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/* ecdh.c - ECDH public key operations used in public key glue code
* Copyright (C) 2010, 2011 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 "gpg.h"
#include "util.h"
#include "pkglue.h"
#include "main.h"
#include "options.h"
/* A table with the default KEK parameters used by GnuPG. */
static const struct
{
unsigned int qbits;
int openpgp_hash_id; /* KEK digest algorithm. */
int openpgp_cipher_id; /* KEK cipher algorithm. */
} kek_params_table[] =
/* Note: Must be sorted by ascending values for QBITS. */
{
{ 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 }
};
/* Return KEK parameters as an opaque MPI The caller must free the
returned value. Returns NULL and sets ERRNO on error. */
gcry_mpi_t
pk_ecdh_default_params (unsigned int qbits)
{
byte *kek_params;
int i;
kek_params = xtrymalloc (4);
if (!kek_params)
return NULL;
kek_params[0] = 3; /* Number of bytes to follow. */
kek_params[1] = 1; /* Version for KDF+AESWRAP. */
/* Search for matching KEK parameter. Defaults to the strongest
possible choices. Performance is not an issue here, only
interoperability. */
for (i=0; i < DIM (kek_params_table); i++)
{
if (kek_params_table[i].qbits >= qbits
|| i+1 == DIM (kek_params_table))
{
kek_params[2] = kek_params_table[i].openpgp_hash_id;
kek_params[3] = kek_params_table[i].openpgp_cipher_id;
break;
}
}
log_assert (i < DIM (kek_params_table));
if (DBG_CRYPTO)
log_printhex ("ECDH KEK params are", kek_params, sizeof(kek_params) );
return gcry_mpi_set_opaque (NULL, kek_params, 4 * 8);
}
/* Encrypts/decrypts DATA using a key derived from the ECC shared
point SHARED_MPI using the FIPS SP 800-56A compliant method
key_derivation+key_wrapping. If IS_ENCRYPT is true the function
encrypts; if false, it decrypts. PKEY is the public key and PK_FP
the fingerprint of this public key. On success the result is
stored at R_RESULT; on failure NULL is stored at R_RESULT and an
error code returned. */
gpg_error_t
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 *r_result)
{
gpg_error_t err;
byte *secret_x;
int secret_x_size;
unsigned int nbits;
const unsigned char *kek_params;
size_t kek_params_size;
int kdf_hash_algo;
int kdf_encr_algo;
unsigned char message[256];
size_t message_size;
*r_result = NULL;
nbits = pubkey_nbits (PUBKEY_ALGO_ECDH, pkey);
if (!nbits)
return gpg_error (GPG_ERR_TOO_SHORT);
{
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 = xtrymalloc_secure (nbytes);
if (!secret_x)
return gpg_error_from_syserror ();
err = gcry_mpi_print (GCRYMPI_FMT_USG, secret_x, nbytes,
&nbytes, shared_mpi);
if (err)
{
xfree (secret_x);
log_error ("ECDH ephemeral export of shared point failed: %s\n",
gpg_strerror (err));
return err;
}
/* Expected size of the x component */
secret_x_size = (nbits+7)/8;
/* Extract X from the result. It must be in the format of:
04 || X || Y
40 || X
41 || X
Since it always comes with the prefix, it's larger than X. In
old experimental version of libgcrypt, there is a case where it
returns X with no prefix of 40, so, nbytes == secret_x_size
is allowed. */
if (nbytes < secret_x_size)
{
xfree (secret_x);
return gpg_error (GPG_ERR_BAD_DATA);
}
/* Remove the prefix. */
if ((nbytes & 1))
memmove (secret_x, secret_x+1, secret_x_size);
/* Clear the rest of data. */
if (nbytes - secret_x_size)
memset (secret_x+secret_x_size, 0, nbytes-secret_x_size);
if (DBG_CRYPTO)
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.
*/
if (!gcry_mpi_get_flag (pkey[2], GCRYMPI_FLAG_OPAQUE))
{
xfree (secret_x);
return gpg_error (GPG_ERR_BUG);
}
kek_params = gcry_mpi_get_opaque (pkey[2], &nbits);
kek_params_size = (nbits+7)/8;
if (DBG_CRYPTO)
log_printhex ("ecdh KDF params:", kek_params, kek_params_size);
/* Expect 4 bytes 03 01 hash_alg symm_alg. */
if (kek_params_size != 4 || kek_params[0] != 3 || kek_params[1] != 1)
{
xfree (secret_x);
return gpg_error (GPG_ERR_BAD_PUBKEY);
}
kdf_hash_algo = kek_params[2];
kdf_encr_algo = kek_params[3];
if (DBG_CRYPTO)
log_debug ("ecdh KDF algorithms %s+%s with aeswrap\n",
openpgp_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)
{
xfree (secret_x);
return gpg_error (GPG_ERR_BAD_PUBKEY);
}
if (kdf_encr_algo != CIPHER_ALGO_AES
&& kdf_encr_algo != CIPHER_ALGO_AES192
&& kdf_encr_algo != CIPHER_ALGO_AES256)
{
xfree (secret_x);
return gpg_error (GPG_ERR_BAD_PUBKEY);
}
/* Build kdf_params. */
{
IOBUF obuf;
obuf = iobuf_temp();
/* variable-length field 1, curve name OID */
err = gpg_mpi_write_nohdr (obuf, pkey[0]);
/* fixed-length field 2 */
iobuf_put (obuf, PUBKEY_ALGO_ECDH);
/* variable-length field 3, KDF params */
err = (err ? err : gpg_mpi_write_nohdr (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);
message_size = iobuf_temp_to_buffer (obuf, message, sizeof message);
iobuf_close (obuf);
if (err)
{
xfree (secret_x);
return err;
}
if(DBG_CRYPTO)
log_printhex ("ecdh KDF message params are:", message, message_size);
}
/* Derive a KEK (key wrapping key) using MESSAGE and SECRET_X. */
{
gcry_md_hd_t h;
int old_size;
err = gcry_md_open (&h, kdf_hash_algo, 0);
if (err)
{
log_error ("gcry_md_open failed for kdf_hash_algo %d: %s",
kdf_hash_algo, gpg_strerror (err));
xfree (secret_x);
return err;
}
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, message, message_size); /* KDF parameters */
gcry_md_final (h);
log_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;
log_assert( old_size >= gcry_cipher_get_algo_keylen( kdf_encr_algo ) );
secret_x_size = gcry_cipher_get_algo_keylen( kdf_encr_algo );
log_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, 0, old_size - secret_x_size);
if (DBG_CRYPTO)
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;
err = gcry_cipher_open (&hd, kdf_encr_algo, GCRY_CIPHER_MODE_AESWRAP, 0);
if (err)
{
log_error ("ecdh failed to initialize AESWRAP: %s\n",
gpg_strerror (err));
xfree (secret_x);
return err;
}
err = gcry_cipher_setkey (hd, secret_x, secret_x_size);
xfree (secret_x);
secret_x = NULL;
if (err)
{
gcry_cipher_close (hd);
log_error ("ecdh failed in gcry_cipher_setkey: %s\n",
gpg_strerror (err));
return err;
}
data_buf_size = (gcry_mpi_get_nbits(data)+7)/8;
if ((data_buf_size & 7) != (is_encrypt ? 0 : 1))
{
log_error ("can't use a shared secret of %d bytes for ecdh\n",
data_buf_size);
return gpg_error (GPG_ERR_BAD_DATA);
}
data_buf = xtrymalloc_secure( 1 + 2*data_buf_size + 8);
if (!data_buf)
{
err = gpg_error_from_syserror ();
gcry_cipher_close (hd);
return err;
}
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. */
err = gcry_mpi_print (GCRYMPI_FMT_USG, in,
data_buf_size, &nbytes, data/*in*/);
if (err)
{
log_error ("ecdh failed to export DEK: %s\n", gpg_strerror (err));
gcry_cipher_close (hd);
xfree (data_buf);
return err;
}
if (DBG_CRYPTO)
log_printhex ("ecdh encrypting :", in, data_buf_size );
err = 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 (err)
{
log_error ("ecdh failed in gcry_cipher_encrypt: %s\n",
gpg_strerror (err));
xfree (data_buf);
return err;
}
data_buf[0] = data_buf_size+8;
if (DBG_CRYPTO)
log_printhex ("ecdh encrypted to:", data_buf+1, data_buf[0] );
result = gcry_mpi_set_opaque (NULL, data_buf, 8 * (1+data_buf[0]));
if (!result)
{
err = gpg_error_from_syserror ();
xfree (data_buf);
log_error ("ecdh failed to create an MPI: %s\n",
gpg_strerror (err));
return err;
}
*r_result = result;
}
else
{
byte *in;
const void *p;
p = gcry_mpi_get_opaque (data, &nbits);
nbytes = (nbits+7)/8;
if (!p || nbytes > data_buf_size || !nbytes)
{
xfree (data_buf);
return gpg_error (GPG_ERR_BAD_MPI);
}
memcpy (data_buf, p, nbytes);
if (data_buf[0] != nbytes-1)
{
log_error ("ecdh inconsistent size\n");
xfree (data_buf);
return gpg_error (GPG_ERR_BAD_MPI);
}
in = data_buf+data_buf_size;
data_buf_size = data_buf[0];
if (DBG_CRYPTO)
log_printhex ("ecdh decrypting :", data_buf+1, data_buf_size);
err = gcry_cipher_decrypt (hd, in, data_buf_size, data_buf+1,
data_buf_size);
gcry_cipher_close (hd);
if (err)
{
log_error ("ecdh failed in gcry_cipher_decrypt: %s\n",
gpg_strerror (err));
xfree (data_buf);
return err;
}
data_buf_size -= 8;
if (DBG_CRYPTO)
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." */
/* " 0x%02x > 8\n", in[data_buf_size-1] ); */
/* return gpg_error (GPG_ERR_BAD_KEY); */
/* } */
err = gcry_mpi_scan (&result, GCRYMPI_FMT_USG, in, data_buf_size, NULL);
xfree (data_buf);
if (err)
{
log_error ("ecdh failed to create a plain text MPI: %s\n",
gpg_strerror (err));
return err;
}
*r_result = result;
}
}
return err;
}
static gcry_mpi_t
gen_k (unsigned nbits)
{
gcry_mpi_t k;
k = gcry_mpi_snew (nbits);
if (DBG_CRYPTO)
log_debug ("choosing a random k of %u bits\n", nbits);
gcry_mpi_randomize (k, nbits-1, GCRY_STRONG_RANDOM);
if (DBG_CRYPTO)
{
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;
}
/* Generate an ephemeral key for the public ECDH key in PKEY. On
success the generated key is stored at R_K; on failure NULL is
stored at R_K and an error code returned. */
gpg_error_t
pk_ecdh_generate_ephemeral_key (gcry_mpi_t *pkey, gcry_mpi_t *r_k)
{
unsigned int nbits;
gcry_mpi_t k;
*r_k = NULL;
nbits = pubkey_nbits (PUBKEY_ALGO_ECDH, pkey);
if (!nbits)
return gpg_error (GPG_ERR_TOO_SHORT);
k = gen_k (nbits);
if (!k)
BUG ();
*r_k = k;
return 0;
}
/* 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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