SASL Programmer's Guide

SASL Programmer's Guide

NOTE: This is a work in progress. Any contributions would be very appreciated

Introduction

About this Guide

This guide gives a tutorial on the use of the Cyrus SASL library for a client or server application. It complies with versions up to and including 1.5.15. The following pages should only be considered a guide, not the final word on programming with the Cyrus SASL library. Consult the sasl.h file in the distribution in the case of ambiguities.

What is SASL?

SASL stands for Simple Authentication Security Layer and is explained in RFC 2222. That document is very difficult to understand however and it should be unnecessary to consult it.

Background

How did the world work before SASL?

Before SASL, when a new protocol was written which required authentication (users proving who they are to an entity), the protocol had to allow explicitly for each individual authentication mechanism. There had to be a distinct way to say "I want to log in with Kerberos V4". There had to be another distinct way to say "I want to log in with CRAM-MD5". There had to be yet a different way to say "I want to log in anonymously," and so on. This was non-ideal for both the protocol and application writers.

Additionally, many programmers were not very familiar with security, so the protocol did support many mechanisms, or worse, they were supported incorrectly. Moreover, when a new authentication method was invented the protocol needed to be modified to support that mechanism.

This system also was not ideal for application writer. She had to have a special case for each mechanism she wished her application to support. Also, the mechanisms were difficult to implement. Even with a good library, an understanding of how the mechanism worked was still necessary. Finally if an application used more than one protocol (for example a mail client might use IMAP, POP, and SMTP) then "Kerberos V4 for IMAP", "Kerberos V4 for POP", "Kerberos V4 for SMTP", "CRAM MD5 for IMAP", "CRAM-MD5 for POP", etc... would need to be written. This could quickly create a huge number of different mechanism-protocol pairs to implement.

SASL to the rescue!

SASL hopefully solves all these problems. In practice it makes many of them easier to deal with.

Protocol designers simply have to support SASL (in particular RFC 2222). Consequently, any mechanism that supports SASL (just about anything you would want to use does now) is supported by the protocol. If a new authentication mechanism is invented the protocol automatically supports it without any modifications.

Application writers, instead of having to support every mechanism for every protocol, only need to support SASL for every protocol. Application writers do not need to understand the authentication mechanisms at all: the SASL library handles all that. Also with the Cyrus SASL library if a new mechanism is invented you do not have rewrite your application at all. You may not even have to restart your application if it is a long running process. This is because the Cyrus SASL library loads each mechanism from a shared library. Simply copying a shared library into a directory will magically make your application support a new mechanism.

Briefly

What is the Cyrus SASL library good for?

The Cyrus SASL library is good for applications that wish to use protocols that support SASL authentication. An non-exhaustive list of these are: IMAP, SMTP, ACAP, and LDAP. Also if you are making a proprietary system and wish to support authentication it is a good way of supporting many different authentication types.

What does the Cyrus SASL library do?

From a client point of view, the Cyrus SASL library, given a list of mechanisms the server supports it will decide the best mechanism to use and tell you what to send to the server at each step of the authentication. From a server perspective, it handles authentication requests from clients.

What doesn't the Cyrus SASL library do?

The Cyrus SASL library is neither network nor protocol aware. It is up to the application to send the data over the wire as well as to send the data in the protocol specific manner. With IMAP this means putting it in the form: + [base64'ed data]\r\n. LDAP just sends data in binary via bind requests. The Cyrus SASL library has utility base64 encode and decode routines to help with this.

Client-only Section

A typical interaction from the client's perspective
  1. A client makes a few calls (explained later) to initialize SASL.
  2. Every time the client application makes a new connection it should make a new context that is kept for the life of the connection.
  3. Ask the server for the list of supported mechanisms
  4. Feed this list to the library
  5. Start the authentication with the mechanism the library chose
  6. The server will return some bytes
  7. Give these to the library
  8. The library returns some bytes to the application
  9. Application sends these bytes over the network
  10. repeat the last 4 steps until the server tells you that the authentication is completed

How does this look in code

 Initialize the library. (done once). 
    
	    int result;

	    /* attempt to start sasl 
	     * See the section on Callbacks and Interactions for an 
	     * explanation of the variable callbacks
	     */ 

	    result=sasl_client_init(callbacks);
            
            /* check to see if that worked */
            if (result!=SASL_OK) [failure]
For every network connection make a new SASL connection: 
	     
            		      
            sasl_conn_t *conn; /* The SASL context kept for the life of the connection */
 
            /* client new connection */
            result=sasl_client_new("imap",     /* The service we are using */
			           serverFQDN, /* The fully qualified domain name
                                                  of the server we're connecting to */
			           NULL,
			           0,
			           &conn);     /* allocated on success */

            /* check to see if that worked */
            if (result!=SASL_OK) [failure]
Next get the list of SASL mechanisms the server supports. This is usually done throught a capability command. Format the list as a single string seperated by spaces. Feed this string into SASL to begin the authentication process. 
       
            sasl_interact_t *client_interact=NULL;
            char *out;
            unsigned outlen;


            do {

              result=sasl_client_start(conn,      /* the same context from above */ 
                                       mechlist,  /* the list of mechanisms from the server */
		  		       NULL, 
                                       &client_interact, /* filled in if an interaction is needed */
			  	       &out,      /* filled in on success */
                                       &outlen,   /* filled in on success */
				       &mechusing);

              if (result==SASL_INTERACT)
              {
                 [deal with the interactions. See interactions section below]
              }


           } while (result==SASL_INTERACT); /* the mechanism may ask us to fill in things
                                               many times. result is SASL_CONTINUE on success */

           if (result!=SASL_CONTINUE) [failure]
If this is sucessful send the protocol specific command to start the authentication process. This may or may not allow for initial data to be sent (see the documentation of the protocol to see). 
        For IMAP this might look like:
          {tag} "AUTHENTICATE" {mechusing}\r\n
          A01 AUTHENTICATE KERBEROS_V4\r\n
           
        SMTP looks like:
         "AUTH" {mechusing}[ {out base64 encoded}]
         AUTH DIGEST-MD5 GHGJJGDDFDKHGHJG=

Check Results
Next, read what the server sent back. It can be one of three things:
  1. Authentication failure. Authentication process is halted. This might look like A01 NO Authentication failure in IMAP or 501 Failed in SMTP. Either retry the authentication or abort.
  2. Authentication success. We're now sucessfully authenticated. This might look like A01 OK Authenticated successful in IMAP or 235 Authentication successful in SMPT. Go here
  3. Another step in the authentication process is necessary. This might look like + HGHDS1HAFJ= in IMAP or 334 PENCeUxFREJoU0NnbmhNWitOMjNGNndAZWx3b29kLmlubm9zb2Z0LmNvbT4= in SMTP. Note it could be an empty string such as + \r\n in IMAP.
Convert the continuation data to binary format (this may include base64 decoding it). Perform another step in the authentication. 

              do {
                result=sasl_client_step(conn,  /* our context */
				        in,    /* the data from the server */
				        inlen, /* it's length */
				        &client_interact,  /* this should be unallocated and NULL */
				        &out,     /* filled in on success */
				        &outlen); /* filled in on success */

                if (result==SASL_INTERACT)
                {
                   [deal with the interactions. See below]
                }


              } while (result==SASL_INTERACT);

              if (result!=SASL_OK) [failure]
Format the output (variable out of lenght outlen) in the protocol specific manner and send it across the network to the server.
Goto here


Authentication Successful

Before we're done we need to call sasl_client_step() one more time to make sure the server isn't trying to fool us. Some protocols include data along with the last step. If so this data should be used here. If not use a length of zero. 


                result=sasl_client_step(conn,  /* our context */
				        in,    /* the data from the server */
				        inlen, /* it's length */
				        &client_interact,  /* this should be unallocated and NULL */
				        &out,     /* filled in on success */
				        &outlen); /* filled in on success */

                if (result!=SASL_OK) [failure]

Congradulations. You have successfully authenticated to the server.

Don't throw away the SASL connection object (sasl_conn_t* ) yet though. If a security layer was negotiated you will need it to encode and decode the data sent over the network.


Done with connection to server. Dispose of SASL connection 
	  
               sasl_dispose(&conn);
Done with SASL forever (application quiting for example). 
            sasl_done();

sasl_client_init

    int sasl_client_init(const sasl_callback_t *callbacks);
    Paramaters:
  • callbacks - List of callbacks. See Callbacks section
This function initializes the SASL library. This must be called before any other SASL calls. See the callbacks section for complete description of callbacks.

sasl_client_new

    int sasl_client_new(const char *service,
		        const char *serverFQDN,
		        const sasl_callback_t *prompt_supp,
		        int secflags,
		        sasl_conn_t **pconn);
    Paramaters:
  • service - the service name being used. This usually is the protocol name (e.g. "ldap")
  • serverFQDN - Fully qualified domain name of server
  • prompt_supp - List off callbacks specific to this connection
  • secflags - security flags ORed together requested (e.g. SASL_SEC_NOPLAINTEXT)
  • pconn - the SASL connection object allocated upon success
This function creates a new SASL connection object. It should be called once for every connection you want to authenticate for.

sasl_client_start

    int sasl_client_start(sasl_conn_t *conn,
		          const char *mechlist,
			  sasl_secret_t *secret,
			  sasl_interact_t **prompt_need,
			  char **clientout,
			  unsigned *clientoutlen,
			  const char **mech);
    Paramaters:
  • conn - the SASL connection object gotten from sasl_client_new()
  • mechlist - the list of mechanisms to try seperated by spaces
  • secret - secret (optional)
  • prompt_need - filled in when a SASL_INTERACT is returned
  • clientout - filled in upon sucess with data to send to server
  • clientoutlen - length of that data
  • mech - filled in with mechanism being used
This function starts an authentication session. It takes a list of possible mechanisms (usually gotten from the server through a capability command) and chooses the "best" mechanism to try. Upon success clientout is filled in with data to send to the server.

sasl_client_step

    int sasl_client_step(sasl_conn_t *conn,
		 const char *serverin,
		 unsigned serverinlen,
		 sasl_interact_t **prompt_need,
		 char **clientout,
		 unsigned *clientoutlen);
    Paramaters:
  • conn - the SASL connection object gotten from sasl_client_new()
  • serverin - data from the server
  • serverinlen - length of data from the server
  • prompt_need - filled in with a SASL_INTERACT is returned
  • clientout - filled in upon sucess with data to send to server
  • clientoutlen - length of that data
This step preforms a step in the authentication process. It takes the data from the server (serverin) and outputs data to send to the server (clientout) upon success. SASL_CONTINUE is returned if another step in the authentication process is necesssary. SASL_OK is returned if we're all done.

Server-only Section

A typical interaction from the server's perspective

 The server makes a few Cyrus SASL calls for initialization. When it gets a new connection it should make a new context for that connection immediatly. The client may then request a list of mechanisms the server suppports. The client also may request to authenticate at some point. The client will specify the mechanism it wishes to use. The server should negotiate this authentication and keep around the context afterwards for encoding and decoding the layers.

How does this look in code?

 Initialization (done once). The application name is used for reading configuration information. 
    
    int result;

    /* Initialize SASL */
    result=sasl_server_init(callbacks,      /* Callbacks supported */
                            "TestServer");  /* Name of the application */
This should be called for each new connection. It probably should be called right when the socket is accepted. The service name is used for PAM authentication if applicable. 

    sasl_conn_t *conn;
    int result;

    /* Make a new context for this connection */
    result=sasl_server_new("smtp", 
		           NULL, /* my fully qualified domain name; 
			            NULL says use gethostname() */
                           NULL, /* The user realm used for password
			            lookups; NULL means default to serverFQDN
                                    Note: This does not affect Kerberos */
		           NULL, /* Callbacks supported only for this connection */
	                   SASL_SECURITY_LAYER, /* I support encryption layers; 
                                                   otherwise pass 0 */
			   &conn);
When a client requests the list of mechanisms supported by the server. This particular call might produce the string: "{PLAIN, KERBEROS_V4, CRAM-MD5, DIGEST-MD5}" 

    result=sasl_listmech(conn,  /* The context for this connection */
			 NULL,  /* not supported */
			 "{",   /* What to prepend the string with */
			 ", ",  /* What to seperate mechanisms with */
			 "}",   /* What to append to the string */
			 &result_string, /* The produced string. Allocated by library */
                         &string_length, /* length of the string */
                         &number_of_mechanisms); /* Number of mechanisms in the string */
When a client requests to authenticate: 
    int result;
    const char *errstr;
    char *out;
    unsigned outlen;

    result=sasl_server_start(conn, /* context */
                             mechanism_client_chose,
                             clientin,    /* the optional string the client gave us */
                             clientinlen, /* and it's length */
                             &out, /* allocated by library on success. Might not be NULL 
                                      terminated */
                             &outlen, 
                             &errstr); /* error string filled in on failure */
    if ((result!=SASL_OK) && (result!=SASL_CONTINUE))
    {
      failure. Send client the protocol specific message that says authentication failed
    }


    if (result==SASL_OK)
    {
      client authentication suceeded. Send client the protocol specific message 
      to say that authentication is complete.
    }
When a response is returned by the client. clientin is the data from the client decoded from protocol specific format to a string of bytes of length clientinlen. This step may occur zero or more times. An application should be able to deal with it occuring an arbitrary number of times. 
    int result;
   
    result=sasl_server_step(conn,
                            clientin,      /* what the client gave */
                            clientinlen,   /* it's length */
                            &out,          /* allocated by library on success. 
                                              Might not be NULL terminated */
                            &outlen,
                            &errstr);      /* error string sometimes filled in on failure */

    if ((result!=SASL_OK) && (result!=SASL_CONTINUE))
    {
      failure. Send client the protocol specific message that says authentication failed
    }

    if (result==SASL_OK)
    {
      client authentication suceeded. Send client the protocol specific message 
      to say that authentication is complete.
    }

    send data 'out' with length 'outlen' over the network in protocol
    specific format

sasl_server_init

 int sasl_server_init(const sasl_callback_t *callbacks, const char *appname);
    Parameters:
  • callbacks - A list of callbacks supported by the application (see Interaction and Callbacks section)
  • appname - A string of the name of the application. This string is what is used when loading configuration options.
sasl_server_init() initializes the session. This should be the first function called. In this function the shared library authentication mechanisms are loaded.

sasl_server_new

 int sasl_server_new(const char *service, const char *serverFQDN, const char *user_realm, const sasl_callback_t *callbacks, int secflags, sasl_conn_t **pconn);
    Parameters:
  • service - The name of the service you are supporting. This might be "acap" or "smtp". This is used by Kerberos mechanisms and possibly other mechanisms. It is also used for PAM authentication.
  • serverFQDN - This is the fully qualified domain name of the server (i.e. your hostname); if NULL, the library calls gethostbyname().
  • user_realm - The realm the connected client is in. The Kerberos mechanisms ignore this parameter and default to the local Kerberos realm. A value of NULL makes the library default, usually to the serverFQDN; a value of "" specifies that the client should specify the realm; this also changes the semantics of "@" in a username for mechanisms that don't support realms.
  • callbacks - Additional callbacks that you wish only to apply to this connection.
  • secflags - security flags. SASL_SECURITY_LAYER if your application supports security layers. 0 otherwise.
  • pconn - Context. Filled in on success.

sasl_server_start

 
     int sasl_server_start(sasl_conn_t *conn,
			  const char *mech,
			  const char *clientin,
			  unsigned clientinlen,
			  char **serverout,
			  unsigned *serveroutlen,
			  const char **errstr);
    Paramaters:
  • conn - The context for the connection
  • mech - The authentication mechanism the client wishes to try (e.g. KERBEROS_V4)
  • clientin - Initial client challenge bytes. Note: some protocols do not allow this. If this is the case passing NULL is valid
  • clientinlen - The length of the challenge. 0 is there is none.
  • serverout - allocated and filled in by the function. These are the bytes that should be encoded as per the protocol and sent over the network back to the client.
  • serveroutlen - length of bytes to send to client
  • errstr - string possibly filled in on error
This function begins the authentication process with a client. If the program returns SASL_CONTINUE that means serverout should be sent to the client. If SASL_OK is returned that means authentication is complete and the application should tell the client the authentication was sucessful. Any other return code means the authentication failed and the client should be notified of this.

sasl_server_step

 
    int sasl_server_step(sasl_conn_t *conn,
		         const char *clientin,
		         unsigned clientinlen,
		         char **serverout,
		         unsigned *serveroutlen,
		         const char **errstr);
    Paramaters:
  • conn - The context for the connection
  • clientin - Data sent by the client.
  • clientinlen - The length of the client data. Note that this may be 0
  • serverout - allocated and filled in by the function. These are the bytes that should be encoded as per the protocol and sent over the network back to the client.
  • serveroutlen - length of bytes to send to client. Note that this may be 0
  • errstr - string possibly filled in on error
This function preforms a step of the authentication. This may need to be called an arbitrary number of times. If the program returns SASL_CONTINUE that means serverout should be sent to the client. If SASL_OK is returned that means authentication is complete and the application should tell the client the authentication was sucessful. Any other return code means the authentication failed and the client should be notified of this.

sasl_listmech

 
    int sasl_listmech(sasl_conn_t *conn,
		      const char *user,
		      const char *prefix,
		      const char *sep,
		      const char *suffix,
		      char **result,
		      unsigned *plen,
		      unsigned *pcount);
    Paramaters:
  • conn - The context for this connection
  • user - Currently not implemented
  • prefix - The string to prepend
  • sep - The string to seperate mechanisms with
  • suffix - The string to end with
  • result - Resultant string
  • plen - Number of characters in the result string
  • pcount - Number of mechanisms listed in the result string
This function is used to create a string with a list of SASL mechanisms supported by the server. This string is often needed for a capability statement.

sasl_checkpass

 
    int sasl_checkpass(sasl_conn_t *conn,
                       const char *user,
                       unsigned userlen,
		       const char *pass,
		       unsigned passlen,
		       const char **errstr);
    Parameters:
  • conn - The context for this connection
  • user - The user trying to check the password for
  • userlen - The user length
  • pass - The password
  • passlen - The password length
  • errstr - Possibly filled in on failure
This checks a plaintext password pass for user user Some protocols have legacy systems for plaintext authentication where this might be used. Note that this has no relationship to the PLAIN mechanism but it does check the password the same way in the Cyrus SASL library.

Common Section

Callbacks and Interactions

 When the application starts and calls sasl_client_init() you must specify for what data you support callbacks and/or interactions. These are for the library getting information needed for authentication from the application. This is needed for things like authentication name and password. If you do not declare supporting a callback you will not be able to use mechanisms that need that data. A callback is for when you have the information before you start the authentication. The SASL library calls a function you specify and your function fills in the requested information. For example if you had the userid of the user already for some reason. An interaction is usually for things you support but will need to ask the user for (e.g. password). sasl_client_start() or sasl_client_step() will return SASL_INTERACT. This will be a list of sasl_interact_t's which contain a human readable string you can prompt the user with, a possible computer readable string, and a default result. The nice thing about interactions is you get them all at once so if you had a GUI application you could bring up a dialog box asking for authentication name and password together instead of one at a time.

Memory management: Any data passed by the application to the library will be copied by the library. The application is responsible for freeing any memory allocated in the callbacks, if any. Interactions have the same memory management requirements as callbacks.

Apparent random exception: The secret returned from SASL_CB_PASS should be allocated with malloc() and will be freed by the library.

For a detailed description of what each of the callback types are see the sasl.h file. Here are some brief explanations:

  • SASL_CB_AUTHNAME - the name of the user authenticating
  • SASL_CB_USER - the name of the user acting for. (for example postman delivering mail for tmartin might have an AUTHNAME of postman and a USER of tmartin)
  • SASL_CB_PASS - password for AUTHNAME
  • SASL_CB_GETREALM - Realm of the server
An example of a way to handle callbacks: 
    /* callbacks we support. This is a global variable at the 
       top of the program */
    static sasl_callback_t callbacks[] = {
    {
      SASL_CB_GETREALM, NULL, NULL  /* we'll just use an interaction if this comes up */
    }, {
      SASL_CB_USER, NULL, NULL      /* we'll just use an interaction if this comes up */
    }, {
      SASL_CB_AUTHNAME, &getauthname_func, NULL /* A mechanism should call getauthname_func
                                                   if it needs the authentication name */
    }, { 
      SASL_CB_PASS, &getsecret_func, NULL      /* Call getsecret_func if need secret */
    }, {
      SASL_CB_LIST_END, NULL, NULL
    }
    };


    static int getsecret_func(sasl_conn_t *conn,
	  void *context __attribute__((unused)),
	  int id,
	  sasl_secret_t **psecret)
    {
       [ask the user for their secret]

       [allocate psecret and insert the secret]

      return SASL_OK;
    }

    static int getauthname_func(void *context,
                                int id,
                                const char **result,
                                unsigned *len)
    {
       if (id!=SASL_CB_AUTHNAME) return SASL_FAIL;

       [fill in result and len]

       return SASL_OK;
     }

    
    in the main program somewhere
    
    sasl_client_init(callbacks);

Example applications that come with the Cyrus SASL library

sample-client and sample-server

 The sample client and server included with this distribution were initially written to help debug mechanisms. They base64 encode all the data and print it out on standard output.

Make sure that you set the IP addresses, the username, the authenticate name, and anything else on the command line (some mechanisms depend on these being present).

Also, sometimes you will receive a get "realm: Information not available" message, or similar; this is due to the fact that some mechanisms do not support realms and therefore never set it.

Cyrus imapd v1.6.0 or later

The Cyrus IMAP server now incorporates SASL for all its authentication needs. It is a good example of a fairly large server application. Also of interest is the prot layer, included in libcyrus. This is a stdio-like interface that automatically takes care of layers using a simple "prot_setsasl()" call.

Cyrus imapd also sets a SASL_CB_PROXY_POLICY callback, which should be of interest to many applications.

imtest, from cyrus imapd 1.6.0 or later

imtest is an application included with Cyrus imapd. It is a very simple IMAP client, but should be of interest to those writing applications. It also uses the prot layer, but it is easy to incorporate similar support without using the prot layer.

Miscelaneous Information

Empty exchanges

 Some SASL mechanisms intentionally send no data; an application should be prepared to either send or receive an empty exchange. The SASL profile for the protocol should define how to send an empty string; make sure to send an empty string when requested, and when receiving an empty string make sure that the "inlength" passed in is 0.

What's not implemented

 Some parts of this API are not implemented by this implementation. A brief outline of these features (and what might come of them) is in this section.

Credentials

 None of the modules support passing credentials. In the future, we hope to add credential passing to the modules that support it (most likely the Kerberos modules). Thus, an application that specifies SASL_SEC_PASS_CREDENTIALS will not receive any mechanisms. The functions sasl_cred_install() and sasl_cred_uninstall() do nothing.

It's likely that the credential API will change, and it's also likely that you'll need the Cyrus SASL library on both sides of the connection to make it work.

Secrets

sasl_client_auth() is unimplemented. It is unclear what this is intended for. If we implement it, it will probably be for generating secrets for fast reauthentication.

The "secret" parameter to sasl_client_start() is unused. It is likely that we will use this for fast reauthentication.

Idle

While the implementation & plugins correctly implement the idle calls, none of them currently do anything.
Timothy L Martin
Last modified: Wed Apr 5 15:24:47 EDT 2000