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/* ecdh.c - ECDH public key operations used in public key glue code
* Copyright (C) 2010 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/>.
*/
#include <config.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <assert.h>
#include "gpg.h"
#include "util.h"
#include "pkglue.h"
#include "main.h"
#include "options.h"
gcry_mpi_t
pk_ecdh_default_params_to_mpi (int qbits)
{
gpg_error_t err;
gcry_mpi_t result;
/* Defaults are the strongest possible choices. Performance is not
an issue here, only interoperability. */
byte kek_params[4] = {
3 /*size of following field*/,
1 /*fixed version for KDF+AESWRAP*/,
DIGEST_ALGO_SHA512 /* KEK MD */,
CIPHER_ALGO_AES256 /*KEK AESWRAP alg*/
};
int i;
static const struct {
int qbits;
int openpgp_hash_id;
int openpgp_cipher_id;
} kek_params_table[] = {
{ 256, DIGEST_ALGO_SHA256, CIPHER_ALGO_AES },
{ 384, DIGEST_ALGO_SHA384, CIPHER_ALGO_AES256 },
/* Note: 528 is 521 rounded to the 8 bit boundary */
{ 528, DIGEST_ALGO_SHA512, CIPHER_ALGO_AES256 }
};
for (i=0; i<sizeof(kek_params_table)/sizeof(kek_params_table[0]); i++)
{
if (kek_params_table[i].qbits >= qbits)
{
kek_params[2] = kek_params_table[i].openpgp_hash_id;
kek_params[3] = kek_params_table[i].openpgp_cipher_id;
break;
}
}
if (DBG_CIPHER)
log_printhex ("ecdh kek params are", kek_params, sizeof(kek_params) );
err = gcry_mpi_scan (&result, GCRYMPI_FMT_USG,
kek_params, sizeof(kek_params), NULL);
if (err)
log_fatal ("mpi_scan failed: %s\n", gpg_strerror (err));
return result;
}
/* Returns allocated (binary) KEK parameters; the size is returned in
* sizeout. The caller must free the returned value with xfree.
* Returns NULL on error.
*/
byte *
pk_ecdh_default_params (int qbits, size_t *sizeout)
{
/* Defaults are the strongest possible choices. Performance is not
an issue here, only interoperability. */
byte kek_params[4] = {
3 /*size of following field*/,
1 /*fixed version for KDF+AESWRAP*/,
DIGEST_ALGO_SHA512 /* KEK MD */,
CIPHER_ALGO_AES256 /* KEK AESWRAP alg */
};
int i;
static const struct {
int qbits;
int openpgp_hash_id;
int openpgp_cipher_id;
} kek_params_table[] = {
{ 256, DIGEST_ALGO_SHA256, CIPHER_ALGO_AES },
{ 384, DIGEST_ALGO_SHA384, CIPHER_ALGO_AES256 },
/* Note: 528 is 521 rounded to the 8 bit boundary */
{ 528, DIGEST_ALGO_SHA512, CIPHER_ALGO_AES256 }
};
byte *p;
*sizeout = 0;
for (i=0; i<sizeof(kek_params_table)/sizeof(kek_params_table[0]); i++)
{
if (kek_params_table[i].qbits >= qbits)
{
kek_params[2] = kek_params_table[i].openpgp_hash_id;
kek_params[3] = kek_params_table[i].openpgp_cipher_id;
break;
}
}
if (DBG_CIPHER )
log_printhex ("ecdh kek params are", kek_params, sizeof(kek_params));
p = xtrymalloc (sizeof(kek_params));
if (!p)
return NULL;
memcpy (p, kek_params, sizeof(kek_params));
*sizeout = sizeof(kek_params);
return p;
}
/* Encrypts/decrypts 'data' with a key derived from shared_mpi ECC
* point using FIPS SP 800-56A compliant method, which is key
* derivation + key wrapping. The direction is determined by the first
* parameter (is_encrypt=1 --> this is encryption). The result is
* returned in out as a size+value MPI.
*
* TODO: memory leaks (x_secret).
*/
static int
pk_ecdh_encrypt_with_shared_point (int is_encrypt, gcry_mpi_t shared_mpi,
const byte pk_fp[MAX_FINGERPRINT_LEN],
gcry_mpi_t data, gcry_mpi_t *pkey,
gcry_mpi_t *out)
{
byte *secret_x;
int secret_x_size;
byte kdf_params[256];
int kdf_params_size=0;
int nbits;
int kdf_hash_algo;
int kdf_encr_algo;
int rc;
*out = NULL;
nbits = pubkey_nbits( PUBKEY_ALGO_ECDH, pkey );
{
size_t nbytes;
/* Extract x component of the shared point: this is the actual
shared secret */
nbytes = (mpi_get_nbits (pkey[1] /* public point */)+7)/8;
secret_x = xmalloc_secure( nbytes );
rc = gcry_mpi_print (GCRYMPI_FMT_USG, secret_x, nbytes,
&nbytes, shared_mpi);
if (rc)
{
xfree (secret_x);
log_error ("ec ephemeral export of shared point failed: %s\n",
gpg_strerror (rc));
return rc;
}
secret_x_size = (nbits+7)/8;
assert (nbytes > secret_x_size);
memmove (secret_x, secret_x+1, secret_x_size);
memset (secret_x+secret_x_size, 0, nbytes-secret_x_size);
if (DBG_CIPHER)
log_printhex ("ecdh shared secret X is:", secret_x, secret_x_size );
}
/*** We have now the shared secret bytes in secret_x. ***/
/* At this point we are done with PK encryption and the rest of the
* function uses symmetric key encryption techniques to protect the
* input 'data'. The following two sections will simply replace
* current secret_x with a value derived from it. This will become
* a KEK.
*/
{
IOBUF obuf = iobuf_temp();
rc = iobuf_write_size_body_mpi ( obuf, pkey[2] ); /* KEK params */
kdf_params_size = iobuf_temp_to_buffer (obuf,
kdf_params, sizeof(kdf_params));
if (DBG_CIPHER)
log_printhex ("ecdh KDF public key params are:",
kdf_params, kdf_params_size );
/* Expect 4 bytes 03 01 hash_alg symm_alg. */
if (kdf_params_size != 4 || kdf_params[0] != 3 || kdf_params[1] != 1)
return GPG_ERR_BAD_PUBKEY;
kdf_hash_algo = kdf_params[2];
kdf_encr_algo = kdf_params[3];
if (DBG_CIPHER)
log_debug ("ecdh KDF algorithms %s+%s with aeswrap\n",
gcry_md_algo_name (kdf_hash_algo),
openpgp_cipher_algo_name (kdf_encr_algo));
if (kdf_hash_algo != GCRY_MD_SHA256
&& kdf_hash_algo != GCRY_MD_SHA384
&& kdf_hash_algo != GCRY_MD_SHA512)
return GPG_ERR_BAD_PUBKEY;
if (kdf_encr_algo != GCRY_CIPHER_AES128
&& kdf_encr_algo != GCRY_CIPHER_AES192
&& kdf_encr_algo != GCRY_CIPHER_AES256)
return GPG_ERR_BAD_PUBKEY;
}
/* Build kdf_params. */
{
IOBUF obuf;
obuf = iobuf_temp();
/* variable-length field 1, curve name OID */
rc = iobuf_write_size_body_mpi ( obuf, pkey[0] );
/* fixed-length field 2 */
iobuf_put (obuf, PUBKEY_ALGO_ECDH);
/* variable-length field 3, KDF params */
rc = (rc ? rc : iobuf_write_size_body_mpi ( obuf, pkey[2] ));
/* fixed-length field 4 */
iobuf_write (obuf, "Anonymous Sender ", 20);
/* fixed-length field 5, recipient fp */
iobuf_write (obuf, pk_fp, 20);
kdf_params_size = iobuf_temp_to_buffer (obuf,
kdf_params, sizeof(kdf_params));
iobuf_close (obuf);
if (rc)
return rc;
if(DBG_CIPHER)
log_printhex ("ecdh KDF message params are:",
kdf_params, kdf_params_size );
}
/* Derive a KEK (key wrapping key) using kdf_params and secret_x. */
{
gcry_md_hd_t h;
int old_size;
rc = gcry_md_open (&h, kdf_hash_algo, 0);
if(rc)
log_bug ("gcry_md_open failed for algo %d: %s",
kdf_hash_algo, gpg_strerror (gcry_error(rc)));
gcry_md_write(h, "\x00\x00\x00\x01", 4); /* counter = 1 */
gcry_md_write(h, secret_x, secret_x_size); /* x of the point X */
gcry_md_write(h, kdf_params, kdf_params_size); /* KDF parameters */
gcry_md_final (h);
assert( gcry_md_get_algo_dlen (kdf_hash_algo) >= 32 );
memcpy (secret_x, gcry_md_read (h, kdf_hash_algo),
gcry_md_get_algo_dlen (kdf_hash_algo));
gcry_md_close (h);
old_size = secret_x_size;
assert( old_size >= gcry_cipher_get_algo_keylen( kdf_encr_algo ) );
secret_x_size = gcry_cipher_get_algo_keylen( kdf_encr_algo );
assert( secret_x_size <= gcry_md_get_algo_dlen (kdf_hash_algo) );
/* We could have allocated more, so clean the tail before returning. */
memset( secret_x+secret_x_size, old_size-secret_x_size, 0 );
if (DBG_CIPHER)
log_printhex ("ecdh KEK is:", secret_x, secret_x_size );
}
/* And, finally, aeswrap with key secret_x. */
{
gcry_cipher_hd_t hd;
size_t nbytes;
byte *data_buf;
int data_buf_size;
gcry_mpi_t result;
rc = gcry_cipher_open (&hd, kdf_encr_algo, GCRY_CIPHER_MODE_AESWRAP, 0);
if (rc)
{
log_error ("ecdh failed to initialize AESWRAP: %s\n",
gpg_strerror (rc));
return rc;
}
rc = gcry_cipher_setkey (hd, secret_x, secret_x_size);
xfree( secret_x );
if (rc)
{
gcry_cipher_close (hd);
log_error ("ecdh failed in gcry_cipher_setkey: %s\n",
gpg_strerror (rc));
return rc;
}
data_buf_size = (gcry_mpi_get_nbits(data)+7)/8;
assert ((data_buf_size & 7) == (is_encrypt ? 0 : 1));
data_buf = xtrymalloc_secure( 1 + 2*data_buf_size + 8);
if (!data_buf)
{
gcry_cipher_close (hd);
return GPG_ERR_ENOMEM;
}
if (is_encrypt)
{
byte *in = data_buf+1+data_buf_size+8;
/* Write data MPI into the end of data_buf. data_buf is size
aeswrap data. */
rc = gcry_mpi_print (GCRYMPI_FMT_USG, in,
data_buf_size, &nbytes, data/*in*/);
if (rc)
{
log_error ("ecdh failed to export DEK: %s\n", gpg_strerror (rc));
gcry_cipher_close (hd);
xfree (data_buf);
return rc;
}
if (DBG_CIPHER)
log_printhex ("ecdh encrypting :", in, data_buf_size );
rc = gcry_cipher_encrypt (hd, data_buf+1, data_buf_size+8,
in, data_buf_size);
memset (in, 0, data_buf_size);
gcry_cipher_close (hd);
if (rc)
{
log_error ("ecdh failed in gcry_cipher_encrypt: %s\n",
gpg_strerror (rc));
xfree (data_buf);
return rc;
}
data_buf[0] = data_buf_size+8;
if (DBG_CIPHER)
log_printhex ("ecdh encrypted to:", data_buf+1, data_buf[0] );
rc = gcry_mpi_scan (&result, GCRYMPI_FMT_USG,
data_buf, 1+data_buf[0], NULL);
/* (byte)size + aeswrap of DEK */
xfree( data_buf );
if (rc)
{
log_error ("ecdh failed to create an MPI: %s\n", gpg_strerror (rc));
return rc;
}
*out = result;
}
else
{
byte *in;
rc = gcry_mpi_print (GCRYMPI_FMT_USG, data_buf, data_buf_size,
&nbytes, data/*in*/);
if (nbytes != data_buf_size || data_buf[0] != data_buf_size-1)
{
log_error ("ecdh inconsistent size\n");
xfree (data_buf);
return GPG_ERR_BAD_MPI;
}
in = data_buf+data_buf_size;
data_buf_size = data_buf[0];
if (DBG_CIPHER)
log_printhex ("ecdh decrypting :", data_buf+1, data_buf_size);
rc = gcry_cipher_decrypt (hd, in, data_buf_size, data_buf+1,
data_buf_size);
gcry_cipher_close (hd);
if (rc)
{
log_error ("ecdh failed in gcry_cipher_decrypt: %s\n",
gpg_strerror (rc));
xfree (data_buf);
return rc;
}
data_buf_size -= 8;
if (DBG_CIPHER)
log_printhex ("ecdh decrypted to :", in, data_buf_size);
/* Padding is removed later. */
/* if (in[data_buf_size-1] > 8 ) */
/* { */
/* log_error("ecdh failed at decryption: invalid padding. %02x > 8\n", */
/* in[data_buf_size-1] ); */
/* return GPG_ERR_BAD_KEY; */
/* } */
rc = gcry_mpi_scan ( &result, GCRYMPI_FMT_USG, in, data_buf_size, NULL);
xfree (data_buf);
if (rc)
{
log_error ("ecdh failed to create a plain text MPI: %s\n",
gpg_strerror (rc));
return rc;
}
*out = result;
}
}
return rc;
}
static gcry_mpi_t
gen_k (unsigned nbits)
{
gcry_mpi_t k;
k = gcry_mpi_snew (nbits);
if (DBG_CIPHER)
log_debug ("choosing a random k of %u bits\n", nbits);
gcry_mpi_randomize (k, nbits-1, GCRY_STRONG_RANDOM);
if (DBG_CIPHER)
{
unsigned char *buffer;
if (gcry_mpi_aprint (GCRYMPI_FMT_HEX, &buffer, NULL, k))
BUG ();
log_debug("ephemeral scalar MPI #0: %s\n", buffer);
gcry_free( buffer );
}
return k;
}
/* Perform ECDH encryption, which involves ECDH key generation. */
int
pk_ecdh_encrypt (gcry_mpi_t *resarr, const byte pk_fp[MAX_FINGERPRINT_LEN],
gcry_mpi_t data, gcry_mpi_t * pkey)
{
gcry_sexp_t s_ciph, s_data, s_pkey;
int nbits;
int rc;
gcry_mpi_t k;
nbits = pubkey_nbits (PUBKEY_ALGO_ECDH, pkey);
/*** Generate an ephemeral key, actually, a scalar. ***/
k = gen_k (nbits);
if( k == NULL )
BUG ();
/*** Done with ephemeral key generation.
* Now use ephemeral secret to get the shared secret. ***/
rc = gcry_sexp_build (&s_pkey, NULL,
"(public-key(ecdh(c%m)(q%m)(p%m)))",
pkey[0], pkey[1], pkey[2]);
if (rc)
BUG ();
/* Put the data into a simple list. */
/* Ephemeral scalar goes as data. */
if (gcry_sexp_build (&s_data, NULL, "%m", k))
BUG ();
/* Pass it to libgcrypt. */
rc = gcry_pk_encrypt (&s_ciph, s_data, s_pkey);
gcry_sexp_release (s_data);
gcry_sexp_release (s_pkey);
if (rc)
return rc;
/* Finally, perform encryption. */
{
/* ... and get the shared point/ */
gcry_mpi_t shared;
shared = mpi_from_sexp (s_ciph, "a");
gcry_sexp_release (s_ciph);
/* Ephemeral public key. */
resarr[0] = mpi_from_sexp (s_ciph, "b");
if (DBG_CIPHER)
{
unsigned char *buffer;
if (gcry_mpi_aprint (GCRYMPI_FMT_HEX, &buffer, NULL, resarr[0]))
BUG ();
log_debug("ephemeral key MPI: %s\n", buffer);
gcry_free( buffer );
}
rc = pk_ecdh_encrypt_with_shared_point (1 /*=encrypton*/, shared,
pk_fp, data, pkey, resarr+1);
mpi_release (shared);
}
return rc;
}
/* Perform ECDH decryption. */
int
pk_ecdh_decrypt (gcry_mpi_t * result, const byte sk_fp[MAX_FINGERPRINT_LEN],
gcry_mpi_t data, gcry_mpi_t shared, gcry_mpi_t * skey)
{
if (!data)
return gpg_error (GPG_ERR_BAD_MPI);
return pk_ecdh_encrypt_with_shared_point (0 /*=decryption*/, shared,
sk_fp, data/*encr data as an MPI*/,
skey, result);
}

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