Some tools to play with GnuPG’s smartcard daemon and OpenPGP smartcards.
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scdtools/src/scdtotp.c

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/*
* scdtools - Tools for Scdaemon and OpenPGP smartcards
* Copyright (C) 2014,2015 Damien Goutte-Gattat
*
* This program 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.
*
* This program 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/>.
*/
#ifdef HAVE_CONFIG_H
#include <config.h>
#endif
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <ctype.h>
#include <time.h>
#include <getopt.h>
#include <err.h>
#include <gcrypt.h>
#include "gpg-util.h"
#define DEFAULT_TIME_STEP 30
#define DEFAULT_DIGITS 6
#define MAX_KEY_SIZE 64
static void
usage(int status)
{
puts("Usage: scdtotp [options]\n\
Generate a time-based one-time password (TOTP) from a seed\n\
found in a OpenPGP smartcard.\n\
");
puts("Options:\n\
-h, --help Display this help message.\n\
-v, --version Display the version message.\n\
");
printf("\
-t, --time SECONDS Generate OTP for the specified time\n\
(in seconds) instead of current time.\n\
-s, --step N Specify the time step factor\n\
(defualt is %d).\n\
-d, --digits N Specify number of digits to output\n\
(default is %d).\n\
\n", DEFAULT_TIME_STEP, DEFAULT_DIGITS);
puts("\
-m, --mac-algo ALG Use the specified HMAC algorithm.\n\
ALG can be 'sha1' (default)',\n\
'sha256' or 'sha512'.\n\
");
printf("\
-n, --private-do N Read key from private data object N\n\
(default is %d).\n\
\n", DEFAULT_PRIVATE_DO);
printf("Report bugs to <%s>.\n", PACKAGE_BUGREPORT);
exit(status);
}
static void
info(void)
{
printf("\
scdtotp (scdtools %s)\n\
Copyright (C) 2014,2015 Damien Goutte-Gattat\n\
\n\
This program is released under the GNU General Public License.\n\
See the COPYING file or <http://www.gnu.org/licenses/gpl.html>.\n\
", VERSION);
exit(EXIT_SUCCESS);
}
/*
* Convert a time value (in seconds) into a moving factor for the
* TOTP algorithm as per RFC 6238.
*
* @param time Elapsed seconds since Unix epoch.
* @param step Time step parameter.
* @param buffer Byte buffer to store the formatted factor. Must be
* at least 8 bytes long.
* @param len Size of the output buffer.
*
* @return The number of bytes written to the buffer (always 8).
*/
static int
time2factor(time_t time, int step, unsigned char *buffer, size_t len)
{
unsigned n_steps;
int i, j;
n_steps = time / step;
for ( i = 0, j = 7; i < 8; i++, j-- )
buffer[i] = (n_steps & (uint64_t)(0xFF << (j * 8))) >> (j * 8);
return 8;
}
/*
* Convert a hexstring into a bytes array.
*
* @param hexstring The string to convert.
* @param buffer The output buffer.
* @param len Size of the output buffer.
*
* @return The number of bytes written to the output buffer, or
* -1 if the buffer is not large enough.
*/
static int
hexstring2bytes(const char *hexstring, unsigned char *buffer, size_t len)
{
int high_nibble, n;
unsigned val;
high_nibble = 1;
n = 0;
while ( *hexstring && n < len ) {
switch ( *hexstring ) {
case '0':
case '1':
case '2':
case '3':
case '4':
case '5':
case '6':
case '7':
case '8':
case '9':
val = *hexstring - '0';
break;
case 'a':
case 'b':
case 'c':
case 'd':
case 'e':
case 'f':
val = *hexstring - 'a' + 10;
break;
case 'A':
case 'B':
case 'C':
case 'D':
case 'E':
case 'F':
val = *hexstring - 'A' + 10;
break;
default:
hexstring += 1;
continue;
}
if ( high_nibble ) {
buffer[n] = val << 4;
high_nibble = 0;
}
else {
buffer[n] |= (val & 0xF);
high_nibble = 1;
n += 1;
}
hexstring += 1;
}
if ( *hexstring )
return -1; /* Buffer too small. */
return n;
}
/*
* Generate a time-based one-time password as per RFC 6238.
*
* @param algo Libgcrypt identifier of the HMAC algorithm to use.
* @param key Hexstring representation of the key.
* @param time Time in seconds.
* @param step Time step parameter.
* @param digits Number of digits of the OTP to generate
* (must be 6, 7, or 8).
* @param otp Pointer to an unsigned integer to store the
* generated OTP value.
*
* @return 0 if OTP generation was successful, or a gpg_error_t
* error code.
*/
static gpg_error_t
generate_totp(int algo,
const char *key,
time_t time,
unsigned step,
int digits,
unsigned *otp)
{
gcry_mac_hd_t mac;
gpg_error_t e;
unsigned char buffer[MAX_KEY_SIZE];
size_t len;
int offset;
static int modulo[] = { 0, 0, 0, 0, 0, 0, 1000000, 10000000, 100000000 };
if ( (len = hexstring2bytes(key, buffer, sizeof(buffer))) == -1 )
return gcry_error(GPG_ERR_INV_KEYLEN);
if ( ! (e = gcry_mac_open(&mac, algo, 0, NULL)) ) {
if ( ! (e = gcry_mac_setkey(mac, buffer, len)) ) {
(void)time2factor(time, step, buffer, sizeof(buffer));
if ( ! (e = gcry_mac_write(mac, buffer, 8)) ) {
len = sizeof(buffer);
if ( ! (e = gcry_mac_read(mac, buffer, &len)) ) {
offset = buffer[len - 1] & 0xF;
*otp =
((buffer[offset] & 0x7F) << 24) |
((buffer[offset + 1] & 0xFF) << 16) |
((buffer[offset + 2] & 0xFF) << 8) |
(buffer[offset + 3] & 0xFF);
*otp = *otp % modulo[digits];
e = gcry_error(GPG_ERR_NO_ERROR);
}
}
}
gcry_mac_close(mac);
}
return e;
}
struct key_data_t {
unsigned char *buffer;
size_t len;
int found;
};
/*
* Callback for the below function.
*/
static gpg_error_t
get_otp_key_cb(void *arg, const char *line)
{
struct key_data_t *key = (struct key_data_t *)arg;
if ( ! strncmp("PRIVATE-DO-", line, 11) ) {
const char *cursor;
size_t n;
cursor = line + 12;
n = 0;
while ( *cursor != '\n' && n < key->len - 1 ) {
if ( isxdigit(*cursor) )
key->buffer[n++] = *cursor;
cursor += 1;
}
key->buffer[n] = '\0';
key->found = 1;
}
return 0;
}
/*
* Retrieve an OTP key from an inserted OpenPGP smartcard.
*
* @param buffer String buffer to store the retrieved key.
* @param len Size of the output buffer.
*
* @return 0 if successful, or a gpg_error_t error code.
*/
static gpg_error_t
get_otp_key(unsigned privatedo, unsigned char *buffer, size_t len)
{
assuan_context_t ctx;
gpg_error_t e;
struct key_data_t key;
char command[32];
key.buffer = buffer;
key.len = len;
key.found = 0;
if ( privatedo < 1 || privatedo > 4 )
return gcry_error(GPG_ERR_INV_ARG);
if ( ! (e = connect_to_scdaemon(&ctx)) ) {
snprintf(command, sizeof(command), "GETATTR PRIVATE-DO-%d", privatedo);
if ( ! (e = assuan_transact(ctx, command, NULL, NULL,
NULL, NULL, get_otp_key_cb, &key)) ) {
if ( key.found )
e = gcry_error(GPG_ERR_NO_ERROR);
else
e = gcry_error(GPG_ERR_NO_DATA);
}
assuan_release(ctx);
}
return e;
}
static gpg_error_t
get_serial_cb(void *arg, const char *line)
{
char *serial = (char *)arg;
if ( ! strncmp("SERIALNO ", line, 9) ) {
strcpy(serial, line + 9);
return GPG_ERR_NO_ERROR;
}
else
return GPG_ERR_NO_DATA;
}
static gpg_error_t
verify_pin(int admin)
{
assuan_context_t ctx;
gpg_error_t e;
char command[64], serial[64];
char *line;
size_t len;
if ( ! (e = connect_to_agent(&ctx, 1)) ) {
if ( ! (e = assuan_transact(ctx, "SCD GETATTR SERIALNO",
NULL, NULL, NULL, NULL, get_serial_cb, serial)) ) {
snprintf(command, sizeof(command), "SCD CHECKPIN %s%s",
serial, admin ? "[CHV3]" : "");
e = assuan_transact(ctx, command, NULL, NULL, NULL, NULL,
NULL, NULL);
}
assuan_release(ctx);
}
return e;
}
static void
print_otp(unsigned otp, unsigned digits)
{
char fmt[] = "%06d\n";
fmt[2] = digits + '0';
printf(fmt, otp);
}
static unsigned long
get_uinteger_or_die(const char *arg)
{
unsigned long val;
char *endptr;
errno = 0;
val = strtoul(arg, &endptr, 10);
if ( errno != 0 || endptr == arg )
errx(EXIT_FAILURE, "Invalid argument, unsigned integer expected: %s", arg);
return val;
}
int
main(int argc, char **argv)
{
int c, algo;
unsigned char key[MAX_KEY_SIZE * 2 + 1];
unsigned otp, step, digits, privatedo;
time_t secs;
gcry_error_t e;
struct option options[] = {
{ "help", 0, NULL, 'h' },
{ "version", 0, NULL, 'v' },
{ "time", 1, NULL, 't' },
{ "step", 1, NULL, 's' },
{ "digits", 1, NULL, 'd' },
{ "mac-algo", 1, NULL, 'm' },
{ "private-do", 1, NULL, 'n' },
{ NULL, 0, NULL, 0 }
};
setprogname(argv[0]);
secs = time(NULL);
step = DEFAULT_TIME_STEP;
digits = DEFAULT_DIGITS;
algo = GCRY_MAC_HMAC_SHA1;
privatedo = DEFAULT_PRIVATE_DO;
while ( (c = getopt_long(argc, argv, "hvt:s:d:m:n:",
options, NULL)) != -1 ) {
switch ( c ) {
case 'h':
usage(EXIT_SUCCESS);
break;
case '?':
usage(EXIT_FAILURE);
break;
case 'v':
info();
break;
case 't':
secs = get_uinteger_or_die(optarg);
break;
case 's':
step = get_uinteger_or_die(optarg);
break;
case 'd':
digits = get_uinteger_or_die(optarg);
if ( digits < 6 || digits > 8 )
errx(EXIT_FAILURE, "digits must be either 6, 7, or 8");
break;
case 'm':
if ( ! strcmp("sha1", optarg) )
algo = GCRY_MAC_HMAC_SHA1;
else if ( ! strcmp("sha256", optarg) )
algo = GCRY_MAC_HMAC_SHA256;
else if ( ! strcmp("sha512", optarg) )
algo = GCRY_MAC_HMAC_SHA512;
else
errx(EXIT_FAILURE, "unsupported HMAC algorithm: %s", optarg);
break;
case 'n':
privatedo = get_uinteger_or_die(optarg);
if ( privatedo < 1 || privatedo > 4 )
errx(EXIT_FAILURE, "DO number must be 1, 2, 3, or 4");
break;
}
}
if ( ! gcry_check_version(GCRYPT_VERSION) )
errx(EXIT_FAILURE, "libgcrypt version mismatch");
gcry_control(GCRYCTL_DISABLE_SECMEM, 0);
gcry_control(GCRYCTL_INITIALIZATION_FINISHED, 0);
if ( privatedo > 2 && (e = verify_pin(privatedo == 4)) )
errx(EXIT_FAILURE, "Cannot get key from token: %s", gcry_strerror(e));
if ( (e = get_otp_key(privatedo, key, sizeof(key))) )
errx(EXIT_FAILURE, "Cannot get key from token: %s", gcry_strerror(e));
if ( (e = generate_totp(algo, key, secs, step, digits, &otp)) )
errx(EXIT_FAILURE, "Cannot generate OTP: %s", gcry_strerror(e));
print_otp(otp, digits);
return EXIT_SUCCESS;
}