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gpg: Clean up ECDH code path (1).

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* g10/ecdh.c (extract_secret_x): New.
(pk_ecdh_encrypt_with_shared_point): Use extract_secret_x.

Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>
This commit is contained in:
NIIBE Yutaka 2020-05-22 10:04:35 +09:00
parent 30eef28bc0
commit 960d37644c
1 changed files with 61 additions and 49 deletions
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108
g10/ecdh.c
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@ -82,6 +82,60 @@ pk_ecdh_default_params (unsigned int qbits)
}
/* Extract xcomponent from the point SHARED_MPI. 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, gcry_mpi_t shared_mpi,
size_t point_nbytes, size_t secret_x_size)
{
gpg_error_t err;
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 ();
err = gcry_mpi_print (GCRYMPI_FMT_USG, secret_x, point_nbytes,
&point_nbytes, shared_mpi);
if (err)
{
xfree (secret_x);
log_error ("ECDH ephemeral export of shared point failed: %s\n",
gpg_strerror (err));
return err;
}
/* 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 err;
}
/* 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
@ -112,55 +166,13 @@ pk_ecdh_encrypt_with_shared_point (int is_encrypt, gcry_mpi_t shared_mpi,
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 (secret_x, secret_x_size, "ECDH shared secret X is:");
}
err = extract_secret_x (&secret_x, shared_mpi,
/* 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. ***/