# SASL Application Programmer's Guide

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

## Contents

- Introduction
  - About this guide
  - What is SASL?
- Background
  - How did the world work before SASL?
  - SASL to the rescue!
- Briefly
  - What is the Cyrus SASL library good for?
  - What does the Cyrus SASL library do?
  - What doesn't the Cyrus SASL library do?
- Client-only Section
  - A typical interaction from the client's perspective
  - How does this look in code?
  - sasl_client_init
  - sasl_client_new
  - sasl_client_start
  - sasl_client_step
- Server-only Section
  - A typical interaction from the server's perspective
  - How does this look in code?
  - sasl_server_init
  - sasl_server_new
  - sasl_server_start
  - sasl_server_step
  - sasl_listmech
  - sasl_checkpass
- Common Section
  - Callbacks and Interaction
  - sasl_encode
  - sasl_decode
- Example applications that come with the Cyrus SASL library
  - `sample-client` and `sample-server`
  - Cyrus imapd v2.1.0 or later
  - `imtest`, from cyrus imapd 2.1.0 or later
- Miscellaneous Information
  - Empty exchanges

## 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 including and after 2.0.0. The following pages should only be considered a guide, not the final word on programming with the Cyrus SASL library. Consult the header files 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.

Cyrus SASL version 2 supports a much improved API over version 1, that allows for much smarter and faster memory allocation for the mechanisms as well as the applications. It is also provides for several new types of plugins to allow for greater overall flexibility. Unfortunately, though similar, this new API is completely incompatible with the old API, and applications will need to be rewritten.

## 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: `

                /* The SASL context kept for the life of the connection */
                sasl_conn_t *conn;

                /* 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, NULL, /* Local and remote IP
                                                      address strings
                                                      (NULL disables mechanisms
                                                       which require this info)*/
                                       NULL,       /* connection-specific
                                                      callbacks */
    			           0,          /* security flags */
    			           &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 through a capability command. Format the list as a single string separated by spaces. Feed this string into SASL to begin the authentication process. `

                sasl_interact_t *client_interact=NULL;
                const char *out, *mechusing;
                unsigned outlen;

                do {

                  result=sasl_client_start(conn,      /* the same context from
                                                         above */
                                           mechlist,  /* the list of mechanisms
                                                         from the server */
                                           &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]

` Note that you do not need to worry about the allocation and freeing of the output buffer out. This is all handled inside of the mechanism. It is important to note, however, that the output buffer is not valid after the next call to `sasl_client_start` or `sasl_client_step`.

If this is successful 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 successfully authenticated. This might look like `A01 OK Authenticated successful` in IMAP or `235 Authentication successful` in SMTP. 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 (for example, 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 || result == SASL_CONTINUE);

                  if (result!=SASL_OK) [failure]

` Format the output (variable out of length outlen) in the protocol specific manner and send it across the network to the server.
Goto here (this process repeats until authentication either succeeds or fails.
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]

Congratulations. 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.

When you are finally done with connection to server, dispose of SASL connection. `

                   sasl_dispose(&conn);

` If you are done with SASL forever (application quiting for example): `

                sasl_done();

`

### sasl_client_init

        int sasl_client_init(const sasl_callback_t *callbacks);

Parameters:
- 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 char *iplocalport,
                            const char *ipremoteport,
    		        const sasl_callback_t *prompt_supp,
    		        unsigned secflags,
    		        sasl_conn_t **pconn);

Parameters:
- service - the service name being used. This usually is the protocol name (e.g. "ldap")
- serverFQDN - Fully qualified domain name of server
- iplocalport and ipremoteport - a string of the format "a.b.c.d;p" detailing the local or remote IP and port, or NULL (which will disable mechanisms that require this information)
- 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_interact_t **prompt_need,
    			  const char **clientout,
    			  unsigned *clientoutlen,
    			  const char **mech);

Parameters:
- conn - the SASL connection object gotten from sasl_client_new()
- mechlist - the list of mechanisms to try (separated by spaces)
- prompt_need - filled in when a SASL_INTERACT is returned
- clientout - filled in upon success 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 points at 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,
    		 const char **clientout,
    		 unsigned *clientoutlen);

Parameters:
- 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 success 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 necessary. 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 immediately. The client may then request a list of mechanisms the server supports. 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.

        sasl_conn_t *conn;
        int result;

        /* Make a new context for this connection */
        result=sasl_server_new("smtp", /* Registered name of service */
    		           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, NULL, /* IP Address information strings */
    		           NULL, /* Callbacks supported only for this connection */
    	                   0, /* security flags (security layers are enabled
                                   * using security properties, separately)
    			   &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 separate mechanisms with */
    			 "}",   /* What to append to the string */
    			 &result_string, /* The produced string. */
                             &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 *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, /* The output of the library.
                                          Might not be NULL terminated */
                                 &outlen);

        if ((result!=SASL_OK) && (result!=SASL_CONTINUE))
          [failure. Send protocol specific message that says authentication failed]
        else if (result==SASL_OK)
          [authentication succeeded. Send client the protocol specific message
           to say that authentication is complete]
        else
          [send data 'out' with length 'outlen' over the network in protocol
           specific format]

` 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 must be able to deal with it occurring 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);

        if ((result!=SASL_OK) && (result!=SASL_CONTINUE))
          [failure. Send protocol specific message that says authentication failed]
        else if (result==SASL_OK)
          [authentication succeeded. Send client the protocol specific message
           to say that authentication is complete]
        else
          [send data 'out' with length 'outlen' over the network in protocol
           specific format]

` This continues until authentication succeeds. When the connection is concluded, make a call to `sasl_dispose` as with the client connection.

### 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 char *iplocalport, const char *ipremoteport, const sasl_callback_t *callbacks, unsigned 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.
- iplocalport and ipremoteport - a string of the format "a.b.c.d;p" detailing the local or remote IP and port, or NULL (which will disable mechanisms that require this information)
- callbacks - Additional callbacks that you wish only to apply to this connection.
- secflags - security flags.
- 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,
    			   const char **serverout,
    			   unsigned *serveroutlen);

Parameters:
- 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

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 successful. 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,
    		         const char **serverout,
    		         unsigned *serveroutlen);

Parameters:
- 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

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 successful. 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,
    		      const char **result,
    		      unsigned *plen,
    		      unsigned *pcount);

Parameters:
- conn - The context for this connection
- user - Currently not implemented
- prefix - The string to prepend
- sep - The string to separate 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);

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

This checks a plaintext password pass for user user Some protocols have legacy systems for plaintext authentication where this might be used.

## 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.

Any memory that is given to the SASL library for the purposes of callbacks and interactions must persist until the exchange completes in either success or failure. That is, the data must persist until `sasl_client_start` or `sasl_client_step` returns something other than `SASL_INTERACT` or `SASL_CONTINUE`.

Memory management:As in the rest of the SASLv2 API, whoever allocates the memory is responsible for freeing it. In almost all cases this should be fairly easy to manage, however a slight exception where the interaction sasl_interact_t structure is allocated and freed by the library, while the results are allocated and freed by the application. As noted above, however, the results may not be freed until after the exchange completes, in either success or failure.

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);

### Security layers

    rough notes
    alls well and good to authenticate, but if you don't use integrity or
    encryption, anyone can hijack your tcp session... thus...

    server sets SASL_SEC_PROPS with a desired buffer size X of the amount of data
    it will be using to read() from the socket.

    libsasl passes this number to the mechanism.

    the mechanism then negotiates with the other size so that "maxbuf" is less
    than or equal to this number X.

    libsasl then returns (via SASL_MAXOUTBUF) the maximum amount of plaintext
    material that can be encoded at any one time, Y.

    Y will be strictly less than X.  If the other side of the connection was good
    with X, Y will probably be only slightly less than X.

    the prot layer now calls sasl_encode every time Y bytes are written (the
    output buffer size is Y).  it will read in sizes of X (the input buffer size
    it told sasl).  since it reads in sizes of X, excluding pathological cases,
    every time it calls read() it will get at least one sasl blob which
    sasl_decode will return the plaintext data for.

    all is happy.

    it's the same for the client.

## 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 v2.1.0 or later

The Cyrus IMAP server now incorporates SASLv2 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 2.1.0 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. Likewise, there are other sample client applications that you can look at including `smtptest` and `pop3test` in the SASL distribution and the Cyrus IMAPd distribution, respectively.

## Miscellaneous 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.

Note especially that the distinction between the empty string "" and the lack of a string (NULL) is extremely important in many cases (most notably, the client-send first scenario), and the application must ensure that it is passing the correct values to the SASL library at all times.

### Idle

While the implementation & plugins correctly implement the idle calls, none of them currently do anything.

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