auth1.c - third_party/openssh-portable - Git at Google

 /*
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
  *                    All rights reserved
  *
  * As far as I am concerned, the code I have written for this software
  * can be used freely for any purpose.  Any derived versions of this
  * software must be clearly marked as such, and if the derived work is
  * incompatible with the protocol description in the RFC file, it must be
  * called by a name other than "ssh" or "Secure Shell".
  */

 #include "includes.h"
 RCSID("$OpenBSD: auth1.c,v 1.44 2002/09/26 11:38:43 markus Exp $");

 #include "xmalloc.h"
 #include "rsa.h"
 #include "ssh1.h"
 #include "packet.h"
 #include "buffer.h"
 #include "mpaux.h"
 #include "log.h"
 #include "servconf.h"
 #include "compat.h"
 #include "auth.h"
 #include "channels.h"
 #include "session.h"
 #include "uidswap.h"
 #include "monitor_wrap.h"

 /* import */
 extern ServerOptions options;

 /*
  * convert ssh auth msg type into description
  */
 static char *
 get_authname(int type)
 {
 	static char buf[1024];
 	switch (type) {
 	case SSH_CMSG_AUTH_PASSWORD:
 		return "password";
 	case SSH_CMSG_AUTH_RSA:
 		return "rsa";
 	case SSH_CMSG_AUTH_RHOSTS_RSA:
 		return "rhosts-rsa";
 	case SSH_CMSG_AUTH_RHOSTS:
 		return "rhosts";
 	case SSH_CMSG_AUTH_TIS:
 	case SSH_CMSG_AUTH_TIS_RESPONSE:
 		return "challenge-response";
 #if defined(KRB4) || defined(KRB5)
 	case SSH_CMSG_AUTH_KERBEROS:
 		return "kerberos";
 #endif
 	}
 	snprintf(buf, sizeof buf, "bad-auth-msg-%d", type);
 	return buf;
 }

 /*
  * read packets, try to authenticate the user and
  * return only if authentication is successful
  */
 static void
 do_authloop(Authctxt *authctxt)
 {
 	int authenticated = 0;
 	u_int bits;
 	Key *client_host_key;
 	BIGNUM *n;
 	char *client_user, *password;
 	char info[1024];
 	u_int dlen;
 	u_int ulen;
 	int type = 0;
 	struct passwd *pw = authctxt->pw;

 	debug("Attempting authentication for %s%.100s.",
 	    authctxt->valid ? "" : "illegal user ", authctxt->user);

 	/* If the user has no password, accept authentication immediately. */
 	if (options.password_authentication &&
 #if defined(KRB4) || defined(KRB5)
 	    (!options.kerberos_authentication || options.kerberos_or_local_passwd) &&
 #endif
 	    PRIVSEP(auth_password(authctxt, ""))) {
 		auth_log(authctxt, 1, "without authentication", "");
 		return;
 	}

 	/* Indicate that authentication is needed. */
 	packet_start(SSH_SMSG_FAILURE);
 	packet_send();
 	packet_write_wait();

 	client_user = NULL;

 	for (;;) {
 		/* default to fail */
 		authenticated = 0;

 		info[0] = '\0';

 		/* Get a packet from the client. */
 		type = packet_read();

 		/* Process the packet. */
 		switch (type) {

 #if defined(KRB4) || defined(KRB5)
 		case SSH_CMSG_AUTH_KERBEROS:
 			if (!options.kerberos_authentication) {
 				verbose("Kerberos authentication disabled.");
 			} else {
 				char *kdata = packet_get_string(&dlen);
 				packet_check_eom();

 				if (kdata[0] == 4) { /* KRB_PROT_VERSION */
 #ifdef KRB4
 					KTEXT_ST tkt, reply;
 					tkt.length = dlen;
 					if (tkt.length < MAX_KTXT_LEN)
 						memcpy(tkt.dat, kdata, tkt.length);

 					if (PRIVSEP(auth_krb4(authctxt, &tkt,
 					    &client_user, &reply))) {
 						authenticated = 1;
 						snprintf(info, sizeof(info),
 						    " tktuser %.100s",
 						    client_user);

 						packet_start(
 						    SSH_SMSG_AUTH_KERBEROS_RESPONSE);
 						packet_put_string((char *)
 						    reply.dat, reply.length);
 						packet_send();
 						packet_write_wait();
 					}
 #endif /* KRB4 */
 				} else {
 #ifdef KRB5
 					krb5_data tkt, reply;
 					tkt.length = dlen;
 					tkt.data = kdata;

 					if (PRIVSEP(auth_krb5(authctxt, &tkt,
 					    &client_user, &reply))) {
 						authenticated = 1;
 						snprintf(info, sizeof(info),
 						    " tktuser %.100s",
 						    client_user);

  						/* Send response to client */
  						packet_start(
 						    SSH_SMSG_AUTH_KERBEROS_RESPONSE);
  						packet_put_string((char *)
 						    reply.data, reply.length);
  						packet_send();
  						packet_write_wait();

  						if (reply.length)
  							xfree(reply.data);
 					}
 #endif /* KRB5 */
 				}
 				xfree(kdata);
 			}
 			break;
 #endif /* KRB4 || KRB5 */

 #if defined(AFS) || defined(KRB5)
 			/* XXX - punt on backward compatibility here. */
 		case SSH_CMSG_HAVE_KERBEROS_TGT:
 			packet_send_debug("Kerberos TGT passing disabled before authentication.");
 			break;
 #ifdef AFS
 		case SSH_CMSG_HAVE_AFS_TOKEN:
 			packet_send_debug("AFS token passing disabled before authentication.");
 			break;
 #endif /* AFS */
 #endif /* AFS || KRB5 */

 		case SSH_CMSG_AUTH_RHOSTS:
 			if (!options.rhosts_authentication) {
 				verbose("Rhosts authentication disabled.");
 				break;
 			}
 			/*
 			 * Get client user name.  Note that we just have to
 			 * trust the client; this is one reason why rhosts
 			 * authentication is insecure. (Another is
 			 * IP-spoofing on a local network.)
 			 */
 			client_user = packet_get_string(&ulen);
 			packet_check_eom();

 			/* Try to authenticate using /etc/hosts.equiv and .rhosts. */
 			authenticated = auth_rhosts(pw, client_user);

 			snprintf(info, sizeof info, " ruser %.100s", client_user);
 			break;

 		case SSH_CMSG_AUTH_RHOSTS_RSA:
 			if (!options.rhosts_rsa_authentication) {
 				verbose("Rhosts with RSA authentication disabled.");
 				break;
 			}
 			/*
 			 * Get client user name.  Note that we just have to
 			 * trust the client; root on the client machine can
 			 * claim to be any user.
 			 */
 			client_user = packet_get_string(&ulen);

 			/* Get the client host key. */
 			client_host_key = key_new(KEY_RSA1);
 			bits = packet_get_int();
 			packet_get_bignum(client_host_key->rsa->e);
 			packet_get_bignum(client_host_key->rsa->n);

 			if (bits != BN_num_bits(client_host_key->rsa->n))
 				verbose("Warning: keysize mismatch for client_host_key: "
 				    "actual %d, announced %d",
 				    BN_num_bits(client_host_key->rsa->n), bits);
 			packet_check_eom();

 			authenticated = auth_rhosts_rsa(pw, client_user,
 			    client_host_key);
 			key_free(client_host_key);

 			snprintf(info, sizeof info, " ruser %.100s", client_user);
 			break;

 		case SSH_CMSG_AUTH_RSA:
 			if (!options.rsa_authentication) {
 				verbose("RSA authentication disabled.");
 				break;
 			}
 			/* RSA authentication requested. */
 			if ((n = BN_new()) == NULL)
 				fatal("do_authloop: BN_new failed");
 			packet_get_bignum(n);
 			packet_check_eom();
 			authenticated = auth_rsa(pw, n);
 			BN_clear_free(n);
 			break;

 		case SSH_CMSG_AUTH_PASSWORD:
 			if (!options.password_authentication) {
 				verbose("Password authentication disabled.");
 				break;
 			}
 			/*
 			 * Read user password.  It is in plain text, but was
 			 * transmitted over the encrypted channel so it is
 			 * not visible to an outside observer.
 			 */
 			password = packet_get_string(&dlen);
 			packet_check_eom();

 			/* Try authentication with the password. */
 			authenticated = PRIVSEP(auth_password(authctxt, password));

 			memset(password, 0, strlen(password));
 			xfree(password);
 			break;

 		case SSH_CMSG_AUTH_TIS:
 			debug("rcvd SSH_CMSG_AUTH_TIS");
 			if (options.challenge_response_authentication == 1) {
 				char *challenge = get_challenge(authctxt);
 				if (challenge != NULL) {
 					debug("sending challenge '%s'", challenge);
 					packet_start(SSH_SMSG_AUTH_TIS_CHALLENGE);
 					packet_put_cstring(challenge);
 					xfree(challenge);
 					packet_send();
 					packet_write_wait();
 					continue;
 				}
 			}
 			break;
 		case SSH_CMSG_AUTH_TIS_RESPONSE:
 			debug("rcvd SSH_CMSG_AUTH_TIS_RESPONSE");
 			if (options.challenge_response_authentication == 1) {
 				char *response = packet_get_string(&dlen);
 				debug("got response '%s'", response);
 				packet_check_eom();
 				authenticated = verify_response(authctxt, response);
 				memset(response, 'r', dlen);
 				xfree(response);
 			}
 			break;

 		default:
 			/*
 			 * Any unknown messages will be ignored (and failure
 			 * returned) during authentication.
 			 */
 			log("Unknown message during authentication: type %d", type);
 			break;
 		}
 #ifdef BSD_AUTH
 		if (authctxt->as) {
 			auth_close(authctxt->as);
 			authctxt->as = NULL;
 		}
 #endif
 		if (!authctxt->valid && authenticated)
 			fatal("INTERNAL ERROR: authenticated invalid user %s",
 			    authctxt->user);

 #ifdef _UNICOS
 		if (type == SSH_CMSG_AUTH_PASSWORD && !authenticated)
 			cray_login_failure(authctxt->user, IA_UDBERR);
 		if (authenticated && cray_access_denied(authctxt->user)) {
 			authenticated = 0;
 			fatal("Access denied for user %s.",authctxt->user);
 		}
 #endif /* _UNICOS */

 #ifdef HAVE_CYGWIN
 		if (authenticated &&
 		    !check_nt_auth(type == SSH_CMSG_AUTH_PASSWORD, pw)) {
 			packet_disconnect("Authentication rejected for uid %d.",
 			pw == NULL ? -1 : pw->pw_uid);
 			authenticated = 0;
 		}
 #else
 		/* Special handling for root */
 		if (!use_privsep &&
 		    authenticated && authctxt->pw->pw_uid == 0 &&
 		    !auth_root_allowed(get_authname(type)))
 			authenticated = 0;
 #endif
 #ifdef USE_PAM
 		if (!use_privsep && authenticated &&
 		    !do_pam_account(pw->pw_name, client_user))
 			authenticated = 0;
 #endif

 		/* Log before sending the reply */
 		auth_log(authctxt, authenticated, get_authname(type), info);

 		if (client_user != NULL) {
 			xfree(client_user);
 			client_user = NULL;
 		}

 		if (authenticated)
 			return;

 		if (authctxt->failures++ > AUTH_FAIL_MAX) {
 			packet_disconnect(AUTH_FAIL_MSG, authctxt->user);
 		}

 		packet_start(SSH_SMSG_FAILURE);
 		packet_send();
 		packet_write_wait();
 	}
 }

 /*
  * Performs authentication of an incoming connection.  Session key has already
  * been exchanged and encryption is enabled.
  */
 Authctxt *
 do_authentication(void)
 {
 	Authctxt *authctxt;
 	u_int ulen;
 	char *user, *style = NULL;

 	/* Get the name of the user that we wish to log in as. */
 	packet_read_expect(SSH_CMSG_USER);

 	/* Get the user name. */
 	user = packet_get_string(&ulen);
 	packet_check_eom();

 	if ((style = strchr(user, ':')) != NULL)
 		*style++ = '\0';

 #ifdef KRB5
 	/* XXX - SSH.com Kerberos v5 braindeath. */
 	if ((datafellows & SSH_BUG_K5USER) &&
 	    options.kerberos_authentication) {
 		char *p;
 		if ((p = strchr(user, '@')) != NULL)
 			*p = '\0';
 	}
 #endif

 	authctxt = authctxt_new();
 	authctxt->user = user;
 	authctxt->style = style;

 	/* Verify that the user is a valid user. */
 	if ((authctxt->pw = PRIVSEP(getpwnamallow(user))) != NULL)
 		authctxt->valid = 1;
 	else
 		debug("do_authentication: illegal user %s", user);

 	setproctitle("%s%s", authctxt->pw ? user : "unknown",
 	    use_privsep ? " [net]" : "");

 #ifdef USE_PAM
 	PRIVSEP(start_pam(authctxt->pw == NULL ? "NOUSER" : user));
 #endif

 	/*
 	 * If we are not running as root, the user must have the same uid as
 	 * the server. (Unless you are running Windows)
 	 */
 #ifndef HAVE_CYGWIN
 	if (!use_privsep && getuid() != 0 && authctxt->pw &&
 	    authctxt->pw->pw_uid != getuid())
 		packet_disconnect("Cannot change user when server not running as root.");
 #endif

 	/*
 	 * Loop until the user has been authenticated or the connection is
 	 * closed, do_authloop() returns only if authentication is successful
 	 */
 	do_authloop(authctxt);

 	/* The user has been authenticated and accepted. */
 	packet_start(SSH_SMSG_SUCCESS);
 	packet_send();
 	packet_write_wait();

 	return (authctxt);
 }
	/*
	* Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
	* All rights reserved
	*
	* As far as I am concerned, the code I have written for this software
	* can be used freely for any purpose. Any derived versions of this
	* software must be clearly marked as such, and if the derived work is
	* incompatible with the protocol description in the RFC file, it must be
	* called by a name other than "ssh" or "Secure Shell".
	*/

	#include "includes.h"
	RCSID("$OpenBSD: auth1.c,v 1.44 2002/09/26 11:38:43 markus Exp $");

	#include "xmalloc.h"
	#include "rsa.h"
	#include "ssh1.h"
	#include "packet.h"
	#include "buffer.h"
	#include "mpaux.h"
	#include "log.h"
	#include "servconf.h"
	#include "compat.h"
	#include "auth.h"
	#include "channels.h"
	#include "session.h"
	#include "uidswap.h"
	#include "monitor_wrap.h"

	/* import */
	extern ServerOptions options;

	/*
	* convert ssh auth msg type into description
	*/
	static char *
	get_authname(int type)
	{
	static char buf[1024];
	switch (type) {
	case SSH_CMSG_AUTH_PASSWORD:
	return "password";
	case SSH_CMSG_AUTH_RSA:
	return "rsa";
	case SSH_CMSG_AUTH_RHOSTS_RSA:
	return "rhosts-rsa";
	case SSH_CMSG_AUTH_RHOSTS:
	return "rhosts";
	case SSH_CMSG_AUTH_TIS:
	case SSH_CMSG_AUTH_TIS_RESPONSE:
	return "challenge-response";
	#if defined(KRB4) \|\| defined(KRB5)
	case SSH_CMSG_AUTH_KERBEROS:
	return "kerberos";
	#endif
	}
	snprintf(buf, sizeof buf, "bad-auth-msg-%d", type);
	return buf;
	}

	/*
	* read packets, try to authenticate the user and
	* return only if authentication is successful
	*/
	static void
	do_authloop(Authctxt *authctxt)
	{
	int authenticated = 0;
	u_int bits;
	Key *client_host_key;
	BIGNUM *n;
	char client_user, password;
	char info[1024];
	u_int dlen;
	u_int ulen;
	int type = 0;
	struct passwd *pw = authctxt->pw;

	debug("Attempting authentication for %s%.100s.",
	authctxt->valid ? "" : "illegal user ", authctxt->user);

	/* If the user has no password, accept authentication immediately. */
	if (options.password_authentication &&
	#if defined(KRB4) \|\| defined(KRB5)
	(!options.kerberos_authentication \|\| options.kerberos_or_local_passwd) &&
	#endif
	PRIVSEP(auth_password(authctxt, ""))) {
	auth_log(authctxt, 1, "without authentication", "");
	return;
	}

	/* Indicate that authentication is needed. */
	packet_start(SSH_SMSG_FAILURE);
	packet_send();
	packet_write_wait();

	client_user = NULL;

	for (;;) {
	/* default to fail */
	authenticated = 0;

	info[0] = '\0';

	/* Get a packet from the client. */
	type = packet_read();

	/* Process the packet. */
	switch (type) {

	#if defined(KRB4) \|\| defined(KRB5)
	case SSH_CMSG_AUTH_KERBEROS:
	if (!options.kerberos_authentication) {
	verbose("Kerberos authentication disabled.");
	} else {
	char *kdata = packet_get_string(&dlen);
	packet_check_eom();

	if (kdata[0] == 4) { /* KRB_PROT_VERSION */
	#ifdef KRB4
	KTEXT_ST tkt, reply;
	tkt.length = dlen;
	if (tkt.length < MAX_KTXT_LEN)
	memcpy(tkt.dat, kdata, tkt.length);

	if (PRIVSEP(auth_krb4(authctxt, &tkt,
	&client_user, &reply))) {
	authenticated = 1;
	snprintf(info, sizeof(info),
	" tktuser %.100s",
	client_user);

	packet_start(
	SSH_SMSG_AUTH_KERBEROS_RESPONSE);
	packet_put_string((char *)
	reply.dat, reply.length);
	packet_send();
	packet_write_wait();
	}
	#endif /* KRB4 */
	} else {
	#ifdef KRB5
	krb5_data tkt, reply;
	tkt.length = dlen;
	tkt.data = kdata;

	if (PRIVSEP(auth_krb5(authctxt, &tkt,
	&client_user, &reply))) {
	authenticated = 1;
	snprintf(info, sizeof(info),
	" tktuser %.100s",
	client_user);

	/* Send response to client */
	packet_start(
	SSH_SMSG_AUTH_KERBEROS_RESPONSE);
	packet_put_string((char *)
	reply.data, reply.length);
	packet_send();
	packet_write_wait();

	if (reply.length)
	xfree(reply.data);
	}
	#endif /* KRB5 */
	}
	xfree(kdata);
	}
	break;
	#endif /* KRB4 \|\| KRB5 */

	#if defined(AFS) \|\| defined(KRB5)
	/* XXX - punt on backward compatibility here. */
	case SSH_CMSG_HAVE_KERBEROS_TGT:
	packet_send_debug("Kerberos TGT passing disabled before authentication.");
	break;
	#ifdef AFS
	case SSH_CMSG_HAVE_AFS_TOKEN:
	packet_send_debug("AFS token passing disabled before authentication.");
	break;
	#endif /* AFS */
	#endif /* AFS \|\| KRB5 */

	case SSH_CMSG_AUTH_RHOSTS:
	if (!options.rhosts_authentication) {
	verbose("Rhosts authentication disabled.");
	break;
	}
	/*
	* Get client user name. Note that we just have to
	* trust the client; this is one reason why rhosts
	* authentication is insecure. (Another is
	* IP-spoofing on a local network.)
	*/
	client_user = packet_get_string(&ulen);
	packet_check_eom();

	/* Try to authenticate using /etc/hosts.equiv and .rhosts. */
	authenticated = auth_rhosts(pw, client_user);

	snprintf(info, sizeof info, " ruser %.100s", client_user);
	break;

	case SSH_CMSG_AUTH_RHOSTS_RSA:
	if (!options.rhosts_rsa_authentication) {
	verbose("Rhosts with RSA authentication disabled.");
	break;
	}
	/*
	* Get client user name. Note that we just have to
	* trust the client; root on the client machine can
	* claim to be any user.
	*/
	client_user = packet_get_string(&ulen);

	/* Get the client host key. */
	client_host_key = key_new(KEY_RSA1);
	bits = packet_get_int();
	packet_get_bignum(client_host_key->rsa->e);
	packet_get_bignum(client_host_key->rsa->n);

	if (bits != BN_num_bits(client_host_key->rsa->n))
	verbose("Warning: keysize mismatch for client_host_key: "
	"actual %d, announced %d",
	BN_num_bits(client_host_key->rsa->n), bits);
	packet_check_eom();

	authenticated = auth_rhosts_rsa(pw, client_user,
	client_host_key);
	key_free(client_host_key);

	snprintf(info, sizeof info, " ruser %.100s", client_user);
	break;

	case SSH_CMSG_AUTH_RSA:
	if (!options.rsa_authentication) {
	verbose("RSA authentication disabled.");
	break;
	}
	/* RSA authentication requested. */
	if ((n = BN_new()) == NULL)
	fatal("do_authloop: BN_new failed");
	packet_get_bignum(n);
	packet_check_eom();
	authenticated = auth_rsa(pw, n);
	BN_clear_free(n);
	break;

	case SSH_CMSG_AUTH_PASSWORD:
	if (!options.password_authentication) {
	verbose("Password authentication disabled.");
	break;
	}
	/*
	* Read user password. It is in plain text, but was
	* transmitted over the encrypted channel so it is
	* not visible to an outside observer.
	*/
	password = packet_get_string(&dlen);
	packet_check_eom();

	/* Try authentication with the password. */
	authenticated = PRIVSEP(auth_password(authctxt, password));

	memset(password, 0, strlen(password));
	xfree(password);
	break;

	case SSH_CMSG_AUTH_TIS:
	debug("rcvd SSH_CMSG_AUTH_TIS");
	if (options.challenge_response_authentication == 1) {
	char *challenge = get_challenge(authctxt);
	if (challenge != NULL) {
	debug("sending challenge '%s'", challenge);
	packet_start(SSH_SMSG_AUTH_TIS_CHALLENGE);
	packet_put_cstring(challenge);
	xfree(challenge);
	packet_send();
	packet_write_wait();
	continue;
	}
	}
	break;
	case SSH_CMSG_AUTH_TIS_RESPONSE:
	debug("rcvd SSH_CMSG_AUTH_TIS_RESPONSE");
	if (options.challenge_response_authentication == 1) {
	char *response = packet_get_string(&dlen);
	debug("got response '%s'", response);
	packet_check_eom();
	authenticated = verify_response(authctxt, response);
	memset(response, 'r', dlen);
	xfree(response);
	}
	break;

	default:
	/*
	* Any unknown messages will be ignored (and failure
	* returned) during authentication.
	*/
	log("Unknown message during authentication: type %d", type);
	break;
	}
	#ifdef BSD_AUTH
	if (authctxt->as) {
	auth_close(authctxt->as);
	authctxt->as = NULL;
	}
	#endif
	if (!authctxt->valid && authenticated)
	fatal("INTERNAL ERROR: authenticated invalid user %s",
	authctxt->user);

	#ifdef _UNICOS
	if (type == SSH_CMSG_AUTH_PASSWORD && !authenticated)
	cray_login_failure(authctxt->user, IA_UDBERR);
	if (authenticated && cray_access_denied(authctxt->user)) {
	authenticated = 0;
	fatal("Access denied for user %s.",authctxt->user);
	}
	#endif /* _UNICOS */

	#ifdef HAVE_CYGWIN
	if (authenticated &&
	!check_nt_auth(type == SSH_CMSG_AUTH_PASSWORD, pw)) {
	packet_disconnect("Authentication rejected for uid %d.",
	pw == NULL ? -1 : pw->pw_uid);
	authenticated = 0;
	}
	#else
	/* Special handling for root */
	if (!use_privsep &&
	authenticated && authctxt->pw->pw_uid == 0 &&
	!auth_root_allowed(get_authname(type)))
	authenticated = 0;
	#endif
	#ifdef USE_PAM
	if (!use_privsep && authenticated &&
	!do_pam_account(pw->pw_name, client_user))
	authenticated = 0;
	#endif

	/* Log before sending the reply */
	auth_log(authctxt, authenticated, get_authname(type), info);

	if (client_user != NULL) {
	xfree(client_user);
	client_user = NULL;
	}

	if (authenticated)
	return;

	if (authctxt->failures++ > AUTH_FAIL_MAX) {
	packet_disconnect(AUTH_FAIL_MSG, authctxt->user);
	}

	packet_start(SSH_SMSG_FAILURE);
	packet_send();
	packet_write_wait();
	}
	}

	/*
	* Performs authentication of an incoming connection. Session key has already
	* been exchanged and encryption is enabled.
	*/
	Authctxt *
	do_authentication(void)
	{
	Authctxt *authctxt;
	u_int ulen;
	char user, style = NULL;

	/* Get the name of the user that we wish to log in as. */
	packet_read_expect(SSH_CMSG_USER);

	/* Get the user name. */
	user = packet_get_string(&ulen);
	packet_check_eom();

	if ((style = strchr(user, ':')) != NULL)
	*style++ = '\0';

	#ifdef KRB5
	/* XXX - SSH.com Kerberos v5 braindeath. */
	if ((datafellows & SSH_BUG_K5USER) &&
	options.kerberos_authentication) {
	char *p;
	if ((p = strchr(user, '@')) != NULL)
	*p = '\0';
	}
	#endif

	authctxt = authctxt_new();
	authctxt->user = user;
	authctxt->style = style;

	/* Verify that the user is a valid user. */
	if ((authctxt->pw = PRIVSEP(getpwnamallow(user))) != NULL)
	authctxt->valid = 1;
	else
	debug("do_authentication: illegal user %s", user);

	setproctitle("%s%s", authctxt->pw ? user : "unknown",
	use_privsep ? " [net]" : "");

	#ifdef USE_PAM
	PRIVSEP(start_pam(authctxt->pw == NULL ? "NOUSER" : user));
	#endif

	/*
	* If we are not running as root, the user must have the same uid as
	* the server. (Unless you are running Windows)
	*/
	#ifndef HAVE_CYGWIN
	if (!use_privsep && getuid() != 0 && authctxt->pw &&
	authctxt->pw->pw_uid != getuid())
	packet_disconnect("Cannot change user when server not running as root.");
	#endif

	/*
	* Loop until the user has been authenticated or the connection is
	* closed, do_authloop() returns only if authentication is successful
	*/
	do_authloop(authctxt);

	/* The user has been authenticated and accepted. */
	packet_start(SSH_SMSG_SUCCESS);
	packet_send();
	packet_write_wait();

	return (authctxt);
	}