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gnupg/g10/ecdh.c

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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 <https://www.gnu.org/licenses/>.
*/
#include <config.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include "gpg.h"
#include "../common/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 (kek_params, sizeof(kek_params), "ECDH KEK params are");
return gcry_mpi_set_opaque (NULL, kek_params, 4 * 8);
}
/* Extract xcomponent from the point SHARED. POINT_NBYTES is the
size to represent an EC point which is determined by the public
key. SECRET_X_SIZE is the size of x component to represent an
integer which is determined by the curve. */
static gpg_error_t
extract_secret_x (byte **r_secret_x,
const char *shared, size_t nshared,
size_t point_nbytes, size_t secret_x_size)
{
byte *secret_x;
*r_secret_x = NULL;
/* Extract X from the result. It must be in the format of:
04 || X || Y
40 || X
41 || X
Since it may come with the prefix, the size of point is larger
than or equals to the size of an integer X. */
if (point_nbytes < secret_x_size)
return gpg_error (GPG_ERR_BAD_DATA);
/* Extract x component of the shared point: this is the actual
shared secret. */
secret_x = xtrymalloc_secure (point_nbytes);
if (!secret_x)
return gpg_error_from_syserror ();
memcpy (secret_x, shared, nshared);
/* Remove the prefix. */
if ((point_nbytes & 1))
memmove (secret_x, secret_x+1, secret_x_size);
/* Clear the rest of data. */
if (point_nbytes - secret_x_size)
memset (secret_x+secret_x_size, 0, point_nbytes-secret_x_size);
if (DBG_CRYPTO)
log_printhex (secret_x, secret_x_size, "ECDH shared secret X is:");
*r_secret_x = secret_x;
return 0;
}
static gpg_error_t
build_kdf_params (unsigned char kdf_params[256], size_t *r_size,
gcry_mpi_t *pkey, const byte pk_fp[MAX_FINGERPRINT_LEN])
{
IOBUF obuf;
gpg_error_t err;
*r_size = 0;
obuf = iobuf_temp();
if (!obuf)
return gpg_error_from_syserror ();
/* 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);
if (!err)
*r_size = iobuf_temp_to_buffer (obuf, kdf_params, 256);
iobuf_close (obuf);
if (!err)
{
if (DBG_CRYPTO)
log_printhex (kdf_params, *r_size, "ecdh KDF message params are:");
}
return err;
}
/* Derive KEK with KEK_SIZE into the memory at SECRET_X. */
static gpg_error_t
derive_kek (size_t kek_size,
int kdf_hash_algo,
byte *secret_x, int secret_x_size,
const unsigned char *kdf_params, size_t kdf_params_size)
{
gpg_error_t err;
gcry_md_hd_t h;
log_assert( gcry_md_get_algo_dlen (kdf_hash_algo) >= 32 );
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));
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, kdf_params, kdf_params_size); /* KDF parameters */
gcry_md_final (h);
memcpy (secret_x, gcry_md_read (h, kdf_hash_algo), kek_size);
gcry_md_close (h);
/* Clean the tail before returning. */
memset (secret_x+kek_size, 0, secret_x_size - kek_size);
if (DBG_CRYPTO)
log_printhex (secret_x, kek_size, "ecdh KEK is:");
return err;
}
/* Prepare ECDH using SHARED, PK_FP fingerprint, and PKEY array.
Returns the cipher handle in R_HD, which needs to be closed by
the caller. */
static gpg_error_t
prepare_ecdh_with_shared_point (const char *shared, size_t nshared,
const byte pk_fp[MAX_FINGERPRINT_LEN],
gcry_mpi_t *pkey, gcry_cipher_hd_t *r_hd)
{
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 kdf_params[256];
size_t kdf_params_size;
size_t kek_size;
gcry_cipher_hd_t hd;
*r_hd = NULL;
if (!gcry_mpi_get_flag (pkey[2], GCRYMPI_FLAG_OPAQUE))
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 (kek_params, kek_params_size, "ecdh KDF params:");
/* Expect 4 bytes 03 01 hash_alg symm_alg. */
if (kek_params_size != 4 || kek_params[0] != 3 || kek_params[1] != 1)
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)
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)
return gpg_error (GPG_ERR_BAD_PUBKEY);
kek_size = gcry_cipher_get_algo_keylen (kdf_encr_algo);
if (kek_size > gcry_md_get_algo_dlen (kdf_hash_algo))
return gpg_error (GPG_ERR_BAD_PUBKEY);
/* Build kdf_params. */
err = build_kdf_params (kdf_params, &kdf_params_size, pkey, pk_fp);
if (err)
return err;
nbits = pubkey_nbits (PUBKEY_ALGO_ECDH, pkey);
if (!nbits)
return gpg_error (GPG_ERR_TOO_SHORT);
secret_x_size = (nbits+7)/8;
if (kek_size > secret_x_size)
return gpg_error (GPG_ERR_BAD_PUBKEY);
err = extract_secret_x (&secret_x, shared, nshared,
/* pkey[1] is the public point */
(mpi_get_nbits (pkey[1])+7)/8,
secret_x_size);
if (err)
return err;
/*** 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.
*/
/* Derive a KEK (key wrapping key) using SECRET_X and KDF_PARAMS. */
err = derive_kek (kek_size, kdf_hash_algo, secret_x,
secret_x_size, kdf_params, kdf_params_size);
if (err)
{
xfree (secret_x);
return err;
}
/* And, finally, aeswrap with key secret_x. */
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, kek_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));
}
else
*r_hd = hd;
return err;
}
/* Encrypts DATA using a key derived from the ECC shared point SHARED
using the FIPS SP 800-56A compliant method
key_derivation+key_wrapping. 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 (const char *shared, size_t nshared,
const byte pk_fp[MAX_FINGERPRINT_LEN],
const byte *data, size_t ndata,
gcry_mpi_t *pkey, gcry_mpi_t *r_result)
{
gpg_error_t err;
gcry_cipher_hd_t hd;
byte *data_buf;
int data_buf_size;
gcry_mpi_t result;
byte *in;
*r_result = NULL;
err = prepare_ecdh_with_shared_point (shared, nshared, pk_fp, pkey, &hd);
if (err)
return err;
data_buf_size = ndata;
if ((data_buf_size & 7) != 0)
{
log_error ("can't use a shared secret of %d bytes for ecdh\n",
data_buf_size);
gcry_cipher_close (hd);
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;
}
in = data_buf+1+data_buf_size+8;
/* Write data MPI into the end of data_buf. data_buf is size
aeswrap data. */
memcpy (in, data, ndata);
if (DBG_CRYPTO)
log_printhex (in, data_buf_size, "ecdh encrypting :");
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 (data_buf+1, data_buf[0], "ecdh encrypted to:");
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;
return err;
}
static gcry_mpi_t
gen_k (unsigned nbits, int little_endian, int is_opaque)
{
gcry_mpi_t k;
if (is_opaque)
{
unsigned char *p;
size_t nbytes = (nbits+7)/8;
p = gcry_random_bytes_secure (nbytes, GCRY_STRONG_RANDOM);
if ((nbits % 8))
{
if (little_endian)
p[nbytes-1] &= ((1 << (nbits % 8)) - 1);
else
p[0] &= ((1 << (nbits % 8)) - 1);
}
k = gcry_mpi_set_opaque (NULL, p, nbits);
return 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;
int is_little_endian = 0;
int require_opaque = 0;
if (openpgp_oid_is_x448 (pkey[0]))
{
is_little_endian = 1;
require_opaque = 1;
}
*r_k = NULL;
nbits = pubkey_nbits (PUBKEY_ALGO_ECDH, pkey);
if (!nbits)
return gpg_error (GPG_ERR_TOO_SHORT);
k = gen_k (nbits, is_little_endian, require_opaque);
if (!k)
BUG ();
*r_k = k;
return 0;
}
/* Perform ECDH decryption. */
int
pk_ecdh_decrypt (gcry_mpi_t *r_result, const byte sk_fp[MAX_FINGERPRINT_LEN],
gcry_mpi_t data,
const byte *shared, size_t nshared, gcry_mpi_t * skey)
{
gpg_error_t err;
gcry_cipher_hd_t hd;
size_t nbytes;
byte *data_buf;
int data_buf_size;
byte *in;
const void *p;
unsigned int nbits;
*r_result = NULL;
err = prepare_ecdh_with_shared_point (shared, nshared, sk_fp, skey, &hd);
if (err)
return err;
p = gcry_mpi_get_opaque (data, &nbits);
nbytes = (nbits+7)/8;
data_buf_size = nbytes;
if ((data_buf_size & 7) != 1)
{
log_error ("can't use a shared secret of %d bytes for ecdh\n",
data_buf_size);
gcry_cipher_close (hd);
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 (!p)
{
xfree (data_buf);
gcry_cipher_close (hd);
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);
gcry_cipher_close (hd);
return gpg_error (GPG_ERR_BAD_MPI);
}
in = data_buf+data_buf_size;
data_buf_size = data_buf[0];
if (DBG_CRYPTO)
log_printhex (data_buf+1, data_buf_size, "ecdh decrypting :");
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 (in, data_buf_size, "ecdh decrypted to :");
/* 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 (r_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;
}
return err;
}
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