heimdal/doc/init-creds

Currently, getting an initial ticket for a user involves many function
calls, especially when a full set of features including password
expiration and challenge preauthentication is desired.  In order to
solve this problem, a new api is proposed.

typedef struct _krb5_prompt {
    char *prompt;
    int hidden;
    krb5_data *reply;
} krb5_prompt;

typedef int (*krb5_prompter_fct)(krb5_context context,
				 void *data,
				 const char *banner,
				 int num_prompts,
				 krb5_prompt prompts[]);

typedef struct _krb5_get_init_creds_opt {
    krb5_flags flags;
    krb5_deltat tkt_life;
    krb5_deltat renew_life;
    int forwardable;
    int proxiable;
    krb5_enctype *etype_list;
    int etype_list_length;
    krb5_address **address_list;
	/* XXX the next three should not be used, as they may be
	removed later */
    krb5_preauthtype *preauth_list;
    int preauth_list_length;
    krb5_data *salt;
} krb5_get_init_creds_opt;

#define KRB5_GET_INIT_CREDS_OPT_TKT_LIFE	0x0001
#define KRB5_GET_INIT_CREDS_OPT_RENEW_LIFE	0x0002
#define KRB5_GET_INIT_CREDS_OPT_FORWARDABLE	0x0004
#define KRB5_GET_INIT_CREDS_OPT_PROXIABLE	0x0008
#define KRB5_GET_INIT_CREDS_OPT_ETYPE_LIST	0x0010
#define KRB5_GET_INIT_CREDS_OPT_ADDRESS_LIST	0x0020
#define KRB5_GET_INIT_CREDS_OPT_PREAUTH_LIST	0x0040
#define KRB5_GET_INIT_CREDS_OPT_SALT		0x0080

void krb5_get_init_creds_opt_init(krb5_get_init_creds_opt *opt);

void krb5_get_init_creds_opt_set_tkt_life(krb5_get_init_creds_opt *opt,
					  krb5_deltat tkt_life);
void krb5_get_init_creds_opt_set_renew_life(krb5_get_init_creds_opt *opt,
					    krb5_deltat renew_life);
void krb5_get_init_creds_opt_set_forwardable(krb5_get_init_creds_opt *opt,
					     int forwardable);
void krb5_get_init_creds_opt_set_proxiable(krb5_get_init_creds_opt *opt,
					   int proxiable);
void krb5_get_init_creds_opt_set_etype_list(krb5_get_init_creds_opt *opt,
					    krb5_enctype *etype_list,
					    int etype_list_length);
void krb5_get_init_creds_opt_set_address_list(krb5_get_init_creds_opt *opt,
					      krb5_address **addresses);
void krb5_get_init_creds_opt_set_preauth_list(krb5_get_init_creds_opt *opt,
					      krb5_preauthtype *preauth_list,
					      int preauth_list_length);
void krb5_get_init_creds_opt_set_salt(krb5_get_init_creds_opt *opt,
				      krb5_data *salt);

krb5_error_code
krb5_get_init_creds_password(krb5_context context,
			     krb5_creds *creds,
			     krb5_principal client,
			     char *password,
			     krb5_prompter_fct prompter,
			     void *data,
			     krb5_deltat start_time,
			     char *in_tkt_service,
			     krb5_get_init_creds_opt *options);

This function will attempt to acquire an initial ticket.  The function
will perform whatever tasks are necessary to do so.  This may include
changing an expired password, preauthentication.

The arguments divide into two types.  Some arguments are basically
invariant and arbitrary across all initial tickets, and if not
specified are determined by configuration or library defaults.  Some
arguments are different for each execution or application, and if not
specified can be determined correctly from system configuration or
environment.  The former arguments are contained in a structure whose
pointer is passed to the function.  A bitmask specifies which elements
of the structure should be used.  In most cases, a NULL pointer can be
used.  The latter arguments are specified as individual arguments to
the function.

If a pointer to a credential is specified, the initial credential is
filled in.  If the caller only wishes to do a simple password check
and will not be doing any other kerberos functions, then a NULL
pointer may be specified, and the credential will be destroyed.

If the client name is non-NULL, the initial ticket requested will be
for that principal.  Otherwise, the principal will be the username
specified by the USER environment variable, or if the USER environment
variable is not set, the username corresponding to the real user id of
the caller.

If the password is non-NULL, then this string is used as the password.
Otherwise, the prompter function will be used to prompt the user for
the password.

If a prompter function is non-NULL, it will be used if additional user
input is required, such as if the user's password has expired and
needs to be changed, or if input preauthentication is necessary.  If
no function is specified and input is required, then the login will
fail.

	The context argument is the same as that passed to krb5_login.
	The data argument is passed unmodified to the prompter
	function and is intended to be used to pass application data
	(such as a display handle) to the prompter function.

	The banner argument, if non-NULL, will indicate what sort of
	input is expected from the user (for example, "Password has
	expired and must be changed" or "Enter Activcard response for
	challenge 012345678"), and should be displayed accordingly.
	
	The num_prompts argument indicates the number of values which
	should be prompted for.  If num_prompts == 0, then the banner
	contains an informational message which should be displayed to
	the user.

	The prompts argument contains an array describing the values
	for which the user should be prompted.  The prompt member
	indicates the prompt for each value ("Enter new
	password"/"Enter it again", or "Challenge response").  The
	hidden member is nonzero if the response should not be
	displayed back to the user.  The reply member is a pointer to
	krb5_data structure which has already been allocated.  The
	prompter should fill in the structure with the NUL-terminated
	response from the user.

	If the response data does not fit, or if any other error
	occurs, then the prompter function should return a non-zero
	value which will be returned by the krb5_get_init_creds
	function. Otherwise, zero should be returned.

	The library function krb5_prompter_posix() implements
	a prompter using a posix terminal for user in.  This function
	does not use the data argument.

If the start_time is zero, then the requested ticket will be valid
beginning immediately.  Otherwise, the start_time indicates how far in
the future the ticket should be postdated.

If the in_tkt_service name is non-NULL, that principal name will be
used as the server name for the initial ticket request.  The realm of
the name specified will be ignored and will be set to the realm of the
client name.  If no in_tkt_service name is specified,
krbtgt/CLIENT-REALM@CLIENT-REALM will be used.

For the rest of arguments, a configuration or library default will be
used if no value is specified in the options structure.

If a tkt_life is specified, that will be the lifetime of the ticket.
The library default is 10 hours; there is no configuration variable
(there should be, but it's not there now).

If a renew_life is specified and non-zero, then the RENEWABLE option
on the ticket will be set, and the value of the argument will be the
the renewable lifetime.  The configuration variable [libdefaults]
"renew_lifetime" is the renewable lifetime if none is passed in.  The
library default is not to set the RENEWABLE option.

If forwardable is specified, the FORWARDABLE option on the ticket will
be set if and only if forwardable is non-zero.  The configuration
variable [libdefaults] "forwardable" is used if no value is passed in.
The option will be set if and only if the variable is "y", "yes",
"true", "t", "1", or "on", case insensitive.  The library default is
not to set the FORWARDABLE option.

If proxiable is specified, the PROXIABLE option on the ticket will be
set if and only if proxiable is non-zero.  The configuration variable
[libdefaults] "proxiable" is used if no value is passed in.  The
option will be set if and only if the variable is "y", "yes", "true",
"t", "1", or "on", case insensitive.  The library default is not to
set the PROXIABLE option.

If etype_list is specified, it will be used as the list of desired
encryption algorithms in the request.  The configuration variable
[libdefaults] "default_tkt_enctypes" is used if no value is passed in.
The library default is "des-cbc-md5 des-cbc-crc".

If address_list is specified, it will be used as the list of addresses
for which the ticket will be valid.  The library default is to use all
local non-loopback addresses.  There is no configuration variable.

If preauth_list is specified, it names preauth data types which will
be included in the request.  The library default is to interact with
the kdc to determine the required preauth types.  There is no
configuration variable.

If salt is specified, it specifies the salt which will be used when
converting the password to a key.  The library default is to interact
with the kdc to determine the correct salt.  There is no configuration
variable.

================================================================

typedef struct _krb5_verify_init_creds_opt {
    krb5_flags flags;
    int ap_req_nofail;
} krb5_verify_init_creds_opt;

#define KRB5_VERIFY_INIT_CREDS_OPT_AP_REQ_NOFAIL	0x0001

void krb5_verify_init_creds_opt_init(krb5_init_creds_opt *options);
void krb5_verify_init_creds_opt_set_ap_req_nofail(krb5_init_creds_opt *options,
						  int ap_req_nofail);

krb5_error_code
krb5_verify_init_creds(krb5_context context,
		       krb5_creds *creds,
		       krb5_principal ap_req_server,
		       krb5_keytab ap_req_keytab,
		       krb5_ccache *ccache,
		       krb5_verify_init_creds_opt *options);

This function will use the initial ticket in creds to make an AP_REQ
and verify it to insure that the AS_REP has not been spoofed.

If the ap_req_server name is non-NULL, then this service name will be
used for the AP_REQ; otherwise, the default host key
(host/hostname.domain@LOCAL-REALM) will be used.

If ap_req_keytab is non-NULL, the service key for the verification
will be read from that keytab; otherwise, the service key will be read
from the default keytab. 

If the service of the ticket in creds is the same as the service name
for the AP_REQ, then this ticket will be used directly.  If the ticket
is a tgt, then it will be used to obtain credentials for the service.
Otherwise, the verification will fail, and return an error.

Other failures of the AP_REQ verification may or may not be considered
errors, as described below.

If a pointer to a credential cache handle is specified, and the handle
is NULL, a credential cache handle referring to all credentials
obtained in the course of verifying the user will be returned.  In
order to avoid potential setuid race conditions and other problems
related to file system access, this handle will refer to a memory
credential cache.  If the handle is non-NULL, then the credentials
will be added to the existing ccache.  If the caller only wishes to
verify the password and will not be doing any other kerberos
functions, then a NULL pointer may be specified, and the credentials
will be deleted before the function returns.

If ap_req_nofail is specified, then failures of the AP_REQ
verification are considered errors if and only if ap_req_nofail is
non-zero.

Whether or not AP_REQ validation is performed and what failures mean
depends on these inputs:

 A) The appropriate keytab exists and contains the named key.

 B) An AP_REQ request to the kdc succeeds, and the resulting AP_REQ
can be decrypted and verified.

 C) The administrator has specified in a configuration file that
AP_REQ validation must succeed.  This is basically a paranoid bit, and
can be overridden by the application based on a command line flag or
other application-specific info.  This flag is especially useful if
the admin is concerned that DNS might be spoofed while determining the
host/FQDN name.  The configuration variable [libdefaults]
"verify_ap_req_nofail" is used if no value is passed in.  The library
default is not to set this option.

Initial ticket verification will succeed if and only if:

 - A && B    or
 - !A && !C

================================================================

For illustrative purposes, here's the invocations I expect some
programs will use.  Of course, error checking needs to be added.

kinit:

    /* Fill in client from the command line || existing ccache, and,
       start_time, and options.{tkt_life,renew_life,forwardable,proxiable}
       from the command line.  Some or all may remain unset. */

    krb5_get_init_creds(context, &creds, client,
			krb5_initial_prompter_posix, NULL,
			start_time, NULL, &options);
    krb5_cc_store_cred(context, ccache, &creds);
    krb5_free_cred_contents(context, &creds);

login:

    krb5_get_init_creds(context, &creds, client,
			krb5_initial_prompter_posix, NULL,
			0, NULL, NULL);
    krb5_verify_init_creds(context, &creds, NULL, NULL, &vcc, NULL);
    /* setuid */
    krb5_cc_store_cred(context, ccache, &creds);
    krb5_cc_copy(context, vcc, ccache);
    krb5_free_cred_contents(context, &creds);
    krb5_cc_destroy(context, vcc);

xdm:

    krb5_get_initial_creds(context, &creds, client,
			   krb5_initial_prompter_xt, (void *) &xtstuff,
			   0, NULL, NULL);
    krb5_verify_init_creds(context, &creds, NULL, NULL, &vcc, NULL);
    /* setuid */
    krb5_cc_store_cred(context, ccache, &creds);
    krb5_free_cred_contents(context, &creds);
    krb5_cc_copy(context, vcc, ccache);
    krb5_cc_destroy(context, vcc);

passwd:

    krb5_init_creds_opt_init(&options);
    krb5_init_creds_opt_set_tkt_life = 300;
    krb5_get_initial_creds(context, &creds, client,
			   krb5_initial_prompter_posix, NULL,
			   0, "kadmin/changepw", &options);
    /* change password */
    krb5_free_cred_contents(context, &creds);

pop3d (simple password validator when no user interation possible):

    krb5_get_initial_creds(context, &creds, client,
			   NULL, NULL, 0, NULL, NULL);
    krb5_verify_init_creds(context, &creds, NULL, NULL, &vcc, NULL);
    krb5_cc_destroy(context, vcc);

================================================================

password expiration has a subtlety.  When a password expires and is
changed, there is a delay between when the master gets the new key
(immediately), and the slaves (propogation interval).  So, when
getting an in_tkt, if the password is expired, the request should be
reissued to the master (this kind of sucks if you have SAM, oh well).
If this says expired, too, then the password should be changed, and
then the initial ticket request should be issued to the master again.
If the master times out, then a message that the password has expired
and cannot be changed due to the master being unreachable should be
displayed.

================================================================

get_init_creds reads config stuff from:

[libdefaults]
	varname1 = defvalue
	REALM = {
		varname1 = value
		varname2 = value
	}

typedef struct _krb5_get_init_creds_opt {
    krb5_flags flags;
    krb5_deltat tkt_life;	/* varname = "ticket_lifetime" */
    krb5_deltat renew_life;	/* varname = "renew_lifetime" */
    int forwardable;		/* varname = "forwardable" */
    int proxiable;		/* varname = "proxiable" */
    krb5_enctype *etype_list;	/* varname = "default_tkt_enctypes" */
    int etype_list_length;
    krb5_address **address_list; /* no varname */
    krb5_preauthtype *preauth_list; /* no varname */
    int preauth_list_length;
    krb5_data *salt;
} krb5_get_init_creds_opt;