command-ssh.c
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	/* command-ssh.c - gpg-agent's ssh-agent emulation layer
	* Copyright (C) 2004, 2005, 2006, 2009 Free Software Foundation, Inc.
	*
	* This file is part of GnuPG.
	*
	* GnuPG is free software; you can redistribute it and/or modify
	* it under the terms of the GNU General Public License as published by
	* the Free Software Foundation; either version 3 of the License, or
	* (at your option) any later version.
	*
	* GnuPG is distributed in the hope that it will be useful,
	* but WITHOUT ANY WARRANTY; without even the implied warranty of
	* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
	* GNU General Public License for more details.
	*
	* You should have received a copy of the GNU General Public License
	* along with this program; if not, see <http://www.gnu.org/licenses/>.
	*/

	/* Only v2 of the ssh-agent protocol is implemented. */

	#include <config.h>

	#include <stdio.h>
	#include <stdlib.h>
	#include <string.h>
	#include <errno.h>
	#include <sys/types.h>
	#include <sys/stat.h>
	#include <dirent.h>
	#include <assert.h>

	#include "agent.h"

	#include "i18n.h"
	#include "../common/ssh-utils.h"




	/* Request types. */
	#define SSH_REQUEST_REQUEST_IDENTITIES 11
	#define SSH_REQUEST_SIGN_REQUEST 13
	#define SSH_REQUEST_ADD_IDENTITY 17
	#define SSH_REQUEST_REMOVE_IDENTITY 18
	#define SSH_REQUEST_REMOVE_ALL_IDENTITIES 19
	#define SSH_REQUEST_LOCK 22
	#define SSH_REQUEST_UNLOCK 23
	#define SSH_REQUEST_ADD_ID_CONSTRAINED 25

	/* Options. */
	#define SSH_OPT_CONSTRAIN_LIFETIME 1
	#define SSH_OPT_CONSTRAIN_CONFIRM 2

	/* Response types. */
	#define SSH_RESPONSE_SUCCESS 6
	#define SSH_RESPONSE_FAILURE 5
	#define SSH_RESPONSE_IDENTITIES_ANSWER 12
	#define SSH_RESPONSE_SIGN_RESPONSE 14

	/* Other constants. */
	#define SSH_DSA_SIGNATURE_PADDING 20
	#define SSH_DSA_SIGNATURE_ELEMS 2
	#define SPEC_FLAG_USE_PKCS1V2 (1 << 0)


	/* The blurb we put into the header of a newly created control file. */
	static const char sshcontrolblurb[] =
	"# List of allowed ssh keys. Only keys present in this file are used\n"
	"# in the SSH protocol. The ssh-add tool may add new entries to this\n"
	"# file to enable them; you may also add them manually. Comment\n"
	"# lines, like this one, as well as empty lines are ignored. Lines do\n"
	"# have a certain length limit but this is not serious limitation as\n"
	"# the format of the entries is fixed and checked by gpg-agent. A\n"
	"# non-comment line starts with optional white spaces, followed by the\n"
	"# keygrip of the key given as 40 hex digits, optionally followed by a\n"
	"# the caching TTL in seconds and another optional field for arbitrary\n"
	"# flags. Prepend the keygrip with an '!' mark to disable it.\n"
	"\n";



	/* Macros. */

	/* Return a new uint32 with b0 being the most significant byte and b3
	being the least significant byte. */
	#define uint32_construct(b0, b1, b2, b3) \
	((b0 << 24) \| (b1 << 16) \| (b2 << 8) \| b3)




	/*
	* Basic types.
	*/

	/* Type for a request handler. */
	typedef gpg_error_t (*ssh_request_handler_t) (ctrl_t ctrl,
	estream_t request,
	estream_t response);

	/* Type, which is used for associating request handlers with the
	appropriate request IDs. */
	typedef struct ssh_request_spec
	{
	unsigned char type;
	ssh_request_handler_t handler;
	const char *identifier;
	unsigned int secret_input;
	} ssh_request_spec_t;

	/* Type for "key modifier functions", which are necessary since
	OpenSSH and GnuPG treat key material slightly different. A key
	modifier is called right after a new key identity has been received
	in order to "sanitize" the material. */
	typedef gpg_error_t (ssh_key_modifier_t) (const char elems,
	gcry_mpi_t *mpis);

	/* The encoding of a generated signature is dependent on the
	algorithm; therefore algorithm specific signature encoding
	functions are necessary. */
	typedef gpg_error_t (*ssh_signature_encoder_t) (estream_t signature_blob,
	gcry_mpi_t *mpis);

	/* Type, which is used for boundling all the algorithm specific
	information together in a single object. */
	typedef struct ssh_key_type_spec
	{
	/* Algorithm identifier as used by OpenSSH. */
	const char *ssh_identifier;

	/* Algorithm identifier as used by GnuPG. */
	const char *identifier;

	/* List of MPI names for secret keys; order matches the one of the
	agent protocol. */
	const char *elems_key_secret;

	/* List of MPI names for public keys; order matches the one of the
	agent protocol. */
	const char *elems_key_public;

	/* List of MPI names for signature data. */
	const char *elems_signature;

	/* List of MPI names for secret keys; order matches the one, which
	is required by gpg-agent's key access layer. */
	const char *elems_sexp_order;

	/* Key modifier function. Key modifier functions are necessary in
	order to fix any inconsistencies between the representation of
	keys on the SSH and on the GnuPG side. */
	ssh_key_modifier_t key_modifier;

	/* Signature encoder function. Signature encoder functions are
	necessary since the encoding of signatures depends on the used
	algorithm. */
	ssh_signature_encoder_t signature_encoder;

	/* Misc flags. */
	unsigned int flags;
	} ssh_key_type_spec_t;


	/* Prototypes. */
	static gpg_error_t ssh_handler_request_identities (ctrl_t ctrl,
	estream_t request,
	estream_t response);
	static gpg_error_t ssh_handler_sign_request (ctrl_t ctrl,
	estream_t request,
	estream_t response);
	static gpg_error_t ssh_handler_add_identity (ctrl_t ctrl,
	estream_t request,
	estream_t response);
	static gpg_error_t ssh_handler_remove_identity (ctrl_t ctrl,
	estream_t request,
	estream_t response);
	static gpg_error_t ssh_handler_remove_all_identities (ctrl_t ctrl,
	estream_t request,
	estream_t response);
	static gpg_error_t ssh_handler_lock (ctrl_t ctrl,
	estream_t request,
	estream_t response);
	static gpg_error_t ssh_handler_unlock (ctrl_t ctrl,
	estream_t request,
	estream_t response);

	static gpg_error_t ssh_key_modifier_rsa (const char elems, gcry_mpi_t mpis);
	static gpg_error_t ssh_signature_encoder_rsa (estream_t signature_blob,
	gcry_mpi_t *mpis);
	static gpg_error_t ssh_signature_encoder_dsa (estream_t signature_blob,
	gcry_mpi_t *mpis);



	/* Global variables. */


	/* Associating request types with the corresponding request
	handlers. */

	static ssh_request_spec_t request_specs[] =
	{
	#define REQUEST_SPEC_DEFINE(id, name, secret_input) \
	{ SSH_REQUEST_##id, ssh_handler_##name, #name, secret_input }

	REQUEST_SPEC_DEFINE (REQUEST_IDENTITIES, request_identities, 1),
	REQUEST_SPEC_DEFINE (SIGN_REQUEST, sign_request, 0),
	REQUEST_SPEC_DEFINE (ADD_IDENTITY, add_identity, 1),
	REQUEST_SPEC_DEFINE (ADD_ID_CONSTRAINED, add_identity, 1),
	REQUEST_SPEC_DEFINE (REMOVE_IDENTITY, remove_identity, 0),
	REQUEST_SPEC_DEFINE (REMOVE_ALL_IDENTITIES, remove_all_identities, 0),
	REQUEST_SPEC_DEFINE (LOCK, lock, 0),
	REQUEST_SPEC_DEFINE (UNLOCK, unlock, 0)
	#undef REQUEST_SPEC_DEFINE
	};


	/* Table holding key type specifications. */
	static ssh_key_type_spec_t ssh_key_types[] =
	{
	{
	"ssh-rsa", "rsa", "nedupq", "en", "s", "nedpqu",
	ssh_key_modifier_rsa, ssh_signature_encoder_rsa,
	SPEC_FLAG_USE_PKCS1V2
	},
	{
	"ssh-dss", "dsa", "pqgyx", "pqgy", "rs", "pqgyx",
	NULL, ssh_signature_encoder_dsa,
	0
	},
	};





	/*
	General utility functions.
	*/

	/* A secure realloc, i.e. it makes sure to allocate secure memory if A
	is NULL. This is required because the standard gcry_realloc does
	not know whether to allocate secure or normal if NULL is passed as
	existing buffer. */
	static void *
	realloc_secure (void *a, size_t n)
	{
	void *p;

	if (a)
	p = gcry_realloc (a, n);
	else
	p = gcry_malloc_secure (n);

	return p;
	}


	/* Create and return a new C-string from DATA/DATA_N (i.e.: add
	NUL-termination); return NULL on OOM. */
	static char *
	make_cstring (const char *data, size_t data_n)
	{
	char *s;

	s = xtrymalloc (data_n + 1);
	if (s)
	{
	memcpy (s, data, data_n);
	s[data_n] = 0;
	}

	return s;
	}




	/*
	Primitive I/O functions.
	*/


	/* Read a byte from STREAM, store it in B. */
	static gpg_error_t
	stream_read_byte (estream_t stream, unsigned char *b)
	{
	gpg_error_t err;
	int ret;

	ret = es_fgetc (stream);
	if (ret == EOF)
	{
	if (es_ferror (stream))
	err = gpg_error_from_syserror ();
	else
	err = gpg_error (GPG_ERR_EOF);
	*b = 0;
	}
	else
	{
	*b = ret & 0xFF;
	err = 0;
	}

	return err;
	}

	/* Write the byte contained in B to STREAM. */
	static gpg_error_t
	stream_write_byte (estream_t stream, unsigned char b)
	{
	gpg_error_t err;
	int ret;

	ret = es_fputc (b, stream);
	if (ret == EOF)
	err = gpg_error_from_syserror ();
	else
	err = 0;

	return err;
	}

	/* Read a uint32 from STREAM, store it in UINT32. */
	static gpg_error_t
	stream_read_uint32 (estream_t stream, u32 *uint32)
	{
	unsigned char buffer[4];
	size_t bytes_read;
	gpg_error_t err;
	int ret;

	ret = es_read (stream, buffer, sizeof (buffer), &bytes_read);
	if (ret)
	err = gpg_error_from_syserror ();
	else
	{
	if (bytes_read != sizeof (buffer))
	err = gpg_error (GPG_ERR_EOF);
	else
	{
	u32 n;

	n = uint32_construct (buffer[0], buffer[1], buffer[2], buffer[3]);
	*uint32 = n;
	err = 0;
	}
	}

	return err;
	}

	/* Write the uint32 contained in UINT32 to STREAM. */
	static gpg_error_t
	stream_write_uint32 (estream_t stream, u32 uint32)
	{
	unsigned char buffer[4];
	gpg_error_t err;
	int ret;

	buffer[0] = uint32 >> 24;
	buffer[1] = uint32 >> 16;
	buffer[2] = uint32 >> 8;
	buffer[3] = uint32 >> 0;

	ret = es_write (stream, buffer, sizeof (buffer), NULL);
	if (ret)
	err = gpg_error_from_syserror ();
	else
	err = 0;

	return err;
	}

	/* Read SIZE bytes from STREAM into BUFFER. */
	static gpg_error_t
	stream_read_data (estream_t stream, unsigned char *buffer, size_t size)
	{
	gpg_error_t err;
	size_t bytes_read;
	int ret;

	ret = es_read (stream, buffer, size, &bytes_read);
	if (ret)
	err = gpg_error_from_syserror ();
	else
	{
	if (bytes_read != size)
	err = gpg_error (GPG_ERR_EOF);
	else
	err = 0;
	}

	return err;
	}

	/* Write SIZE bytes from BUFFER to STREAM. */
	static gpg_error_t
	stream_write_data (estream_t stream, const unsigned char *buffer, size_t size)
	{
	gpg_error_t err;
	int ret;

	ret = es_write (stream, buffer, size, NULL);
	if (ret)
	err = gpg_error_from_syserror ();
	else
	err = 0;

	return err;
	}

	/* Read a binary string from STREAM into STRING, store size of string
	in STRING_SIZE; depending on SECURE use secure memory for
	string. */
	static gpg_error_t
	stream_read_string (estream_t stream, unsigned int secure,
	unsigned char *string, u32 string_size)
	{
	gpg_error_t err;
	unsigned char *buffer = NULL;
	u32 length = 0;

	/* Read string length. */
	err = stream_read_uint32 (stream, &length);
	if (err)
	goto out;

	/* Allocate space. */
	if (secure)
	buffer = xtrymalloc_secure (length + 1);
	else
	buffer = xtrymalloc (length + 1);
	if (! buffer)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	/* Read data. */
	err = stream_read_data (stream, buffer, length);
	if (err)
	goto out;

	/* Finalize string object. */
	buffer[length] = 0;
	*string = buffer;
	if (string_size)
	*string_size = length;

	out:

	if (err)
	xfree (buffer);

	return err;
	}

	/* Read a C-string from STREAM, store copy in STRING. */
	static gpg_error_t
	stream_read_cstring (estream_t stream, char **string)
	{
	unsigned char *buffer;
	gpg_error_t err;

	err = stream_read_string (stream, 0, &buffer, NULL);
	if (err)
	goto out;

	string = (char ) buffer;

	out:

	return err;
	}


	/* Write a binary string from STRING of size STRING_N to STREAM. */
	static gpg_error_t
	stream_write_string (estream_t stream,
	const unsigned char *string, u32 string_n)
	{
	gpg_error_t err;

	err = stream_write_uint32 (stream, string_n);
	if (err)
	goto out;

	err = stream_write_data (stream, string, string_n);

	out:

	return err;
	}

	/* Write a C-string from STRING to STREAM. */
	static gpg_error_t
	stream_write_cstring (estream_t stream, const char *string)
	{
	gpg_error_t err;

	err = stream_write_string (stream,
	(const unsigned char *) string, strlen (string));

	return err;
	}

	/* Read an MPI from STREAM, store it in MPINT. Depending on SECURE
	use secure memory. */
	static gpg_error_t
	stream_read_mpi (estream_t stream, unsigned int secure, gcry_mpi_t *mpint)
	{
	unsigned char *mpi_data;
	u32 mpi_data_size;
	gpg_error_t err;
	gcry_mpi_t mpi;

	mpi_data = NULL;

	err = stream_read_string (stream, secure, &mpi_data, &mpi_data_size);
	if (err)
	goto out;

	/* To avoid excessive use of secure memory we check that an MPI is
	not too large. */
	if (mpi_data_size > 520)
	{
	log_error (_("ssh keys greater than %d bits are not supported\n"), 4096);
	err = GPG_ERR_TOO_LARGE;
	goto out;
	}

	err = gcry_mpi_scan (&mpi, GCRYMPI_FMT_STD, mpi_data, mpi_data_size, NULL);
	if (err)
	goto out;

	*mpint = mpi;

	out:

	xfree (mpi_data);

	return err;
	}

	/* Write the MPI contained in MPINT to STREAM. */
	static gpg_error_t
	stream_write_mpi (estream_t stream, gcry_mpi_t mpint)
	{
	unsigned char *mpi_buffer;
	size_t mpi_buffer_n;
	gpg_error_t err;

	mpi_buffer = NULL;

	err = gcry_mpi_aprint (GCRYMPI_FMT_STD, &mpi_buffer, &mpi_buffer_n, mpint);
	if (err)
	goto out;

	err = stream_write_string (stream, mpi_buffer, mpi_buffer_n);

	out:

	xfree (mpi_buffer);

	return err;
	}

	/* Copy data from SRC to DST until EOF is reached. */
	static gpg_error_t
	stream_copy (estream_t dst, estream_t src)
	{
	char buffer[BUFSIZ];
	size_t bytes_read;
	gpg_error_t err;
	int ret;

	err = 0;
	while (1)
	{
	ret = es_read (src, buffer, sizeof (buffer), &bytes_read);
	if (ret \|\| (! bytes_read))
	{
	if (ret)
	err = gpg_error_from_syserror ();
	break;
	}
	ret = es_write (dst, buffer, bytes_read, NULL);
	if (ret)
	{
	err = gpg_error_from_syserror ();
	break;
	}
	}

	return err;
	}


	/* Read the content of the file specified by FILENAME into a newly
	create buffer, which is to be stored in BUFFER; store length of
	buffer in BUFFER_N. */
	static gpg_error_t
	file_to_buffer (const char filename, unsigned char buffer, size_t buffer_n)
	{
	unsigned char *buffer_new;
	struct stat statbuf;
	estream_t stream;
	gpg_error_t err;
	int ret;

	*buffer = NULL;
	*buffer_n = 0;

	buffer_new = NULL;
	err = 0;

	stream = es_fopen (filename, "r");
	if (! stream)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	ret = fstat (es_fileno (stream), &statbuf);
	if (ret)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	buffer_new = xtrymalloc (statbuf.st_size);
	if (! buffer_new)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	err = stream_read_data (stream, buffer_new, statbuf.st_size);
	if (err)
	goto out;

	*buffer = buffer_new;
	*buffer_n = statbuf.st_size;

	out:

	if (stream)
	es_fclose (stream);

	if (err)
	xfree (buffer_new);

	return err;
	}




	/* Open the ssh control file and create it if not available. With
	APPEND passed as true the file will be opened in append mode,
	otherwise in read only mode. On success a file pointer is stored
	at the address of R_FP. */
	static gpg_error_t
	open_control_file (FILE **r_fp, int append)
	{
	gpg_error_t err;
	char *fname;
	FILE *fp;

	/* Note: As soon as we start to use non blocking functions here
	(i.e. where Pth might switch threads) we need to employ a
	mutex. */
	*r_fp = NULL;
	fname = make_filename (opt.homedir, "sshcontrol", NULL);
	/* FIXME: With "a+" we are not able to check whether this will will
	be created and thus the blurb needs to be written first. */
	fp = fopen (fname, append? "a+":"r");
	if (!fp && errno == ENOENT)
	{
	estream_t stream = es_fopen (fname, "wx,mode=-rw-r");
	if (!stream)
	{
	err = gpg_error_from_syserror ();
	log_error (_("can't create `%s': %s\n"), fname, gpg_strerror (err));
	xfree (fname);
	return err;
	}
	es_fputs (sshcontrolblurb, stream);
	es_fclose (stream);
	fp = fopen (fname, append? "a+":"r");
	}

	if (!fp)
	{
	err = gpg_error (gpg_err_code_from_errno (errno));
	log_error (_("can't open `%s': %s\n"), fname, gpg_strerror (err));
	xfree (fname);
	return err;
	}

	*r_fp = fp;

	return 0;
	}


	/* Search the file at stream FP from the beginning until a matching
	HEXGRIP is found; return success in this case and store true at
	DISABLED if the found key has been disabled. If R_TTL is not NULL
	a specified TTL for that key is stored there. If R_CONFIRM is not
	NULL it is set to 1 if the key has the confirm flag set. */
	static gpg_error_t
	search_control_file (FILE fp, const char hexgrip,
	int r_disabled, int r_ttl, int *r_confirm)
	{
	int c, i, n;
	char p, pend, line[256];
	long ttl;
	int lnr = 0;
	const char fname[] = "sshcontrol";

	assert (strlen (hexgrip) == 40 );

	if (r_confirm)
	*r_confirm = 0;

	fseek (fp, 0, SEEK_SET);
	clearerr (fp);
	*r_disabled = 0;
	next_line:
	do
	{
	if (!fgets (line, DIM(line)-1, fp) )
	{
	if (feof (fp))
	return gpg_error (GPG_ERR_EOF);
	return gpg_error (gpg_err_code_from_errno (errno));
	}
	lnr++;

	if (!*line \|\| line[strlen(line)-1] != '\n')
	{
	/* Eat until end of line */
	while ( (c=getc (fp)) != EOF && c != '\n')
	;
	return gpg_error (*line? GPG_ERR_LINE_TOO_LONG
	: GPG_ERR_INCOMPLETE_LINE);
	}

	/* Allow for empty lines and spaces */
	for (p=line; spacep (p); p++)
	;
	}
	while (!p \|\| p == '\n' \|\| *p == '#');

	*r_disabled = 0;
	if (*p == '!')
	{
	*r_disabled = 1;
	for (p++; spacep (p); p++)
	;
	}

	for (i=0; hexdigitp (p) && i < 40; p++, i++)
	if (hexgrip[i] != (p >= 'a'? (p & 0xdf): *p))
	goto next_line;
	if (i != 40 \|\| !(spacep (p) \|\| *p == '\n'))
	{
	log_error ("invalid formatted line in `%s', line %d\n", fname, lnr);
	return gpg_error (GPG_ERR_BAD_DATA);
	}

	ttl = strtol (p, &pend, 10);
	p = pend;
	if (!(spacep (p) \|\| *p == '\n') \|\| ttl < -1)
	{
	log_error ("invalid TTL value in `%s', line %d; assuming 0\n",
	fname, lnr);
	ttl = 0;
	}
	if (r_ttl)
	*r_ttl = ttl;

	/* Now check for key-value pairs of the form NAME[=VALUE]. */
	while (*p)
	{
	for (; spacep (p) && *p != '\n'; p++)
	;
	if (!p \|\| p == '\n')
	break;
	n = strcspn (p, "= \t\n");
	if (p[n] == '=')
	{
	log_error ("assigning a value to a flag is not yet supported; "
	"in `%s', line %d; flag ignored\n", fname, lnr);
	p++;
	}
	else if (n == 7 && !memcmp (p, "confirm", 7))
	{
	if (r_confirm)
	*r_confirm = 1;
	}
	else
	log_error ("invalid flag `%.*s' in `%s', line %d; ignored\n",
	n, p, fname, lnr);
	p += n;
	}

	return 0; /* Okay: found it. */
	}



	/* Add an entry to the control file to mark the key with the keygrip
	HEXGRIP as usable for SSH; i.e. it will be returned when ssh asks
	for it. FMTFPR is the fingerprint string. This function is in
	general used to add a key received through the ssh-add function.
	We can assume that the user wants to allow ssh using this key. */
	static gpg_error_t
	add_control_entry (ctrl_t ctrl, const char hexgrip, const char fmtfpr,
	int ttl, int confirm)
	{
	gpg_error_t err;
	FILE *fp;
	int disabled;

	(void)ctrl;

	err = open_control_file (&fp, 1);
	if (err)
	return err;

	err = search_control_file (fp, hexgrip, &disabled, NULL, NULL);
	if (err && gpg_err_code(err) == GPG_ERR_EOF)
	{
	struct tm *tp;
	time_t atime = time (NULL);

	/* Not yet in the file - add it. Because the file has been
	opened in append mode, we simply need to write to it. */
	tp = localtime (&atime);
	fprintf (fp, ("# Key added on: %04d-%02d-%02d %02d:%02d:%02d\n"
	"# Fingerprint: %s\n"
	"%s %d%s\n"),
	1900+tp->tm_year, tp->tm_mon+1, tp->tm_mday,
	tp->tm_hour, tp->tm_min, tp->tm_sec,
	fmtfpr, hexgrip, ttl, confirm? " confirm":"");

	}
	fclose (fp);
	return 0;
	}


	/* Scan the sshcontrol file and return the TTL. */
	static int
	ttl_from_sshcontrol (const char *hexgrip)
	{
	FILE *fp;
	int disabled, ttl;

	if (!hexgrip \|\| strlen (hexgrip) != 40)
	return 0; /* Wrong input: Use global default. */

	if (open_control_file (&fp, 0))
	return 0; /* Error: Use the global default TTL. */

	if (search_control_file (fp, hexgrip, &disabled, &ttl, NULL)
	\|\| disabled)
	ttl = 0; /* Use the global default if not found or disabled. */

	fclose (fp);

	return ttl;
	}


	/* Scan the sshcontrol file and return the confirm flag. */
	static int
	confirm_flag_from_sshcontrol (const char *hexgrip)
	{
	FILE *fp;
	int disabled, confirm;

	if (!hexgrip \|\| strlen (hexgrip) != 40)
	return 1; /* Wrong input: Better ask for confirmation. */

	if (open_control_file (&fp, 0))
	return 1; /* Error: Better ask for confirmation. */

	if (search_control_file (fp, hexgrip, &disabled, NULL, &confirm)
	\|\| disabled)
	confirm = 0; /* If not found or disabled, there is no reason to
	ask for confirmation. */

	fclose (fp);

	return confirm;
	}





	/*

	MPI lists.

	*/

	/* Free the list of MPIs MPI_LIST. */
	static void
	mpint_list_free (gcry_mpi_t *mpi_list)
	{
	if (mpi_list)
	{
	unsigned int i;

	for (i = 0; mpi_list[i]; i++)
	gcry_mpi_release (mpi_list[i]);
	xfree (mpi_list);
	}
	}

	/* Receive key material MPIs from STREAM according to KEY_SPEC;
	depending on SECRET expect a public key or secret key. The newly
	allocated list of MPIs is stored in MPI_LIST. Returns usual error
	code. */
	static gpg_error_t
	ssh_receive_mpint_list (estream_t stream, int secret,
	ssh_key_type_spec_t key_spec, gcry_mpi_t **mpi_list)
	{
	const char *elems_public;
	unsigned int elems_n;
	const char *elems;
	int elem_is_secret;
	gcry_mpi_t *mpis;
	gpg_error_t err;
	unsigned int i;

	mpis = NULL;
	err = 0;

	if (secret)
	elems = key_spec.elems_key_secret;
	else
	elems = key_spec.elems_key_public;
	elems_n = strlen (elems);

	elems_public = key_spec.elems_key_public;

	mpis = xtrycalloc (elems_n + 1, sizeof *mpis );
	if (!mpis)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	elem_is_secret = 0;
	for (i = 0; i < elems_n; i++)
	{
	if (secret)
	elem_is_secret = ! strchr (elems_public, elems[i]);
	err = stream_read_mpi (stream, elem_is_secret, &mpis[i]);
	if (err)
	break;
	}
	if (err)
	goto out;

	*mpi_list = mpis;

	out:

	if (err)
	mpint_list_free (mpis);

	return err;
	}



	/* Key modifier function for RSA. */
	static gpg_error_t
	ssh_key_modifier_rsa (const char elems, gcry_mpi_t mpis)
	{
	gcry_mpi_t p;
	gcry_mpi_t q;
	gcry_mpi_t u;

	if (strcmp (elems, "nedupq"))
	/* Modifying only necessary for secret keys. */
	goto out;

	u = mpis[3];
	p = mpis[4];
	q = mpis[5];

	if (gcry_mpi_cmp (p, q) > 0)
	{
	/* P shall be smaller then Q! Swap primes. iqmp becomes u. */
	gcry_mpi_t tmp;

	tmp = mpis[4];
	mpis[4] = mpis[5];
	mpis[5] = tmp;
	}
	else
	/* U needs to be recomputed. */
	gcry_mpi_invm (u, p, q);

	out:

	return 0;
	}

	/* Signature encoder function for RSA. */
	static gpg_error_t
	ssh_signature_encoder_rsa (estream_t signature_blob, gcry_mpi_t *mpis)
	{
	unsigned char *data;
	size_t data_n;
	gpg_error_t err;
	gcry_mpi_t s;

	s = mpis[0];

	err = gcry_mpi_aprint (GCRYMPI_FMT_USG, &data, &data_n, s);
	if (err)
	goto out;

	err = stream_write_string (signature_blob, data, data_n);
	xfree (data);

	out:

	return err;
	}


	/* Signature encoder function for DSA. */
	static gpg_error_t
	ssh_signature_encoder_dsa (estream_t signature_blob, gcry_mpi_t *mpis)
	{
	unsigned char buffer[SSH_DSA_SIGNATURE_PADDING * SSH_DSA_SIGNATURE_ELEMS];
	unsigned char *data;
	size_t data_n;
	gpg_error_t err;
	int i;

	data = NULL;

	for (i = 0; i < 2; i++)
	{
	err = gcry_mpi_aprint (GCRYMPI_FMT_USG, &data, &data_n, mpis[i]);
	if (err)
	break;

	if (data_n > SSH_DSA_SIGNATURE_PADDING)
	{
	err = gpg_error (GPG_ERR_INTERNAL); /* FIXME? */
	break;
	}

	memset (buffer + (i * SSH_DSA_SIGNATURE_PADDING), 0,
	SSH_DSA_SIGNATURE_PADDING - data_n);
	memcpy (buffer + (i * SSH_DSA_SIGNATURE_PADDING)
	+ (SSH_DSA_SIGNATURE_PADDING - data_n), data, data_n);

	xfree (data);
	data = NULL;
	}
	if (err)
	goto out;

	err = stream_write_string (signature_blob, buffer, sizeof (buffer));

	out:

	xfree (data);

	return err;
	}

	/*
	S-Expressions.
	*/


	/* This function constructs a new S-Expression for the key identified
	by the KEY_SPEC, SECRET, MPIS and COMMENT, which is to be stored in
	SEXP. Returns usual error code. /
	static gpg_error_t
	sexp_key_construct (gcry_sexp_t *sexp,
	ssh_key_type_spec_t key_spec, int secret,
	gcry_mpi_t mpis, const char comment)
	{
	const char *key_identifier[] = { "public-key", "private-key" };
	gcry_sexp_t sexp_new;
	char *sexp_template;
	size_t sexp_template_n;
	gpg_error_t err;
	const char *elems;
	size_t elems_n;
	unsigned int i;
	unsigned int j;
	void **arg_list;

	err = 0;
	sexp_new = NULL;
	arg_list = NULL;
	if (secret)
	elems = key_spec.elems_sexp_order;
	else
	elems = key_spec.elems_key_public;
	elems_n = strlen (elems);

	/*
	Calculate size for sexp_template_n:

	"(%s(%s<mpis>)(comment%s))" -> 20 + sizeof (<mpis>).

	mpi: (X%m) -> 5.

	*/
	sexp_template_n = 20 + (elems_n * 5);
	sexp_template = xtrymalloc (sexp_template_n);
	if (! sexp_template)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	/* Key identifier, algorithm identifier, mpis, comment. */
	arg_list = xtrymalloc (sizeof (arg_list) (2 + elems_n + 1));
	if (! arg_list)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	i = 0;
	arg_list[i++] = &key_identifier[secret];
	arg_list[i++] = &key_spec.identifier;

	*sexp_template = 0;
	sexp_template_n = 0;
	sexp_template_n = sprintf (sexp_template + sexp_template_n, "(%%s(%%s");
	for (i = 0; i < elems_n; i++)
	{
	sexp_template_n += sprintf (sexp_template + sexp_template_n, "(%c%%m)",
	elems[i]);
	if (secret)
	{
	for (j = 0; j < elems_n; j++)
	if (key_spec.elems_key_secret[j] == elems[i])
	break;
	}
	else
	j = i;
	arg_list[i + 2] = &mpis[j];
	}
	sexp_template_n += sprintf (sexp_template + sexp_template_n,
	")(comment%%s))");

	arg_list[i + 2] = &comment;

	err = gcry_sexp_build_array (&sexp_new, NULL, sexp_template, arg_list);
	if (err)
	goto out;

	*sexp = sexp_new;

	out:

	xfree (arg_list);
	xfree (sexp_template);

	return err;
	}

	/* This functions breaks up the key contained in the S-Expression SEXP
	according to KEY_SPEC. The MPIs are bundled in a newly create
	list, which is to be stored in MPIS; a newly allocated string
	holding the comment will be stored in COMMENT; SECRET will be
	filled with a boolean flag specifying what kind of key it is.
	Returns usual error code. */
	static gpg_error_t
	sexp_key_extract (gcry_sexp_t sexp,
	ssh_key_type_spec_t key_spec, int *secret,
	gcry_mpi_t mpis, char comment)
	{
	gpg_error_t err;
	gcry_sexp_t value_list;
	gcry_sexp_t value_pair;
	gcry_sexp_t comment_list;
	unsigned int i;
	char *comment_new;
	const char *data;
	size_t data_n;
	int is_secret;
	size_t elems_n;
	const char *elems;
	gcry_mpi_t *mpis_new;
	gcry_mpi_t mpi;

	err = 0;
	value_list = NULL;
	value_pair = NULL;
	comment_list = NULL;
	comment_new = NULL;
	mpis_new = NULL;

	data = gcry_sexp_nth_data (sexp, 0, &data_n);
	if (! data)
	{
	err = gpg_error (GPG_ERR_INV_SEXP);
	goto out;
	}

	if ((data_n == 10 && !strncmp (data, "public-key", 10))
	\|\| (data_n == 21 && !strncmp (data, "protected-private-key", 21))
	\|\| (data_n == 20 && !strncmp (data, "shadowed-private-key", 20)))
	{
	is_secret = 0;
	elems = key_spec.elems_key_public;
	}
	else if (data_n == 11 && !strncmp (data, "private-key", 11))
	{
	is_secret = 1;
	elems = key_spec.elems_key_secret;
	}
	else
	{
	err = gpg_error (GPG_ERR_INV_SEXP);
	goto out;
	}

	elems_n = strlen (elems);
	mpis_new = xtrycalloc (elems_n + 1, sizeof *mpis_new );
	if (!mpis_new)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	value_list = gcry_sexp_find_token (sexp, key_spec.identifier, 0);
	if (! value_list)
	{
	err = gpg_error (GPG_ERR_INV_SEXP);
	goto out;
	}

	for (i = 0; i < elems_n; i++)
	{
	value_pair = gcry_sexp_find_token (value_list, elems + i, 1);
	if (! value_pair)
	{
	err = gpg_error (GPG_ERR_INV_SEXP);
	break;
	}

	/* Note that we need to use STD format; i.e. prepend a 0x00 to
	indicate a positive number if the high bit is set. */
	mpi = gcry_sexp_nth_mpi (value_pair, 1, GCRYMPI_FMT_STD);
	if (! mpi)
	{
	err = gpg_error (GPG_ERR_INV_SEXP);
	break;
	}
	mpis_new[i] = mpi;
	gcry_sexp_release (value_pair);
	value_pair = NULL;
	}
	if (err)
	goto out;

	/* We do not require a comment sublist to be present here. */
	data = NULL;
	data_n = 0;

	comment_list = gcry_sexp_find_token (sexp, "comment", 0);
	if (comment_list)
	data = gcry_sexp_nth_data (comment_list, 1, &data_n);
	if (! data)
	{
	data = "(none)";
	data_n = 6;
	}

	comment_new = make_cstring (data, data_n);
	if (! comment_new)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	if (secret)
	*secret = is_secret;
	*mpis = mpis_new;
	*comment = comment_new;

	out:

	gcry_sexp_release (value_list);
	gcry_sexp_release (value_pair);
	gcry_sexp_release (comment_list);

	if (err)
	{
	xfree (comment_new);
	mpint_list_free (mpis_new);
	}

	return err;
	}

	/* Extract the car from SEXP, and create a newly created C-string
	which is to be stored in IDENTIFIER. */
	static gpg_error_t
	sexp_extract_identifier (gcry_sexp_t sexp, char **identifier)
	{
	char *identifier_new;
	gcry_sexp_t sublist;
	const char *data;
	size_t data_n;
	gpg_error_t err;

	identifier_new = NULL;
	err = 0;

	sublist = gcry_sexp_nth (sexp, 1);
	if (! sublist)
	{
	err = gpg_error (GPG_ERR_INV_SEXP);
	goto out;
	}

	data = gcry_sexp_nth_data (sublist, 0, &data_n);
	if (! data)
	{
	err = gpg_error (GPG_ERR_INV_SEXP);
	goto out;
	}

	identifier_new = make_cstring (data, data_n);
	if (! identifier_new)
	{
	err = gpg_err_code_from_errno (errno);
	goto out;
	}

	*identifier = identifier_new;

	out:

	gcry_sexp_release (sublist);

	return err;
	}



	/*

	Key I/O.

	*/

	/* Search for a key specification entry. If SSH_NAME is not NULL,
	search for an entry whose "ssh_name" is equal to SSH_NAME;
	otherwise, search for an entry whose "name" is equal to NAME.
	Store found entry in SPEC on success, return error otherwise. */
	static gpg_error_t
	ssh_key_type_lookup (const char ssh_name, const char name,
	ssh_key_type_spec_t *spec)
	{
	gpg_error_t err;
	unsigned int i;

	for (i = 0; i < DIM (ssh_key_types); i++)
	if ((ssh_name && (! strcmp (ssh_name, ssh_key_types[i].ssh_identifier)))
	\|\| (name && (! strcmp (name, ssh_key_types[i].identifier))))
	break;

	if (i == DIM (ssh_key_types))
	err = gpg_error (GPG_ERR_NOT_FOUND);
	else
	{
	*spec = ssh_key_types[i];
	err = 0;
	}

	return err;
	}

	/* Receive a key from STREAM, according to the key specification given
	as KEY_SPEC. Depending on SECRET, receive a secret or a public
	key. If READ_COMMENT is true, receive a comment string as well.
	Constructs a new S-Expression from received data and stores it in
	KEY_NEW. Returns zero on success or an error code. */
	static gpg_error_t
	ssh_receive_key (estream_t stream, gcry_sexp_t *key_new, int secret,
	int read_comment, ssh_key_type_spec_t *key_spec)
	{
	gpg_error_t err;
	char *key_type = NULL;
	char *comment = NULL;
	gcry_sexp_t key = NULL;
	ssh_key_type_spec_t spec;
	gcry_mpi_t *mpi_list = NULL;
	const char *elems;

	err = stream_read_cstring (stream, &key_type);
	if (err)
	goto out;

	err = ssh_key_type_lookup (key_type, NULL, &spec);
	if (err)
	goto out;

	err = ssh_receive_mpint_list (stream, secret, spec, &mpi_list);
	if (err)
	goto out;

	if (read_comment)
	{
	err = stream_read_cstring (stream, &comment);
	if (err)
	goto out;
	}

	if (secret)
	elems = spec.elems_key_secret;
	else
	elems = spec.elems_key_public;

	if (spec.key_modifier)
	{
	err = (*spec.key_modifier) (elems, mpi_list);
	if (err)
	goto out;
	}

	err = sexp_key_construct (&key, spec, secret, mpi_list, comment? comment:"");
	if (err)
	goto out;

	if (key_spec)
	*key_spec = spec;
	*key_new = key;

	out:

	mpint_list_free (mpi_list);
	xfree (key_type);
	xfree (comment);

	return err;
	}

	/* Converts a key of type TYPE, whose key material is given in MPIS,
	into a newly created binary blob, which is to be stored in
	BLOB/BLOB_SIZE. Returns zero on success or an error code. */
	static gpg_error_t
	ssh_convert_key_to_blob (unsigned char *blob, size_t blob_size,
	const char type, gcry_mpi_t mpis)
	{
	unsigned char *blob_new;
	long int blob_size_new;
	estream_t stream;
	gpg_error_t err;
	unsigned int i;

	*blob = NULL;
	*blob_size = 0;

	blob_new = NULL;
	stream = NULL;
	err = 0;

	stream = es_mopen (NULL, 0, 0, 1, NULL, NULL, "r+");
	if (! stream)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	err = stream_write_cstring (stream, type);
	if (err)
	goto out;

	for (i = 0; mpis[i] && (! err); i++)
	err = stream_write_mpi (stream, mpis[i]);
	if (err)
	goto out;

	blob_size_new = es_ftell (stream);
	if (blob_size_new == -1)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	err = es_fseek (stream, 0, SEEK_SET);
	if (err)
	goto out;

	blob_new = xtrymalloc (blob_size_new);
	if (! blob_new)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	err = stream_read_data (stream, blob_new, blob_size_new);
	if (err)
	goto out;

	*blob = blob_new;
	*blob_size = blob_size_new;

	out:

	if (stream)
	es_fclose (stream);
	if (err)
	xfree (blob_new);

	return err;
	}


	/* Write the public key KEY_PUBLIC to STREAM in SSH key format. If
	OVERRIDE_COMMENT is not NULL, it will be used instead of the
	comment stored in the key. */
	static gpg_error_t
	ssh_send_key_public (estream_t stream, gcry_sexp_t key_public,
	const char *override_comment)
	{
	ssh_key_type_spec_t spec;
	gcry_mpi_t *mpi_list;
	char *key_type;
	char *comment;
	unsigned char *blob;
	size_t blob_n;
	gpg_error_t err;

	key_type = NULL;
	mpi_list = NULL;
	comment = NULL;
	blob = NULL;

	err = sexp_extract_identifier (key_public, &key_type);
	if (err)
	goto out;

	err = ssh_key_type_lookup (NULL, key_type, &spec);
	if (err)
	goto out;

	err = sexp_key_extract (key_public, spec, NULL, &mpi_list, &comment);
	if (err)
	goto out;

	err = ssh_convert_key_to_blob (&blob, &blob_n,
	spec.ssh_identifier, mpi_list);
	if (err)
	goto out;

	err = stream_write_string (stream, blob, blob_n);
	if (err)
	goto out;

	err = stream_write_cstring (stream,
	override_comment? override_comment : comment);

	out:

	mpint_list_free (mpi_list);
	xfree (key_type);
	xfree (comment);
	xfree (blob);

	return err;
	}

	/* Read a public key out of BLOB/BLOB_SIZE according to the key
	specification given as KEY_SPEC, storing the new key in KEY_PUBLIC.
	Returns zero on success or an error code. */
	static gpg_error_t
	ssh_read_key_public_from_blob (unsigned char *blob, size_t blob_size,
	gcry_sexp_t *key_public,
	ssh_key_type_spec_t *key_spec)
	{
	estream_t blob_stream;
	gpg_error_t err;

	err = 0;

	blob_stream = es_mopen (NULL, 0, 0, 1, NULL, NULL, "r+");
	if (! blob_stream)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	err = stream_write_data (blob_stream, blob, blob_size);
	if (err)
	goto out;

	err = es_fseek (blob_stream, 0, SEEK_SET);
	if (err)
	goto out;

	err = ssh_receive_key (blob_stream, key_public, 0, 0, key_spec);

	out:

	if (blob_stream)
	es_fclose (blob_stream);

	return err;
	}



	/* This function calculates the key grip for the key contained in the
	S-Expression KEY and writes it to BUFFER, which must be large
	enough to hold it. Returns usual error code. */
	static gpg_error_t
	ssh_key_grip (gcry_sexp_t key, unsigned char *buffer)
	{
	if (!gcry_pk_get_keygrip (key, buffer))
	return gpg_error (GPG_ERR_INTERNAL);

	return 0;
	}


	/* Converts the secret key KEY_SECRET into a public key, storing it in
	KEY_PUBLIC. SPEC is the according key specification. Returns zero
	on success or an error code. */
	static gpg_error_t
	key_secret_to_public (gcry_sexp_t *key_public,
	ssh_key_type_spec_t spec, gcry_sexp_t key_secret)
	{
	char *comment;
	gcry_mpi_t *mpis;
	gpg_error_t err;
	int is_secret;

	comment = NULL;
	mpis = NULL;

	err = sexp_key_extract (key_secret, spec, &is_secret, &mpis, &comment);
	if (err)
	goto out;

	err = sexp_key_construct (key_public, spec, 0, mpis, comment);

	out:

	mpint_list_free (mpis);
	xfree (comment);

	return err;
	}


	/* Check whether a smartcard is available and whether it has a usable
	key. Store a copy of that key at R_PK and return 0. If no key is
	available store NULL at R_PK and return an error code. If CARDSN
	is not NULL, a string with the serial number of the card will be
	a malloced and stored there. */
	static gpg_error_t
	card_key_available (ctrl_t ctrl, gcry_sexp_t r_pk, char *cardsn)
	{
	gpg_error_t err;
	char *authkeyid;
	char *serialno = NULL;
	unsigned char *pkbuf;
	size_t pkbuflen;
	gcry_sexp_t s_pk;
	unsigned char grip[20];

	*r_pk = NULL;
	if (cardsn)
	*cardsn = NULL;

	/* First see whether a card is available and whether the application
	is supported. */
	err = agent_card_getattr (ctrl, "$AUTHKEYID", &authkeyid);
	if ( gpg_err_code (err) == GPG_ERR_CARD_REMOVED )
	{
	/* Ask for the serial number to reset the card. */
	err = agent_card_serialno (ctrl, &serialno);
	if (err)
	{
	if (opt.verbose)
	log_info (_("error getting serial number of card: %s\n"),
	gpg_strerror (err));
	return err;
	}
	log_info (_("detected card with S/N: %s\n"), serialno);
	err = agent_card_getattr (ctrl, "$AUTHKEYID", &authkeyid);
	}
	if (err)
	{
	log_error (_("error getting default authentication keyID of card: %s\n"),
	gpg_strerror (err));
	xfree (serialno);
	return err;
	}

	/* Get the S/N if we don't have it yet. Use the fast getattr method. */
	if (!serialno && (err = agent_card_getattr (ctrl, "SERIALNO", &serialno)) )
	{
	log_error (_("error getting serial number of card: %s\n"),
	gpg_strerror (err));
	xfree (authkeyid);
	return err;
	}

	/* Read the public key. */
	err = agent_card_readkey (ctrl, authkeyid, &pkbuf);
	if (err)
	{
	if (opt.verbose)
	log_info (_("no suitable card key found: %s\n"), gpg_strerror (err));
	xfree (serialno);
	xfree (authkeyid);
	return err;
	}

	pkbuflen = gcry_sexp_canon_len (pkbuf, 0, NULL, NULL);
	err = gcry_sexp_sscan (&s_pk, NULL, (char*)pkbuf, pkbuflen);
	if (err)
	{
	log_error ("failed to build S-Exp from received card key: %s\n",
	gpg_strerror (err));
	xfree (pkbuf);
	xfree (serialno);
	xfree (authkeyid);
	return err;
	}

	err = ssh_key_grip (s_pk, grip);
	if (err)
	{
	log_debug ("error computing keygrip from received card key: %s\n",
	gcry_strerror (err));
	xfree (pkbuf);
	gcry_sexp_release (s_pk);
	xfree (serialno);
	xfree (authkeyid);
	return err;
	}

	if ( agent_key_available (grip) )
	{
	/* (Shadow)-key is not available in our key storage. */
	unsigned char *shadow_info;
	unsigned char *tmp;

	shadow_info = make_shadow_info (serialno, authkeyid);
	if (!shadow_info)
	{
	err = gpg_error_from_syserror ();
	xfree (pkbuf);
	gcry_sexp_release (s_pk);
	xfree (serialno);
	xfree (authkeyid);
	return err;
	}
	err = agent_shadow_key (pkbuf, shadow_info, &tmp);
	xfree (shadow_info);
	if (err)
	{
	log_error (_("shadowing the key failed: %s\n"), gpg_strerror (err));
	xfree (pkbuf);
	gcry_sexp_release (s_pk);
	xfree (serialno);
	xfree (authkeyid);
	return err;
	}
	xfree (pkbuf);
	pkbuf = tmp;
	pkbuflen = gcry_sexp_canon_len (pkbuf, 0, NULL, NULL);
	assert (pkbuflen);

	err = agent_write_private_key (grip, pkbuf, pkbuflen, 0);
	if (err)
	{
	log_error (_("error writing key: %s\n"), gpg_strerror (err));
	xfree (pkbuf);
	gcry_sexp_release (s_pk);
	xfree (serialno);
	xfree (authkeyid);
	return err;
	}
	}

	if (cardsn)
	{
	char *dispsn;

	/* If the card handler is able to return a short serialnumber,
	use that one, else use the complete serialno. */
	if (!agent_card_getattr (ctrl, "$DISPSERIALNO", &dispsn))
	{
	*cardsn = xtryasprintf ("cardno:%s", dispsn);
	xfree (dispsn);
	}
	else
	*cardsn = xtryasprintf ("cardno:%s", serialno);
	if (!*cardsn)
	{
	err = gpg_error_from_syserror ();
	xfree (pkbuf);
	gcry_sexp_release (s_pk);
	xfree (serialno);
	xfree (authkeyid);
	return err;
	}
	}

	xfree (pkbuf);
	xfree (serialno);
	xfree (authkeyid);
	*r_pk = s_pk;
	return 0;
	}




	/*

	Request handler. Each handler is provided with a CTRL context, a
	REQUEST object and a RESPONSE object. The actual request is to be
	read from REQUEST, the response needs to be written to RESPONSE.

	*/


	/* Handler for the "request_identities" command. */
	static gpg_error_t
	ssh_handler_request_identities (ctrl_t ctrl,
	estream_t request, estream_t response)
	{
	char *key_type;
	ssh_key_type_spec_t spec;
	struct dirent *dir_entry;
	char *key_directory;
	size_t key_directory_n;
	char *key_path;
	unsigned char *buffer;
	size_t buffer_n;
	u32 key_counter;
	estream_t key_blobs;
	gcry_sexp_t key_secret;
	gcry_sexp_t key_public;
	DIR *dir;
	gpg_error_t err;
	int ret;
	FILE *ctrl_fp = NULL;
	char *cardsn;
	gpg_error_t ret_err;

	(void)request;

	/* Prepare buffer stream. */

	key_directory = NULL;
	key_secret = NULL;
	key_public = NULL;
	key_type = NULL;
	key_path = NULL;
	key_counter = 0;
	buffer = NULL;
	dir = NULL;
	err = 0;

	key_blobs = es_mopen (NULL, 0, 0, 1, NULL, NULL, "r+");
	if (! key_blobs)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	/* Open key directory. */
	key_directory = make_filename (opt.homedir, GNUPG_PRIVATE_KEYS_DIR, NULL);
	if (! key_directory)
	{
	err = gpg_err_code_from_errno (errno);
	goto out;
	}
	key_directory_n = strlen (key_directory);

	key_path = xtrymalloc (key_directory_n + 46);
	if (! key_path)
	{
	err = gpg_err_code_from_errno (errno);
	goto out;
	}

	sprintf (key_path, "%s/", key_directory);
	sprintf (key_path + key_directory_n + 41, ".key");

	dir = opendir (key_directory);
	if (! dir)
	{
	err = gpg_err_code_from_errno (errno);
	goto out;
	}



	/* First check whether a key is currently available in the card
	reader - this should be allowed even without being listed in
	sshcontrol. */

	if (!card_key_available (ctrl, &key_public, &cardsn))
	{
	err = ssh_send_key_public (key_blobs, key_public, cardsn);
	gcry_sexp_release (key_public);
	key_public = NULL;
	xfree (cardsn);
	if (err)
	goto out;

	key_counter++;
	}


	/* Then look at all the registered an allowed keys. */


	/* Fixme: We should better iterate over the control file and check
	whether the key file is there. This is better in resepct to
	performance if tehre are a lot of key sin our key storage. */
	/* FIXME: make sure that buffer gets deallocated properly. */
	err = open_control_file (&ctrl_fp, 0);
	if (err)
	goto out;

	while ( (dir_entry = readdir (dir)) )
	{
	if ((strlen (dir_entry->d_name) == 44)
	&& (! strncmp (dir_entry->d_name + 40, ".key", 4)))
	{
	char hexgrip[41];
	int disabled;

	/* We do only want to return keys listed in our control
	file. */
	strncpy (hexgrip, dir_entry->d_name, 40);
	hexgrip[40] = 0;
	if ( strlen (hexgrip) != 40 )
	continue;
	if (search_control_file (ctrl_fp, hexgrip, &disabled, NULL, NULL)
	\|\| disabled)
	continue;

	strncpy (key_path + key_directory_n + 1, dir_entry->d_name, 40);

	/* Read file content. */
	err = file_to_buffer (key_path, &buffer, &buffer_n);
	if (err)
	goto out;

	err = gcry_sexp_sscan (&key_secret, NULL, (char*)buffer, buffer_n);
	if (err)
	goto out;

	xfree (buffer);
	buffer = NULL;

	err = sexp_extract_identifier (key_secret, &key_type);
	if (err)
	goto out;

	err = ssh_key_type_lookup (NULL, key_type, &spec);
	if (err)
	goto out;

	xfree (key_type);
	key_type = NULL;

	err = key_secret_to_public (&key_public, spec, key_secret);
	if (err)
	goto out;

	gcry_sexp_release (key_secret);
	key_secret = NULL;

	err = ssh_send_key_public (key_blobs, key_public, NULL);
	if (err)
	goto out;

	gcry_sexp_release (key_public);
	key_public = NULL;

	key_counter++;
	}
	}

	ret = es_fseek (key_blobs, 0, SEEK_SET);
	if (ret)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	out:

	/* Send response. */

	gcry_sexp_release (key_secret);
	gcry_sexp_release (key_public);

	if (! err)
	{
	ret_err = stream_write_byte (response, SSH_RESPONSE_IDENTITIES_ANSWER);
	if (ret_err)
	goto leave;
	ret_err = stream_write_uint32 (response, key_counter);
	if (ret_err)
	goto leave;
	ret_err = stream_copy (response, key_blobs);
	if (ret_err)
	goto leave;
	}
	else
	{
	ret_err = stream_write_byte (response, SSH_RESPONSE_FAILURE);
	goto leave;
	};

	leave:

	if (key_blobs)
	es_fclose (key_blobs);
	if (dir)
	closedir (dir);

	if (ctrl_fp)
	fclose (ctrl_fp);

	xfree (key_directory);
	xfree (key_path);
	xfree (buffer);
	xfree (key_type);

	return ret_err;
	}


	/* This function hashes the data contained in DATA of size DATA_N
	according to the message digest algorithm specified by MD_ALGORITHM
	and writes the message digest to HASH, which needs to large enough
	for the digest. */
	static gpg_error_t
	data_hash (unsigned char *data, size_t data_n,
	int md_algorithm, unsigned char *hash)
	{
	gcry_md_hash_buffer (md_algorithm, hash, data, data_n);

	return 0;
	}

	/* This function signs the data contained in CTRL, stores the created
	signature in newly allocated memory in SIG and it's size in SIG_N;
	SIG_ENCODER is the signature encoder to use. */
	static gpg_error_t
	data_sign (ctrl_t ctrl, ssh_signature_encoder_t sig_encoder,
	unsigned char *sig, size_t sig_n)
	{
	gpg_error_t err;
	gcry_sexp_t signature_sexp = NULL;
	estream_t stream = NULL;
	gcry_sexp_t valuelist = NULL;
	gcry_sexp_t sublist = NULL;
	gcry_mpi_t sig_value = NULL;
	unsigned char *sig_blob = NULL;
	size_t sig_blob_n = 0;
	char *identifier = NULL;
	const char *identifier_raw;
	size_t identifier_n;
	ssh_key_type_spec_t spec;
	int ret;
	unsigned int i;
	const char *elems;
	size_t elems_n;
	gcry_mpi_t *mpis = NULL;
	char hexgrip[40+1];

	*sig = NULL;
	*sig_n = 0;

	/* Quick check to see whether we have a valid keygrip and convert it
	to hex. */
	if (!ctrl->have_keygrip)
	{
	err = gpg_error (GPG_ERR_NO_SECKEY);
	goto out;
	}
	bin2hex (ctrl->keygrip, 20, hexgrip);

	/* Ask for confirmation if needed. */
	if (confirm_flag_from_sshcontrol (hexgrip))
	{
	gcry_sexp_t key;
	char fpr, prompt;
	char *comment = NULL;

	err = agent_raw_key_from_file (ctrl, ctrl->keygrip, &key);
	if (err)
	goto out;
	err = ssh_get_fingerprint_string (key, &fpr);
	if (!err)
	{
	gcry_sexp_t tmpsxp = gcry_sexp_find_token (key, "comment", 0);
	if (tmpsxp)
	comment = gcry_sexp_nth_string (tmpsxp, 1);
	gcry_sexp_release (tmpsxp);
	}
	gcry_sexp_release (key);
	if (err)
	goto out;
	prompt = xtryasprintf (_("An ssh process requested the use of key%%0A"
	" %s%%0A"
	" (%s)%%0A"
	"Do you want to allow this?"),
	fpr, comment? comment:"");
	xfree (fpr);
	gcry_free (comment);
	err = agent_get_confirmation (ctrl, prompt, _("Allow"), _("Deny"), 0);
	xfree (prompt);
	if (err)
	goto out;
	}

	/* Create signature. */
	ctrl->use_auth_call = 1;
	err = agent_pksign_do (ctrl, NULL,
	_("Please enter the passphrase "
	"for the ssh key%%0A %F%%0A (%c)"),
	&signature_sexp,
	CACHE_MODE_SSH, ttl_from_sshcontrol);
	ctrl->use_auth_call = 0;
	if (err)
	goto out;

	valuelist = gcry_sexp_nth (signature_sexp, 1);
	if (! valuelist)
	{
	err = gpg_error (GPG_ERR_INV_SEXP);
	goto out;
	}

	stream = es_mopen (NULL, 0, 0, 1, NULL, NULL, "r+");
	if (! stream)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	identifier_raw = gcry_sexp_nth_data (valuelist, 0, &identifier_n);
	if (! identifier_raw)
	{
	err = gpg_error (GPG_ERR_INV_SEXP);
	goto out;
	}

	identifier = make_cstring (identifier_raw, identifier_n);
	if (! identifier)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	err = ssh_key_type_lookup (NULL, identifier, &spec);
	if (err)
	goto out;

	err = stream_write_cstring (stream, spec.ssh_identifier);
	if (err)
	goto out;

	elems = spec.elems_signature;
	elems_n = strlen (elems);

	mpis = xtrycalloc (elems_n + 1, sizeof *mpis);
	if (!mpis)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	for (i = 0; i < elems_n; i++)
	{
	sublist = gcry_sexp_find_token (valuelist, spec.elems_signature + i, 1);
	if (! sublist)
	{
	err = gpg_error (GPG_ERR_INV_SEXP);
	break;
	}

	sig_value = gcry_sexp_nth_mpi (sublist, 1, GCRYMPI_FMT_USG);
	if (! sig_value)
	{
	err = gpg_error (GPG_ERR_INTERNAL); /* FIXME? */
	break;
	}
	gcry_sexp_release (sublist);
	sublist = NULL;

	mpis[i] = sig_value;
	}
	if (err)
	goto out;

	err = (*sig_encoder) (stream, mpis);
	if (err)
	goto out;

	sig_blob_n = es_ftell (stream);
	if (sig_blob_n == -1)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	sig_blob = xtrymalloc (sig_blob_n);
	if (! sig_blob)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	ret = es_fseek (stream, 0, SEEK_SET);
	if (ret)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	err = stream_read_data (stream, sig_blob, sig_blob_n);
	if (err)
	goto out;

	*sig = sig_blob;
	*sig_n = sig_blob_n;

	out:

	if (err)
	xfree (sig_blob);

	if (stream)
	es_fclose (stream);
	gcry_sexp_release (valuelist);
	gcry_sexp_release (signature_sexp);
	gcry_sexp_release (sublist);
	mpint_list_free (mpis);
	xfree (identifier);

	return err;
	}

	/* Handler for the "sign_request" command. */
	static gpg_error_t
	ssh_handler_sign_request (ctrl_t ctrl, estream_t request, estream_t response)
	{
	gcry_sexp_t key;
	ssh_key_type_spec_t spec;
	unsigned char hash[MAX_DIGEST_LEN];
	unsigned int hash_n;
	unsigned char key_grip[20];
	unsigned char *key_blob;
	u32 key_blob_size;
	unsigned char *data;
	unsigned char *sig;
	size_t sig_n;
	u32 data_size;
	u32 flags;
	gpg_error_t err;
	gpg_error_t ret_err;

	key_blob = NULL;
	data = NULL;
	sig = NULL;
	key = NULL;

	/* Receive key. */

	err = stream_read_string (request, 0, &key_blob, &key_blob_size);
	if (err)
	goto out;

	err = ssh_read_key_public_from_blob (key_blob, key_blob_size, &key, &spec);
	if (err)
	goto out;

	/* Receive data to sign. */
	err = stream_read_string (request, 0, &data, &data_size);
	if (err)
	goto out;

	/* FIXME? */
	err = stream_read_uint32 (request, &flags);
	if (err)
	goto out;

	/* Hash data. */
	hash_n = gcry_md_get_algo_dlen (GCRY_MD_SHA1);
	if (! hash_n)
	{
	err = gpg_error (GPG_ERR_INTERNAL);
	goto out;
	}
	err = data_hash (data, data_size, GCRY_MD_SHA1, hash);
	if (err)
	goto out;

	/* Calculate key grip. */
	err = ssh_key_grip (key, key_grip);
	if (err)
	goto out;

	/* Sign data. */

	ctrl->digest.algo = GCRY_MD_SHA1;
	memcpy (ctrl->digest.value, hash, hash_n);
	ctrl->digest.valuelen = hash_n;
	ctrl->digest.raw_value = ! (spec.flags & SPEC_FLAG_USE_PKCS1V2);
	ctrl->have_keygrip = 1;
	memcpy (ctrl->keygrip, key_grip, 20);

	err = data_sign (ctrl, spec.signature_encoder, &sig, &sig_n);

	out:

	/* Done. */

	if (! err)
	{
	ret_err = stream_write_byte (response, SSH_RESPONSE_SIGN_RESPONSE);
	if (ret_err)
	goto leave;
	ret_err = stream_write_string (response, sig, sig_n);
	if (ret_err)
	goto leave;
	}
	else
	{
	ret_err = stream_write_byte (response, SSH_RESPONSE_FAILURE);
	if (ret_err)
	goto leave;
	}

	leave:

	gcry_sexp_release (key);
	xfree (key_blob);
	xfree (data);
	xfree (sig);

	return ret_err;
	}

	/* This function extracts the comment contained in the key
	S-Expression KEY and stores a copy in COMMENT. Returns usual error
	code. */
	static gpg_error_t
	ssh_key_extract_comment (gcry_sexp_t key, char **comment)
	{
	gcry_sexp_t comment_list;
	char *comment_new;
	const char *data;
	size_t data_n;
	gpg_error_t err;

	comment_list = gcry_sexp_find_token (key, "comment", 0);
	if (! comment_list)
	{
	err = gpg_error (GPG_ERR_INV_SEXP);
	goto out;
	}

	data = gcry_sexp_nth_data (comment_list, 1, &data_n);
	if (! data)
	{
	err = gpg_error (GPG_ERR_INV_SEXP);
	goto out;
	}

	comment_new = make_cstring (data, data_n);
	if (! comment_new)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	*comment = comment_new;
	err = 0;

	out:

	gcry_sexp_release (comment_list);

	return err;
	}

	/* This function converts the key contained in the S-Expression KEY
	into a buffer, which is protected by the passphrase PASSPHRASE.
	Returns usual error code. */
	static gpg_error_t
	ssh_key_to_protected_buffer (gcry_sexp_t key, const char *passphrase,
	unsigned char *buffer, size_t buffer_n)
	{
	unsigned char *buffer_new;
	unsigned int buffer_new_n;
	gpg_error_t err;

	err = 0;
	buffer_new_n = gcry_sexp_sprint (key, GCRYSEXP_FMT_CANON, NULL, 0);
	buffer_new = xtrymalloc_secure (buffer_new_n);
	if (! buffer_new)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	gcry_sexp_sprint (key, GCRYSEXP_FMT_CANON, buffer_new, buffer_new_n);
	/* FIXME: guarantee? */

	err = agent_protect (buffer_new, passphrase, buffer, buffer_n, 0);

	out:

	xfree (buffer_new);

	return err;
	}



	/* Callback function to compare the first entered PIN with the one
	currently being entered. */
	static int
	reenter_compare_cb (struct pin_entry_info_s *pi)
	{
	const char *pin1 = pi->check_cb_arg;

	if (!strcmp (pin1, pi->pin))
	return 0; /* okay */
	return -1;
	}

	/* Store the ssh KEY into our local key storage and protect it after
	asking for a passphrase. Cache that passphrase. TTL is the
	maximum caching time for that key. If the key already exists in
	our key storage, don't do anything. When entering a new key also
	add an entry to the sshcontrol file. */
	static gpg_error_t
	ssh_identity_register (ctrl_t ctrl, gcry_sexp_t key, int ttl, int confirm)
	{
	gpg_error_t err;
	unsigned char key_grip_raw[20];
	char key_grip[41];
	unsigned char *buffer = NULL;
	size_t buffer_n;
	char *description = NULL;
	const char *description2 = _("Please re-enter this passphrase");
	char *comment = NULL;
	char *key_fpr = NULL;
	const char *initial_errtext = NULL;
	unsigned int i;
	struct pin_entry_info_s pi = NULL, pi2;

	err = ssh_key_grip (key, key_grip_raw);
	if (err)
	goto out;

	/* Check whether the key is already in our key storage. Don't do
	anything then. */
	if ( !agent_key_available (key_grip_raw) )
	goto out; /* Yes, key is available. */

	err = ssh_get_fingerprint_string (key, &key_fpr);
	if (err)
	goto out;

	err = ssh_key_extract_comment (key, &comment);
	if (err)
	goto out;

	if ( asprintf (&description,
	_("Please enter a passphrase to protect"
	" the received secret key%%0A"
	" %s%%0A"
	" %s%%0A"
	"within gpg-agent's key storage"),
	key_fpr, comment ? comment : "") < 0)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}


	pi = gcry_calloc_secure (2, sizeof (*pi) + 100 + 1);
	if (!pi)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}
	pi2 = pi + (sizeof *pi + 100 + 1);
	pi->max_length = 100;
	pi->max_tries = 1;
	pi2->max_length = 100;
	pi2->max_tries = 1;
	pi2->check_cb = reenter_compare_cb;
	pi2->check_cb_arg = pi->pin;

	next_try:
	err = agent_askpin (ctrl, description, NULL, initial_errtext, pi);
	initial_errtext = NULL;
	if (err)
	goto out;

	/* Unless the passphrase is empty, ask to confirm it. */
	if (pi->pin && *pi->pin)
	{
	err = agent_askpin (ctrl, description2, NULL, NULL, pi2);
	if (err == -1)
	{ /* The re-entered one did not match and the user did not
	hit cancel. */
	initial_errtext = _("does not match - try again");
	goto next_try;
	}
	}

	err = ssh_key_to_protected_buffer (key, pi->pin, &buffer, &buffer_n);
	if (err)
	goto out;

	/* Store this key to our key storage. */
	err = agent_write_private_key (key_grip_raw, buffer, buffer_n, 0);
	if (err)
	goto out;

	/* Cache this passphrase. */
	for (i = 0; i < 20; i++)
	sprintf (key_grip + 2 * i, "%02X", key_grip_raw[i]);

	err = agent_put_cache (key_grip, CACHE_MODE_SSH, pi->pin, ttl);
	if (err)
	goto out;

	/* And add an entry to the sshcontrol file. */
	err = add_control_entry (ctrl, key_grip, key_fpr, ttl, confirm);


	out:
	if (pi && pi->max_length)
	wipememory (pi->pin, pi->max_length);
	xfree (pi);
	xfree (buffer);
	xfree (comment);
	xfree (key_fpr);
	xfree (description);

	return err;
	}


	/* This function removes the key contained in the S-Expression KEY
	from the local key storage, in case it exists there. Returns usual
	error code. FIXME: this function is a stub. */
	static gpg_error_t
	ssh_identity_drop (gcry_sexp_t key)
	{
	unsigned char key_grip[21] = { 0 };
	gpg_error_t err;

	err = ssh_key_grip (key, key_grip);
	if (err)
	goto out;

	key_grip[sizeof (key_grip) - 1] = 0;

	/* FIXME: What to do here - forgetting the passphrase or deleting
	the key from key cache? */

	out:

	return err;
	}

	/* Handler for the "add_identity" command. */
	static gpg_error_t
	ssh_handler_add_identity (ctrl_t ctrl, estream_t request, estream_t response)
	{
	gpg_error_t ret_err;
	gpg_error_t err;
	gcry_sexp_t key;
	unsigned char b;
	int confirm;
	int ttl;

	confirm = 0;
	key = NULL;
	ttl = 0;

	/* FIXME? */
	err = ssh_receive_key (request, &key, 1, 1, NULL);
	if (err)
	goto out;

	while (1)
	{
	err = stream_read_byte (request, &b);
	if (gpg_err_code (err) == GPG_ERR_EOF)
	{
	err = 0;
	break;
	}

	switch (b)
	{
	case SSH_OPT_CONSTRAIN_LIFETIME:
	{
	u32 n = 0;

	err = stream_read_uint32 (request, &n);
	if (! err)
	ttl = n;
	break;
	}

	case SSH_OPT_CONSTRAIN_CONFIRM:
	{
	confirm = 1;
	break;
	}

	default:
	/* FIXME: log/bad? */
	break;
	}
	}
	if (err)
	goto out;

	err = ssh_identity_register (ctrl, key, ttl, confirm);

	out:

	gcry_sexp_release (key);

	if (! err)
	ret_err = stream_write_byte (response, SSH_RESPONSE_SUCCESS);
	else
	ret_err = stream_write_byte (response, SSH_RESPONSE_FAILURE);

	return ret_err;
	}

	/* Handler for the "remove_identity" command. */
	static gpg_error_t
	ssh_handler_remove_identity (ctrl_t ctrl,
	estream_t request, estream_t response)
	{
	unsigned char *key_blob;
	u32 key_blob_size;
	gcry_sexp_t key;
	gpg_error_t ret_err;
	gpg_error_t err;

	(void)ctrl;

	/* Receive key. */

	key_blob = NULL;
	key = NULL;

	err = stream_read_string (request, 0, &key_blob, &key_blob_size);
	if (err)
	goto out;

	err = ssh_read_key_public_from_blob (key_blob, key_blob_size, &key, NULL);
	if (err)
	goto out;

	err = ssh_identity_drop (key);

	out:

	xfree (key_blob);
	gcry_sexp_release (key);

	if (! err)
	ret_err = stream_write_byte (response, SSH_RESPONSE_SUCCESS);
	else
	ret_err = stream_write_byte (response, SSH_RESPONSE_FAILURE);

	return ret_err;
	}

	/* FIXME: stub function. Actually useful? */
	static gpg_error_t
	ssh_identities_remove_all (void)
	{
	gpg_error_t err;

	err = 0;

	/* FIXME: shall we remove _all_ cache entries or only those
	registered through the ssh emulation? */

	return err;
	}

	/* Handler for the "remove_all_identities" command. */
	static gpg_error_t
	ssh_handler_remove_all_identities (ctrl_t ctrl,
	estream_t request, estream_t response)
	{
	gpg_error_t ret_err;
	gpg_error_t err;

	(void)ctrl;
	(void)request;

	err = ssh_identities_remove_all ();

	if (! err)
	ret_err = stream_write_byte (response, SSH_RESPONSE_SUCCESS);
	else
	ret_err = stream_write_byte (response, SSH_RESPONSE_FAILURE);

	return ret_err;
	}

	/* Lock agent? FIXME: stub function. */
	static gpg_error_t
	ssh_lock (void)
	{
	gpg_error_t err;

	/* FIXME */
	log_error ("ssh-agent's lock command is not implemented\n");
	err = 0;

	return err;
	}

	/* Unock agent? FIXME: stub function. */
	static gpg_error_t
	ssh_unlock (void)
	{
	gpg_error_t err;

	log_error ("ssh-agent's unlock command is not implemented\n");
	err = 0;

	return err;
	}

	/* Handler for the "lock" command. */
	static gpg_error_t
	ssh_handler_lock (ctrl_t ctrl, estream_t request, estream_t response)
	{
	gpg_error_t ret_err;
	gpg_error_t err;

	(void)ctrl;
	(void)request;

	err = ssh_lock ();

	if (! err)
	ret_err = stream_write_byte (response, SSH_RESPONSE_SUCCESS);
	else
	ret_err = stream_write_byte (response, SSH_RESPONSE_FAILURE);

	return ret_err;
	}

	/* Handler for the "unlock" command. */
	static gpg_error_t
	ssh_handler_unlock (ctrl_t ctrl, estream_t request, estream_t response)
	{
	gpg_error_t ret_err;
	gpg_error_t err;

	(void)ctrl;
	(void)request;

	err = ssh_unlock ();

	if (! err)
	ret_err = stream_write_byte (response, SSH_RESPONSE_SUCCESS);
	else
	ret_err = stream_write_byte (response, SSH_RESPONSE_FAILURE);

	return ret_err;
	}



	/* Return the request specification for the request identified by TYPE
	or NULL in case the requested request specification could not be
	found. */
	static ssh_request_spec_t *
	request_spec_lookup (int type)
	{
	ssh_request_spec_t *spec;
	unsigned int i;

	for (i = 0; i < DIM (request_specs); i++)
	if (request_specs[i].type == type)
	break;
	if (i == DIM (request_specs))
	{
	if (opt.verbose)
	log_info ("ssh request %u is not supported\n", type);
	spec = NULL;
	}
	else
	spec = request_specs + i;

	return spec;
	}

	/* Process a single request. The request is read from and the
	response is written to STREAM_SOCK. Uses CTRL as context. Returns
	zero in case of success, non zero in case of failure. */
	static int
	ssh_request_process (ctrl_t ctrl, estream_t stream_sock)
	{
	ssh_request_spec_t *spec;
	estream_t response;
	estream_t request;
	unsigned char request_type;
	gpg_error_t err;
	int send_err;
	int ret;
	unsigned char *request_data;
	u32 request_data_size;
	u32 response_size;

	request_data = NULL;
	response = NULL;
	request = NULL;
	send_err = 0;

	/* Create memory streams for request/response data. The entire
	request will be stored in secure memory, since it might contain
	secret key material. The response does not have to be stored in
	secure memory, since we never give out secret keys.

	Note: we only have little secure memory, but there is NO
	possibility of DoS here; only trusted clients are allowed to
	connect to the agent. What could happen is that the agent
	returns out-of-secure-memory errors on requests in case the
	agent's owner floods his own agent with many large messages.
	-moritz */

	/* Retrieve request. */
	err = stream_read_string (stream_sock, 1, &request_data, &request_data_size);
	if (err)
	goto out;

	if (opt.verbose > 1)
	log_info ("received ssh request of length %u\n",
	(unsigned int)request_data_size);

	if (! request_data_size)
	{
	send_err = 1;
	goto out;
	/* Broken request; FIXME. */
	}

	request_type = request_data[0];
	spec = request_spec_lookup (request_type);
	if (! spec)
	{
	send_err = 1;
	goto out;
	/* Unknown request; FIXME. */
	}

	if (spec->secret_input)
	request = es_mopen (NULL, 0, 0, 1, realloc_secure, gcry_free, "r+");
	else
	request = es_mopen (NULL, 0, 0, 1, gcry_realloc, gcry_free, "r+");
	if (! request)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}
	ret = es_setvbuf (request, NULL, _IONBF, 0);
	if (ret)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}
	err = stream_write_data (request, request_data + 1, request_data_size - 1);
	if (err)
	goto out;
	es_rewind (request);

	response = es_mopen (NULL, 0, 0, 1, NULL, NULL, "r+");
	if (! response)
	{
	err = gpg_error_from_syserror ();
	goto out;
	}

	if (opt.verbose)
	log_info ("ssh request handler for %s (%u) started\n",
	spec->identifier, spec->type);

	err = (*spec->handler) (ctrl, request, response);

	if (opt.verbose)
	{
	if (err)
	log_info ("ssh request handler for %s (%u) failed: %s\n",
	spec->identifier, spec->type, gpg_strerror (err));
	else
	log_info ("ssh request handler for %s (%u) ready\n",
	spec->identifier, spec->type);
	}

	if (err)
	{
	send_err = 1;
	goto out;
	}

	response_size = es_ftell (response);
	if (opt.verbose > 1)
	log_info ("sending ssh response of length %u\n",
	(unsigned int)response_size);

	err = es_fseek (response, 0, SEEK_SET);
	if (err)
	{
	send_err = 1;
	goto out;
	}

	err = stream_write_uint32 (stream_sock, response_size);
	if (err)
	{
	send_err = 1;
	goto out;
	}

	err = stream_copy (stream_sock, response);
	if (err)
	goto out;

	err = es_fflush (stream_sock);
	if (err)
	goto out;

	out:

	if (err && es_feof (stream_sock))
	log_error ("error occured while processing request: %s\n",
	gpg_strerror (err));

	if (send_err)
	{
	if (opt.verbose > 1)
	log_info ("sending ssh error response\n");
	err = stream_write_uint32 (stream_sock, 1);
	if (err)
	goto leave;
	err = stream_write_byte (stream_sock, SSH_RESPONSE_FAILURE);
	if (err)
	goto leave;
	}

	leave:

	if (request)
	es_fclose (request);
	if (response)
	es_fclose (response);
	xfree (request_data); /* FIXME? */

	return !!err;
	}

	/* Start serving client on SOCK_CLIENT. */
	void
	start_command_handler_ssh (ctrl_t ctrl, gnupg_fd_t sock_client)
	{
	estream_t stream_sock = NULL;
	gpg_error_t err = 0;
	int ret;

	/* Because the ssh protocol does not send us information about the
	the current TTY setting, we resort here to use those from startup
	or those explictly set. */
	{
	static const char *names[] =
	{"GPG_TTY", "DISPLAY", "TERM", "XAUTHORITY", "PINENTRY_USER_DATA", NULL};
	int idx;
	const char *value;

	for (idx=0; !err && names[idx]; idx++)
	if (!session_env_getenv (ctrl->session_env, names[idx])
	&& (value = session_env_getenv (opt.startup_env, names[idx])))
	err = session_env_setenv (ctrl->session_env, names[idx], value);

	if (!err && !ctrl->lc_ctype && opt.startup_lc_ctype)
	if (!(ctrl->lc_ctype = xtrystrdup (opt.startup_lc_ctype)))
	err = gpg_error_from_syserror ();

	if (!err && !ctrl->lc_messages && opt.startup_lc_messages)
	if (!(ctrl->lc_messages = xtrystrdup (opt.startup_lc_messages)))
	err = gpg_error_from_syserror ();

	if (err)
	{
	log_error ("error setting default session environment: %s\n",
	gpg_strerror (err));
	goto out;
	}
	}


	/* Create stream from socket. */
	stream_sock = es_fdopen (FD2INT(sock_client), "r+");
	if (!stream_sock)
	{
	err = gpg_error_from_syserror ();
	log_error (_("failed to create stream from socket: %s\n"),
	gpg_strerror (err));
	goto out;
	}
	/* We have to disable the estream buffering, because the estream
	core doesn't know about secure memory. */
	ret = es_setvbuf (stream_sock, NULL, _IONBF, 0);
	if (ret)
	{
	err = gpg_error_from_syserror ();
	log_error ("failed to disable buffering "
	"on socket stream: %s\n", gpg_strerror (err));
	goto out;
	}

	/* Main processing loop. */
	while ( !ssh_request_process (ctrl, stream_sock) )
	{
	/* Check wether we have reached EOF before trying to read
	another request. */
	int c;

	c = es_fgetc (stream_sock);
	if (c == EOF)
	break;
	es_ungetc (c, stream_sock);
	}

	/* Reset the SCD in case it has been used. */
	agent_reset_scd (ctrl);


	out:
	if (stream_sock)
	es_fclose (stream_sock);
	}

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