krb_mk_req, krb_rd_req, krb_kntoln, krb_set_key, krb_get_cred, krb_mk_priv, krb_rd_priv, krb_mk_safe, krb_rd_safe, krb_mk_err, krb_rd_err, krb_ck_repl - Kerberos authentication library

#include <des.h>
#include <krb.h>

extern char *krb_err_txt[];

int krb_mk_req(authent,service,instance,realm,checksum)
KTEXT authent;
char *service;
char *instance;
char *realm;
u_long checksum;

int krb_rd_req(authent,service,instance,from_addr,ad,fn)
KTEXT authent;
char *service;
char *instance;
u_long from_addr;
AUTH_DAT *ad;
char *fn;

int krb_kntoln(ad,lname)
AUTH_DAT *ad;
char *lname;

int krb_set_key(key,cvt)
char *key;
int cvt;

int krb_get_cred(service,instance,realm,c)
char *service;
char *instance;
char *realm;
CREDENTIALS *c;

long krb_mk_priv(in,out,in_length,schedule,key,sender,receiver)
u_char *in;
u_char *out;
u_long in_length;
des_cblock key;
des_key_schedule schedule;
struct sockaddr_in *sender;
struct sockaddr_in *receiver;

long krb_rd_priv(in,in_length,schedule,key,sender,receiver,msg_data)
u_char *in;
u_long in_length;
Key_schedule schedule;
des_cblock key;
struct sockaddr_in *sender;
struct sockaddr_in *receiver;
MSG_DAT *msg_data;

long krb_mk_safe(in,out,in_length,key,sender,receiver)
u_char *in;
u_char *out;
u_long in_length;
des_cblock key;
struct sockaddr_in *sender;
struct sockaddr_in *receiver;

long krb_rd_safe(in,length,key,sender,receiver,msg_data)
u_char *in;
u_long length;
des_cblock key;
struct sockaddr_in *sender;
struct sockaddr_in *receiver;
MSG_DAT *msg_data;

long krb_mk_err(out,code,string)
u_char *out;
long code;
char *string;

long krb_rd_err(in,length,code,msg_data)
u_char *in;
u_long length;
long code;
MSG_DAT *msg_data;

This library supports network authentication and various related operations. The library contains many routines beyond those described in this man page, but they are not intended to be used directly. Instead, they are called by the routines that are described, the authentication server and the login program.

krb_err_txt[] contains text string descriptions of various Kerberos error codes returned by some of the routines below.

krb_mk_req takes a pointer to a text structure in which an authenticator is to be built. It also takes the name, instance, and realm of the service to be used and an optional checksum. It is up to the application to decide how to generate the checksum. krb_mk_req then retrieves a ticket for the desired service and creates an authenticator. The authenticator is built in authent and is accessible to the calling procedure.

It is up to the application to get the authenticator to the service where it will be read by krb_rd_req. Unless an attacker posesses the session key contained in the ticket, it will be unable to modify the authenticator. Thus, the checksum can be used to verify the authenticity of the other data that will pass through a connection.

krb_rd_req takes an authenticator of type KTEXT, a service name, an instance, the address of the host originating the request, and a pointer to a structure of type AUTH_DAT which is filled in with information obtained from the authenticator. It also optionally takes the name of the file in which it will find the secret key(s) for the service. If the supplied instance contains "*", then the first service key with the same service name found in the service key file will be used, and the instance argument will be filled in with the chosen instance. This means that the caller must provide space for such an instance name.

It is used to find out information about the principal when a request has been made to a service. It is up to the application protocol to get the authenticator from the client to the service. The authenticator is then passed to krb_rd_req to extract the desired information.

krb_rd_req returns zero (RD_AP_OK) upon successful authentication. If a packet was forged, modified, or replayed, authentication will fail. If the authentication fails, a non-zero value is returned indicating the particular problem encountered. See krb.h for the list of error codes.

If the last argument is the null string (""), krb_rd_req will use the file /etc/srvtab to find its keys. If the last argument is NULL, it will assume that the key has been set by krb_set_key and will not bother looking further.

krb_kntoln converts a Kerberos name to a local name. It takes a structure of type AUTH_DAT and uses the name and instance to look in the database /etc/aname to find the corresponding local name. The local name is returned and can be used by an application to change uids, directories, or other parameters. It is not an integral part of Kerberos, but is instead provided to support the use of Kerberos in existing utilities.

krb_set_key takes as an argument a des key. It then creates a key schedule from it and saves the original key to be used as an initialization vector. It is used to set the server's key which must be used to decrypt tickets.

If called with a non-zero second argument, krb_set_key will first convert the input from a string of arbitrary length to a DES key by encrypting it with a one-way function.

In most cases it should not be necessary to call krb_set_key. The necessary keys will usually be obtained and set inside krb_rd_req. krb_set_key is provided for those applications that do not wish to place the application keys on disk.

krb_get_cred searches the caller's ticket file for a ticket for the given service, instance, and realm; and, if a ticket is found, fills in the given CREDENTIALS structure with the ticket information.

If the ticket was found, krb_get_cred returns GC_OK. If the ticket file can't be found, can't be read, doesn't belong to the user (other than root), isn't a regular file, or is in the wrong mode, the error GC_TKFIL is returned.

krb_mk_priv creates an encrypted, authenticated message from any arbitrary application data, pointed to by in and in_length bytes long. The private session key, pointed to by key and the key schedule, schedule, are used to encrypt the data and some header information using pcbc_encrypt. sender and receiver point to the Internet address of the two parties. In addition to providing privacy, this protocol message protects against modifications, insertions or replays. The encapsulated message and header are placed in the area pointed to by out and the routine returns the length of the output, or -1 indicating an error.

krb_rd_priv decrypts and authenticates a received krb_mk_priv message. in points to the beginning of the received message, whose length is specified in in_length. The private session key, pointed to by key, and the key schedule, schedule, are used to decrypt and verify the received message. msg_data is a pointer to a MSG_DAT struct, defined in krb.h. The routine fills in the app_data field with a pointer to the decrypted application data, app_length with the length of the app_data field, time_sec and time_5ms with the timestamps in the message, and swap with a 1 if the byte order of the receiver is different than that of the sender. (The application must still determine if it is appropriate to byte-swap application data; the Kerberos protocol fields are already taken care of). The hash field returns a value useful as input to the krb_ck_repl routine.

The routine returns zero if ok, or a Kerberos error code. Modified messages and old messages cause errors, but it is up to the caller to check the time sequence of messages, and to check against recently replayed messages using krb_ck_repl if so desired.

krb_mk_safe creates an authenticated, but unencrypted message from any arbitrary application data, pointed to by in and in_length bytes long. The private session key, pointed to by key, is used to seed the quad_cksum() checksum algorithm used as part of the authentication. sender and receiver point to the Internet address of the two parties. This message does not provide privacy, but does protect (via detection) against modifications, insertions or replays. The encapsulated message and header are placed in the area pointed to by out and the routine returns the length of the output, or -1 indicating an error. The authentication provided by this routine is not as strong as that provided by krb_mk_priv or by computing the checksum using cbc_cksum instead, both of which authenticate via DES.

krb_rd_safe authenticates a received krb_mk_safe message. in points to the beginning of the received message, whose length is specified in in_length. The private session key, pointed to by key, is used to seed the quad_cksum() routine as part of the authentication. msg_data is a pointer to a MSG_DAT struct, defined in krb.h . The routine fills in these MSG_DAT fields: the app_data field with a pointer to the application data, app_length with the length of the app_data field, time_sec and time_5ms with the timestamps in the message, and swap with a 1 if the byte order of the receiver is different than that of the sender. (The application must still determine if it is appropriate to byte-swap application data; the Kerberos protocol fields are already taken care of). The hash field returns a value useful as input to the krb_ck_repl routine.

The routine returns zero if ok, or a Kerberos error code. Modified messages and old messages cause errors, but it is up to the caller to check the time sequence of messages, and to check against recently replayed messages using krb_ck_repl if so desired.

krb_mk_err constructs an application level error message that may be used along with krb_mk_priv or krb_mk_safe. out is a pointer to the output buffer, code is an application specific error code, and string is an application specific error string.

krb_rd_err unpacks a received krb_mk_err message. in points to the beginning of the received message, whose length is specified in in_length. code is a pointer to a value to be filled in with the error value provided by the application. msg_data is a pointer to a MSG_DAT struct, defined in krb.h . The routine fills in these MSG_DAT fields: the app_data field with a pointer to the application error text, app_length with the length of the app_data field, and swap with a 1 if the byte order of the receiver is different than that of the sender. (The application must still determine if it is appropriate to byte-swap application data; the Kerberos protocol fields are already taken care of).

The routine returns zero if the error message has been successfully received, or a Kerberos error code.

The KTEXT structure is used to pass around text of varying lengths. It consists of a buffer for the data, and a length. krb_rd_req takes an argument of this type containing the authenticator, and krb_mk_req returns the authenticator in a structure of this type. KTEXT itself is really a pointer to the structure. The actual structure is of type KTEXT_ST.

The AUTH_DAT structure is filled in by krb_rd_req. It must be allocated before calling krb_rd_req, and a pointer to it is passed. The structure is filled in with data obtained from Kerberos. MSG_DAT structure is filled in by either krb_rd_priv, krb_rd_safe, or krb_rd_err. It must be allocated before the call and a pointer to it is passed. The structure is filled in with data obtained from Kerberos.

/usr/include/krb.h

/usr/lib/libkrb.a

/usr/include/des.h

/usr/lib/libdes.a

/etc/aname

/etc/srvtab

/tmp/tkt[uid]

kerberos(1), des_crypt(3)

The caller of krb_rd_req, krb_rd_priv, and krb_rd_safe must check time order and for replay attempts. krb_ck_repl is not implemented yet.

Clifford Neuman, MIT Project Athena

Steve Miller, MIT Project Athena/Digital Equipment Corporation

COPYRIGHT 1985,1986,1989 Massachusetts Institute of Technology