Module Netmech_crammd5_sasl.CRAM_MD5

module CRAM_MD5: Netsys_sasl_types.SASL_MECHANISM

The CRAM-MD5 SASL mechanism (RFC 2195), which is obsolete and only provided for completeness.

Key facts:

The password is checked with a challenge-response mechanism, and does not appear in the clear.
The mechanism is vulnerable to man-in-the-middle attacks.
The client does not authenticate the server in any way.
The hash function MD5 counts as broken.
There is no support for channel binding within the mechanism.
There is no support for authorization names.
The mechanism provides at best medium security, and should only be used over channels that are otherwise secured.

This implementation checks whether the server receives user names and passwords in UTF-8 encoding. Note that the mechanism predates the widespread use of Unicode, so this may cause interoperability issues with old implementations.

Parameters:

Both create_client_session and create_server_session accept the boolean parameter "mutual". If true, however, authentication fails immediately, as mutual authentication cannot be supported.
The boolean parameter "secure" is ignored (i.e. we regard this mechanism as secure)

As for all SASL mechanisms in OCamlnet, SASLprep is not automatically called. Users of CRAM-MD5 should pass user names and passwords through Netsaslprep.saslprep.

val mechanism_name : string

val client_first : [ `No | `Optional | `Required ]

Whether the client sends the first message:

`Required: always
`Optional: the client may choose to do so if the protocol permits this
`No: the server sends the first message

val server_sends_final_data : bool

Whether final data from the server must be interpreted by the mechanism

val supports_authz : bool

whether the authorization name can be transmitted

val available : unit -> bool

Whether the mechanism is available, in particular whether the required crypto support is linked in

type credentials

val init_credentials : (string * string * (string * string) list) list ->
       credentials

Supply the mechanism with credentials. These are given as list (type,value,params). The mechanism may pick any element of this list which are considered as equivalent.

Types are defined per mechanism. All mechanisms understand the "password" type, which is just the cleartext password, e.g.

            [ "password", "ThE sEcReT", [] ]

Another common type is derived from the LDAP authPassword scheme (RFC 3112):

             [ "authPassword-" ^ scheme, authValue, [ "info", authInfo ] ]

The "info" attribute is optional. For instance, if you want to provide MD5-hashed passwords:

             [ "authPassword-MD5", hashed_password, [ "info", salt ] ]

Another common type is derived from the (older) LDAP userPassword scheme (RFC 2307):

            [ "userPassword-" ^ scheme, value, [] ]

More information can be found here: Credentials for SASL

type server_session

val server_state : server_session ->
       Netsys_sasl_types.server_state

val create_server_session : lookup:(string ->
               string -> credentials option) ->
       params:(string * string * bool) list ->
       unit -> server_session

Create a new server session. The lookup function is used to get the credentials for a given user name and a given authorization name (which is the empty string if not applicable). If the lookup function returns None, the user can either not be found, or the user does not have the privileges for the authorization name.

User name and authorization name are passed in UTF-8 encoding.

The parameters are given as list (name,value,critical). Critical parameters must be interpreted by the mechanism, and unknown critical parameters must be rejected by a Failure exception. Non-critical parameters are ignored if they are unknown to the mechanism.

val server_configure_channel_binding : server_session ->
       (string * string) list -> server_session

Configures acceptable channel bindings wiht a list of pairs (type,data).

val server_process_response : server_session ->
       string -> server_session

Process the response from the client. This function must generally only be called when the session state is `Wait. As an exception, however, this function may also be invoked with the initial client response, even if the session state is `Emit, so far the mechanism permits at least optionally that the client starts the protocol.

val server_process_response_restart : server_session ->
       string -> bool -> server_session * bool

Process the response from the client when another session can be continued. The string argument is the initial client response. This function must only be called when the state reaches `Restart id after server_process_response, and in this case the old session with id can be restarted. This function should be called with the same message string as server_process_repsonse was just called with.

If the bool arg is true, a stale response is created. This is a special restart which forces the client to run through the authentication process again, although everything else was successful. (If the protocol does not support the stale flag, it is silently ignored.)

Returns true if the restart is successful. If not, false is returned. In this case, the server_session object can (and should) still be used, but the caller must treat it as new session. In particular, the session ID may change.

All in all, the idea of this function is best illustrated by this authentication snippet how to process responses (the session cache functions need to be written by the user of this module):

  let update_cache() =
    match server_session_id session with
      | None -> ()
      | Some id ->
          replace_in_session_cache id (server_stash_session session) in

  let rec check_state_after_response() =
    match server_state session with
      | `Restart id ->
           let old_session_s, time = find_in_session_cache id in
           let old_session = server_resume_session ~lookup old_session_s in
           let set_stale = current_time - time > limit in
           let session, cont =
             server_process_response_restart session msg set_stale in
           if not cont then 
             delete_in_session_cache id;
           (* Now check server_state again, should be `Emit now *)
           check_state_after_response()
      | `Emit ->
           let session, out_msg = server_emit_challenge session in
           update_cache();
           ...
      | ... ->
  in
  server_process_response session msg;

val server_emit_challenge : server_session ->
       server_session * string

Emit a server challenge. This function must only be called when the session state is `Emit.

val server_stash_session : server_session -> string

Serializes the session as string

val server_resume_session : lookup:(string ->
               string -> credentials option) ->
       string -> server_session

Unserializes the session, and connect with the lookup function.

val server_session_id : server_session -> string option

Optionally return a string that can be used to identify the server session. Not all mechanisms support this.

val server_prop : server_session -> string -> string

Get a mechanism-specific property of the session. E.g. this can be the "digest-uri" sent by the client.

val server_user_name : server_session -> string

The name the client has authenticated as (or Not_found)

val server_authz_name : server_session -> string

The name the client authorizes as (or Not_found)

val server_channel_binding : server_session -> Netsys_sasl_types.cb

Whether the client suggests or demands channel binding

val server_gssapi_props : server_session -> Netsys_gssapi.server_props

Return the GSSAPI properties of the server, after the authentication is successful (and raises Not_found up to then).

type client_session

val client_state : client_session ->
       Netsys_sasl_types.client_state

val create_client_session : user:string ->
       authz:string ->
       creds:credentials ->
       params:(string * string * bool) list ->
       unit -> client_session

The new client session authenticate as user and authorizes as authz (empty string if not applicable). The credentials are creds.

user and authz must be encoded in UTF-8.

val client_configure_channel_binding : client_session ->
       Netsys_sasl_types.cb -> client_session

Configure GS2-style channel binding

val client_restart : client_session ->
       client_session

Restart the session for another authentication round. The session must be in state `OK.

val client_process_challenge : client_session ->
       string -> client_session

Process the challenge from the server. The state must be `Wait. As an exception, this function can also be called for the initial challenge from the server, even if the state is `Emit.

val client_emit_response : client_session ->
       client_session * string

Emit a new response. The state must be `Emit.

val client_channel_binding : client_session -> Netsys_sasl_types.cb

Whether the client suggests or demands channel binding

val client_user_name : client_session -> string

The user name

val client_authz_name : client_session -> string

The authorization name

val client_stash_session : client_session -> string

Serializes the session as string

val client_resume_session : string -> client_session

Unserializes the session

val client_session_id : client_session -> string option

Optionally return a string that can be used to identify the client session. Not all mechanisms support this.

val client_prop : client_session -> string -> string

Get a mechanism-specific property of the session. E.g. this can be the "realm" sent by the server.

val client_gssapi_props : client_session -> Netsys_gssapi.client_props

Return the GSSAPI properties of the client, after the authentication is successful (and raises Not_found up to then).

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