opaque - simple command-line frontend for libopaque

       Written by Stefan Marsiske.

       Copyright  ©  2023  Stefan  Marsiske.   License  LGPLv3+:   GNU   Lesser   GPL   version   3   or   later
       https://gnu.org/licenses/lgpl.html.   This  is free software: you are free to change and redistribute it.
       There is NO WARRANTY, to the extent permitted by law.

       The OPAQUE protocol is an asymmetric password-authenticated key-exchange.  Essentially it allows a client
       to  establish  a shared secret with a server based on only having a password.  The client doesn’t need to
       store any state.  The protocol has two phases:

       • In the initialization phase a client registers with the server.

       • In the AKE phase the client and server establish a shared secret.

       The initialization only needs to be executed once, the key-exchange can be  executed  as  many  times  as
       necessary.

   Initialization
       Initializing  OPAQUE  (registration)  can  be  done either online or offline.  The online variant has the
       benefit that the server never learns anything about the users  password,  with  the  drawback  that  this
       requires 3 messages to be exchanged by the client and the server.

       The  offline  initialization  is  much  easier, however either the user learns the servers secret, or the
       server learns the users password.  The latter might be useful if some organisation wants to enforce  some
       password  quality  rules  and  check  those  upon  registration.   The  drawback is that either way, some
       sensitive information leaks to the other party.

   OfflineRegistration
              echo -n password | ./opaque init user server >record 3>export_key

   OnlineRegistrationsocatstyle
       On the server:

              socat tcp-l:23523,reuseaddr,fork system:"bash -c \'opaque server-reg user server 3>record\'"

       On the client:

              socat tcp:127.0.0.1:23523 exec:'bash -c \"opaque user-reg user server 3< <(echo -n password) 4>export_key\"'

   tcpserverstyle
       On the server:

              s6-tcpserver 127.0.0.1 23523 bash -c 'opaque server-reg user server 3>record'

       On the client:

              s6-tcpclient 127.0.0.1 23523 bash -c "opaque user-reg user server <&6 >&7 3< <(echo -n password) 4>export_key"

   Manually
       It’s possible to do all 4 steps seperately, in case you cannot connect to the server directly, then:

       The user initiates with:

              echo -n password | opaque register >msg 3>ctx

       The server gets msg and responds with rpub, while keeping rsec secret:

              cat msg | opaque respond >rpub 3>rsec

       The user receives rpub and creates stub record and optionally uses the export key to encrypt more data:

              cat ctx | opaque finalize user server 4<rpub >record 3>export_key

       the server finalizes the record by completing the stub record from the client:

              cat rec | opaque store user server >record 3<rsec

   RunningOPAQUEtcpserverstyle
       On the server:

              s6-tcpserver 127.0.0.1 23523 bash -c './opaque server user server context 3<record 4>shared_secret'

       On the client:

              s6-tcpclient 127.0.0.1 23523 bash -c "./opaque user user server context <&6 >&7 3< <(echo -n password) 4>export_key 5>shared_secret"

   socatstyle
       On the server:

              socat tcp-l:23523,reuseaddr,fork system:"bash -c \'./opaque server user server context 3<record 4>shared_secret\'"

       On the client:

              socat tcp:127.0.0.1:23523 exec:'bash -c \"./opaque user user server context 3< <(echo -n password) 4>export_key  5>shared_secret\"'

       opaque - simple command-line frontend for libopaque

       https://github.com/stef/libopaque/issues/

       https://ctrlc.hu/~stef/blog/tags/opaque/

       socat(1), tcpserver(1)

                                                                                                       opaque(1)

       Create new OPAQUE records

       Create new opaque record - offline

              echo -n password | opaque init idU idS 3>export_key [4<skS] >record

       Initiate new online registration

              echo -n password | opaque register >msg 3>ctx

       Respond to new online registration request

              opaque respond <msg >rpub 3>rsec [4<skS]

       Finalize online registration

              opaque finalize idU idS <ctx 4<rpub 3>export_key >record

       Complete online record

              opaque store <rec 3<rsec >record

       Server portion of online registration

              socat | opaque server-reg 3>record [4<skS]

       User portion of online registration

              socat | opaque user-reg idU idS 3< <(echo -n password) 4>export_key

       Run OPAQUE

       Server portion of OPAQUE session

              socat | opaque server idU idS context 3<record 4>shared_key

       User portion of OPAQUE session

              socat | opaque user idU idS context 3< <(echo -n password) 4>export_key 5>shared_key [6<pkS]