Programming with Kerberos
*************************

First you need to know how the Kerberos model works, go read the introduction text (see What is Kerberos?).

Kerberos 5 API Overview
=======================

All functions are documented in manual pages.  This section tries to give an overview of the major components used in Kerberos library, and point to where to look for a specific function.

Kerberos context
----------------

A kerberos context (krb5_context) holds all per thread state. All global variables that are context specific are stored in this structure, including default encryption types, credential cache (for example, a ticket file), and default realms.

See the manual pages for krb5_context,3 and krb5_init_context,3.

Kerberos authentication context
-------------------------------

Kerberos authentication context (krb5_auth_context) holds all context related to an authenticated connection, in a similar way to the kerberos context that holds the context for the thread or process.

The krb5_auth_context is used by various functions that are directly related to authentication between the server/client. Example of data that this structure contains are various flags, addresses of client and server, port numbers, keyblocks (and subkeys), sequence numbers, replay cache, and checksum types.

See the manual page for krb5_auth_context,3.

Kerberos principal
------------------

The Kerberos principal is the structure that identifies a user or service in Kerberos. The structure that holds the principal is the krb5_principal. There are function to extract the realm and elements of the principal, but most applications have no reason to inspect the content of the structure.

The are several ways to create a principal (with different degree of portability), and one way to free it.

See manual page for krb5_principal,3 for more information about the functions.

Credential cache
----------------

A credential cache holds the tickets for a user. A given user can have several credential caches, one for each realm where the user have the initial tickets (the first krbtgt).

The credential cache data can be stored internally in different way, each of them for different proposes.  File credential (FILE) caches and processes based (KCM) caches are for permanent storage. While memory caches (MEMORY) are local caches to the local process.

Caches are opened with krb5_cc_resolve,3 or created with krb5_cc_gen_unique,3.

If the cache needs to be opened again (using krb5_cc_resolve,3) krb5_cc_close,3 will close the handle, but not the remove the cache. krb5_cc_destroy,3 will zero out the cache, remove the cache so it can no longer be referenced.

See also manual page for krb5_ccache,3

Kerberos errors
---------------

Kerberos errors are based on the com_err library.  All error codes are 32-bit signed numbers, the first 24 bits define what subsystem the error originates from, and last 8 bits are 255 error codes within the library.  Each error code have fixed string associated with it.  For example, the error-code -1765328383 have the symbolic name KRB5KDC_ERR_NAME_EXP, and associated error string ``Client's entry in database has expired''.

This is a great improvement compared to just getting one of the unix error-codes back.  However, Heimdal have an extention to pass back customised errors messages.  Instead of getting ``Key table entry not found'', the user might back ``failed to find host/host.example.com@EXAMLE.COM(kvno 3) in keytab /etc/krb5.keytab (des-cbc-crc)''.  This improves the chance that the user find the cause of the error so you should use the customised error message whenever it's available.

See also manual page for krb5_get_error_string,3 and krb5_get_err_text,3.

Keytab management
-----------------

A keytab is a storage for locally stored keys. Heimdal includes keytab support for Kerberos 5 keytabs, Kerberos 4 srvtab, AFS-KeyFile's, and for storing keys in memory.

Keytabs are used for servers and long-running services.

See also manual page for krb5_keytab,3

Kerberos crypto
---------------

Heimdal includes a implementation of the Kerberos crypto framework, all crypto operations.

See also manual page for krb5_crypto_init,3, krb5_keyblock,3, krb5_create_checksum,3, and krb5_encrypt,3.

Walkthrough of a sample Kerberos 5 client
=========================================

This example contains parts of a sample TCP Kerberos 5 clients, if you want a real working client, please look in appl/test directory in the Heimdal distribution.

All Kerberos error-codes that are returned from kerberos functions in this program are passed to krb5_err, that will print a descriptive text of the error code and exit. Graphical programs can convert error-code to a human readable error-string with the krb5_get_err_text,3 function.

Note that you should not use any Kerberos function before krb5_init_context() have completed successfully. That is the reason err() is used when krb5_init_context() fails.

First the client needs to call krb5_init_context to initialise the Kerberos 5 library. This is only needed once per thread in the program. If the function returns a non-zero value it indicates that either the Kerberos implementation is failing or it's disabled on this host.

     #include <krb5.h>

     int
     main(int argc, char **argv)
     {
             krb5_context context;

             if (krb5_context(&context))
                     errx (1, "krb5_context");

Now the client wants to connect to the host at the other end. The preferred way of doing this is using getaddrinfo,3 (for operating system that have this function implemented), since getaddrinfo is neutral to the address type and can use any protocol that is available.

             struct addrinfo *ai, *a;
             struct addrinfo hints;
             int error;

             memset (&hints, 0, sizeof(hints));
             hints.ai_socktype = SOCK_STREAM;
             hints.ai_protocol = IPPROTO_TCP;

             error = getaddrinfo (hostname, "pop3", &hints, &ai);
             if (error)
                     errx (1, "%s: %s", hostname, gai_strerror(error));

             for (a = ai; a != NULL; a = a->ai_next) {
                     int s;

                     s = socket (a->ai_family, a->ai_socktype, a->ai_protocol);
                     if (s < 0)
                             continue;
                     if (connect (s, a->ai_addr, a->ai_addrlen) < 0) {
                             warn ("connect(%s)", hostname);
                                 close (s);
                                 continue;
                     }
                     freeaddrinfo (ai);
                     ai = NULL;
             }
             if (ai) {
                         freeaddrinfo (ai);
                         errx ("failed to contact %s", hostname);
             }

Before authenticating, an authentication context needs to be created. This context keeps all information for one (to be) authenticated connection (see krb5_auth_context,3).

             status = krb5_auth_con_init (context, &auth_context);
             if (status)
                     krb5_err (context, 1, status, "krb5_auth_con_init");

For setting the address in the authentication there is a help function krb5_auth_con_setaddrs_from_fd that does everything that is needed when given a connected file descriptor to the socket.

             status = krb5_auth_con_setaddrs_from_fd (context,
                                                      auth_context,
                                                      &sock);
             if (status)
                     krb5_err (context, 1, status,
                               "krb5_auth_con_setaddrs_from_fd");

The next step is to build a server principal for the service we want to connect to. (See also krb5_sname_to_principal,3.)

             status = krb5_sname_to_principal (context,
                                               hostname,
                                               service,
                                               KRB5_NT_SRV_HST,
                                               &server);
             if (status)
                     krb5_err (context, 1, status, "krb5_sname_to_principal");

The client principal is not passed to krb5_sendauth,3 function, this causes the krb5_sendauth function to try to figure it out itself.

The server program is using the function krb5_recvauth,3 to receive the Kerberos 5 authenticator.

In this case, mutual authentication will be tried. That means that the server will authenticate to the client. Using mutual authentication is good since it enables the user to verify that they are talking to the right server (a server that knows the key).

If you are using a non-blocking socket you will need to do all work of krb5_sendauth yourself. Basically you need to send over the authenticator from krb5_mk_req,3 and, in case of mutual authentication, verifying the result from the server with krb5_rd_rep,3.

             status = krb5_sendauth (context,
                                     &auth_context,
                                     &sock,
                                     VERSION,
                                     NULL,
                                     server,
                                     AP_OPTS_MUTUAL_REQUIRED,
                                     NULL,
                                     NULL,
                                     NULL,
                                     NULL,
                                     NULL,
                                     NULL);
             if (status)
                     krb5_err (context, 1, status, "krb5_sendauth");

Once authentication has been performed, it is time to send some data. First we create a krb5_data structure, then we sign it with krb5_mk_safe,3 using the auth_context that contains the session-key that was exchanged in the krb5_sendauth,3/krb5_recvauth,3 authentication sequence.

             data.data   = "hej";
             data.length = 3;

             krb5_data_zero (&packet);

             status = krb5_mk_safe (context,
                                    auth_context,
                                    &data,
                                    &packet,
                                    NULL);
             if (status)
                     krb5_err (context, 1, status, "krb5_mk_safe");

And send it over the network.

             len = packet.length;
             net_len = htonl(len);

             if (krb5_net_write (context, &sock, &net_len, 4) != 4)
                     err (1, "krb5_net_write");
             if (krb5_net_write (context, &sock, packet.data, len) != len)
                     err (1, "krb5_net_write");

To send encrypted (and signed) data krb5_mk_priv,3 should be used instead. krb5_mk_priv,3 works the same way as krb5_mk_safe,3, with the exception that it encrypts the data in addition to signing it.

             data.data   = "hemligt";
             data.length = 7;

             krb5_data_free (&packet);

             status = krb5_mk_priv (context,
                                    auth_context,
                                    &data,
                                    &packet,
                                    NULL);
             if (status)
                     krb5_err (context, 1, status, "krb5_mk_priv");

And send it over the network.

             len = packet.length;
             net_len = htonl(len);

             if (krb5_net_write (context, &sock, &net_len, 4) != 4)
                     err (1, "krb5_net_write");
             if (krb5_net_write (context, &sock, packet.data, len) != len)
                     err (1, "krb5_net_write");

The server is using krb5_rd_safe,3 and krb5_rd_priv,3 to verify the signature and decrypt the packet.

Validating a password in an application
=======================================

See the manual page for krb5_verify_user,3.

API differences to MIT Kerberos
===============================

This section is somewhat disorganised, but so far there is no overall structure to the differences, though some of the have their root in that Heimdal uses an ASN.1 compiler and MIT doesn't.

Principal and realms
--------------------

Heimdal stores the realm as a krb5_realm, that is a char *. MIT Kerberos uses a krb5_data to store a realm.

In Heimdal krb5_principal doesn't contain the component name_type; it's instead stored in component name.name_type. To get and set the nametype in Heimdal, use krb5_principal_get_type,3 and krb5_principal_set_type,3.

For more information about principal and realms, see krb5_principal,3.

Error messages
--------------

To get the error string, Heimdal uses krb5_get_error_string,3 or, if NULL is returned, krb5_get_err_text,3. This is to return custom error messages (like ``Can't find host/datan.example.com@EXAMPLE.COM in /etc/krb5.conf.'' instead of a ``Key table entry not found'' that error_message,3 returns.

Heimdal uses a threadsafe(r) version of the com_err interface; the global com_err table isn't initialised.  Then error_message,3 returns quite a boring error string (just the error code itself).

File formats
============

This section documents the diffrent file formats that are used in Heimdal and other Kerberos implementations.

keytab
------

The keytab binary format is not a standard format. The format has evolved and may continue to. It is however understood by several Kerberos implementations including Heimdal, MIT, Sun's Java ktab and are created by the ktpass.exe utility from Windows. So it has established itself as the defacto format for storing Kerberos keys.

The following C-like structure definitions illustrate the MIT keytab file format. All values are in network byte order. All text is ASCII.

       keytab {
           uint16_t file_format_version;                    /* 0x502 */
           keytab_entry entries[*];
       };

       keytab_entry {
           int32_t size;
           uint16_t num_components;   /* subtract 1 if version 0x501 */
           counted_octet_string realm;
           counted_octet_string components[num_components];
           uint32_t name_type;       /* not present if version 0x501 */
           uint32_t timestamp;
           uint8_t vno8;
           keyblock key;
           uint32_t vno; /* only present if >= 4 bytes left in entry */
       };

       counted_octet_string {
           uint16_t length;
           uint8_t data[length];
       };

       keyblock {
           uint16_t type;
           counted_octet_string;
       };

All numbers are stored in network byteorder (big endian) format.

The keytab file format begins with the 16 bit file_format_version which at the time this document was authored is 0x502. The format of older keytabs is described at the end of this document.

The file_format_version is immediately followed by an array of keytab_entry structures which are prefixed with a 32 bit size indicating the number of bytes that follow in the entry. Note that the size should be evaluated as signed. This is because a negative value indicates that the entry is in fact empty (e.g. it has been deleted) and that the negative value of that negative value (which is of course a positive value) is the offset to the next keytab_entry. Based on these size values alone the entire keytab file can be traversed.

The size is followed by a 16 bit num_components field indicating the number of counted_octet_string components in the components array.

The num_components field is followed by a counted_octet_string representing the realm of the principal.

A counted_octet_string is simply an array of bytes prefixed with a 16 bit length. For the realm and name components, the counted_octet_string bytes are ASCII encoded text with no zero terminator.

Following the realm is the components array that represents the name of the principal. The text of these components may be joined with slashs to construct the typical SPN representation. For example, the service principal HTTP/www.foo.net@FOO.NET would consist of name components "HTTP" followed by "www.foo.net".

Following the components array is the 32 bit name_type (e.g. 1 is KRB5_NT_PRINCIPAL, 2 is KRB5_NT_SRV_INST, 5 is KRB5_NT_UID, etc). In practice the name_type is almost certainly 1 meaning KRB5_NT_PRINCIPAL.

The 32 bit timestamp indicates the time the key was established for that principal. The value represents the number of seconds since Jan 1, 1970.

The 8 bit vno8 field is the version number of the key. This value is overridden by the 32 bit vno field if it is present. The vno8 field is filled with the lower 8 bits of the 32 bit protocol kvno field.

The keyblock structure consists of a 16 bit value indicating the encryption type and is a counted_octet_string containing the key.  The encryption type is the same as the Kerberos standard (e.g. 3 is des-cbc-md5, 23 is arcfour-hmac-md5, etc).

The last field of the keytab_entry structure is optional. If the size of the keytab_entry indicates that there are at least 4 bytes remaining, a 32 bit value representing the key version number is present. This value supersedes the 8 bit vno8 value preceeding the keyblock.

Older keytabs with a file_format_version of 0x501 are different in three ways:

All integers are in host byte order [1].

The num_components field is 1 too large (i.e. after decoding, decrement by 1).

The 32 bit name_type field is not present.

[1] The file_format_version field should really be treated as two separate 8 bit quantities representing the major and minor version number respectively.

Heimdal database dump file
--------------------------

Format of the Heimdal text dump file as of Heimdal 0.6.3:

Each line in the dump file is one entry in the database.

Each field of a line is separated by one or more spaces, with the exception of fields consisting of principals containing spaces, where space can be quoted with \ and \ is quoted by \.

Fields and their types are:

     	Quoted princial (quote character is \) [string]
     	Keys [keys]
     	Created by [event]
     	Modified by [event optional]
     	Valid start time [time optional]
     	Valid end time [time optional]
     	Password end valid time [time optional]
     	Max lifetime of ticket [time optional]
     	Max renew time of ticket [integer optional]
     	Flags [hdb flags]
     	Generation number [generation optional]
     	Extensions [extentions optional]

Fields following these silently are ignored.

All optional fields will be skipped if they fail to parse (or comprise the optional field marker of "-", w/o quotes).

Example:

     fred@EXAMPLE.COM 27:1:16:e8b4c8fc7e60b9e641dcf4cff3f08a701d982a2f89ba373733d26ca59ba6c789666f6b8bfcf169412bb1e5dceb9b33cda29f3412:-:1:3:4498a933881178c744f4232172dcd774c64e81fa6d05ecdf643a7e390624a0ebf3c7407a:-:1:2:b01934b13eb795d76f3a80717d469639b4da0cfb644161340ef44fdeb375e54d684dbb85:-:1:1:ea8e16d8078bf60c781da90f508d4deccba70595258b9d31888d33987cd31af0c9cced2e:- 20020415130120:admin@EXAMPLE.COM 20041221112428:fred@EXAMPLE.COM - - - 86400 604800 126 20020415130120:793707:28 -

Encoding of types are as follows:

keys

          kvno:[masterkvno:keytype:keydata:salt]{zero or more separated by :}

     kvno is the key version number.

     keydata is hex-encoded

     masterkvno is the kvno of the database master key.  If this field is empty, the kadmin load and merge operations will encrypt the key data with the master key if there is one.  Otherwise the key data will be imported asis.

     salt is encoded as "-" (no/default salt) or

          salt-type /
          salt-type / "string"
          salt-type / hex-encoded-data

     keytype is the protocol enctype number; see enum ENCTYPE in include/krb5_asn1.h for values.

     Example:

          27:1:16:e8b4c8fc7e60b9e641dcf4cff3f08a701d982a2f89ba373733d26ca59ba6c789666f6b8bfcf169412bb1e5dceb9b33cda29f3412:-:1:3:4498a933881178c744f4232172dcd774c64e81fa6d05ecdf643a7e390624a0ebf3c7407a:-:1:2:b01934b13eb795d76f3a80717d469639b4da0cfb644161340ef44fdeb375e54d684dbb85:-:1:1:ea8e16d8078bf60c781da90f508d4deccba70595258b9d31888d33987cd31af0c9cced2e:-

          kvno=27,{key: masterkvno=1,keytype=des3-cbc-sha1,keydata=..., default salt}...

time

     Format of the time is: YYYYmmddHHMMSS, corresponding to strftime format "%Y%m%d%k%M%S".

     Time is expressed in UTC.

     Time can be optional (using -), when the time 0 is used.

     Example:

          20041221112428

event

          	time:principal

     time is as given in format time

     principal is a string.  Not quoting it may not work in earlier versions of Heimdal.

     Example:

          20041221112428:bloggs@EXAMPLE.COM

hdb flags

     Integer encoding of HDB flags, see HDBFlags in lib/hdb/hdb.asn1. Each bit in the integer is the same as the bit in the specification.

generation:

          time:usec:gen

     usec is a the microsecond, integer. gen is generation number, integer.

     The generation can be defaulted (using '-') or the empty string

extensions:

          first-hex-encoded-HDB-Extension[:second-...]

     HDB-extension is encoded the DER encoded HDB-Extension from lib/hdb/hdb.asn1. Consumers HDB extensions should be aware that unknown entires needs to be preserved even thought the ASN.1 data content might be unknown. There is a critical flag in the data to show to the KDC that the entry MUST be understod if the entry is to be used.