# Module Netmech_scram

    module Netmech_scram: sig .. end

SCRAM mechanism for authentication (RFC 5802)

This implements SCRAM-SHA-1 for GSSAPI. Other profiles may be added later.

As we do not implement SASLprep, usernames and passwords are restricted to US-ASCII.

    type ptype = [ `GSSAPI ]

Currently only the variant for ``GSSAPI` is supported

    type mechanism = [ `SHA_1 ]

`type ``profile = {`

`` | `ptype : `ptype`;`
`` | `mechanism : `mechanism`;` | `(*` | Which mechanism | `*)`
`` | `return_unknown_user : `bool`;` | `(*` | Whether servers exhibit the fact that the user is unknown | `*)`
`` | `iteration_count_limit : `int`;` | `(*` | Largest supported iteration number | `*)`

}
Profile

    type server_error = [ `Channel_binding_not_supported
           | `Channel_bindings_dont_match
           | `Extension of string
           | `Extensions_not_supported
           | `Invalid_encoding
           | `Invalid_proof
           | `Invalid_username_encoding
           | `No_resources
           | `Other_error
           | `Server_does_support_channel_binding
           | `Unknown_user
           | `Unsupported_channel_binding_type ]

Error codes of this protocol

    type client_session

Session context for clients

    type server_session

Session context for servers

    exception Invalid_encoding of string * string

Raised by clients when something cannot be decoded. First string is an error message, the second string the raw message that cannot be decoded

    exception Invalid_username_encoding of string * string

Raised by clients when the username does not match the requirements. Arguments as for `Invalid_encoding`.

    exception Extensions_not_supported of string * string

Raised by clients when the server enables an unsupported extension. Arguments as for `Invalid_encoding`.

    exception Protocol_error of string

Raised by clients when the server violates the protocol. The argument is a message.

    exception Invalid_server_signature

Raised by clients when the signature sent by the server is invalid (i.e. the server does not know the client password)

    exception Server_error of server_error

Raised by clients when the server sent an error code

    val profile : ?return_unknown_user:bool ->
           ?iteration_count_limit:int -> ptype -> profile

Creates a profile

    val string_of_server_error : server_error -> string

    val server_error_of_string : string -> server_error

Conversion

## Clients

The idea is to create a client session `s` first. The functions `client_emit_flag` and `client_recv_flag` indicate now whether the client needs to emit a new message, or whether it needs to receive a message, respectively. Emission is done by `client_emit_message`, reception by `client_recv_message`. If everything goes well, the protocol state advances, and finally `client_finish_flag` is true. This indicates that the client is authenticated and that the server knows the client's password. If an error occurs, an exception is raised (see above for possibilities), and `client_error_flag` signals `true`.

    val create_client_session : profile -> string -> string -> client_session

`create_client_session p username password`: Creates a new client session for profile `p` so that the client authenticates as user `username`, and proves its identify with the given `password`.

    val client_configure_channel_binding : client_session -> string -> unit

Instruct the client to require a channel binding. The passed string is the `c` parameter (before encoding it via Base64. The function needs to be called before sending the second message to the server. It fails if called too late.

    val client_emit_flag : client_session -> bool

Whether `client_emit_message` can now be called

    val client_recv_flag : client_session -> bool

Whether `client_recv_message` can now be called

    val client_finish_flag : client_session -> bool

Whether the client is authenticated and the server verified

    val client_error_flag : client_session -> bool

Whether an error occurred, and the protocol cannot advance anymore

    val client_channel_binding : client_session -> string

Returns the channel binding ("" of none)

    val client_emit_message : client_session -> string

Emits the next message to be sent to the server

    val client_recv_message : client_session -> string -> unit

Receives the next message from the server

    val client_protocol_key : client_session -> string option

The 128-bit protocol key for encrypting messages. This is available as soon as the second client message is emitted.

    val client_user_name : client_session -> string

The user name

    val client_export : client_session -> string

    val client_import : string -> client_session

Exports a client session as string, and imports the string again. Only established sessions are allowed to be exported (for which `client_finish_flag` is true).

The export format is just a marshalled Ocaml value.

## Servers

The idea is to create a server session `s` first. The functions `server_emit_flag` and `server_recv_flag` indicate now whether the server needs to emit a new message, or whether it needs to receive a message, respectively. Emission is done by `server_emit_message`, reception by `server_recv_message`. If everything goes well, the protocol state advances, and finally `server_finish_flag` is true. This indicates that the client could be authenticated.

If an error occurs, no exception is raised, and the protocol advances nevertheless, and finally the server sends an error token to the client. After this, `server_error_flag` returns true.

    val create_server_session : profile ->
           (string -> string * string * int) -> server_session

`create_server_session p auth`: Creates a new server session with profile `p` and authenticator function `auth`.

The function is `auth` is called when the credentials of the client have been received to check whether the client can be authenticated. It is called as

          let (salted_password, salt, iteration_count) = auth username

where `username` is the user name. The function can now raise `Not_found` if the user is unknown, or it can return the shown triple. Note that the cleartext password needs not to be known. `salt` is a random string, and `iteration_count` a security parameter that should be at least 4096. Whereas `salt` should be different for each user, the `iteration_count` can be chosen as a constant (e.g. 4096). Now `salted_password` can be computed from the cleartext password and these two extra parameters. See `salt_password` below.

    val create_salt : unit -> string

Creates a random string suited as salt

    val salt_password : string -> string -> int -> string

`let salted_password = salt_password password salt iteration_count`

As we do not implement `SASLprep` only passwords consisting of US-ASCII characters are accepted (`Invalid_encoding` otherwise).

    val server_emit_flag : server_session -> bool

Whether `server_emit_message` can now be called

    val server_recv_flag : server_session -> bool

Whether `server_recv_message` can now be called

    val server_finish_flag : server_session -> bool

Whether the client is authenticated

    val server_error_flag : server_session -> bool

Whether an error occurred, and the protocol cannot advance anymore

    val server_emit_message : server_session -> string

Emits the next message to be sent to the client

    val server_recv_message : server_session -> string -> unit

Receives the next message from the client

    val server_protocol_key : server_session -> string option

The 128-bit protocol key for encrypting messages. This is available as soon as the second client message has been received.

    val server_channel_binding : server_session -> string option

Returns the channel binding requirement (the "c" parameter). It is up to the application to enforce the binding. This information is available as soon as the second client message has been received

    val server_user_name : server_session -> string option

The user name as transmitted from the client. This is returned here even before the authentication is completed!

    val server_export : server_session -> string

    val server_import : string -> server_session

Exports a server session as string, and imports the string again. Only established sessions are allowed to be exported (for which `server_finish_flag` is true).

The export format is just a marshalled Ocaml value.

## Confidentiality

`type ``specific_keys = {`

`` | `kc : `string`;`
`` | `ke : `string`;`
`` | `ki : `string`;`

}
The specific keys to use

    module AES_CTS: sig .. end

This module implements AES in Ciphertext Stealing mode (see RFC 3962)

    module Cryptosystem: sig .. end

This is the cryptosystem as defined in RFC 3961, so far needed here.

    module Debug: sig .. end