, sign_fun := {Mod, Fun}}, ecdsa) ->
+    Mod:Fun(Msg);
+do_digitally_signed({3, 3}, Hash, sha256, #{algorithm := ecdsa, sign_fun := {Mod, Fun}}, ecdsa) ->
+    Mod:Fun({digest, Hash});
 do_digitally_signed({3, Minor}, Msg, HashAlgo, {#'RSAPrivateKey'{} = Key,
                                                  #'RSASSA-PSS-params'{}}, SignAlgo) when Minor >= 3 ->
     Options = signature_options(SignAlgo, HashAlgo),