*
* @param public_key Public key to use.
* @param m Plaintext to encrypt.
+ * @param desired_ops How many homomorphic ops the caller intends to use
* @param[out] ciphertext Encrytion of @a plaintext with @a public_key.
- * @return guaranteed number of supported homomorphic operations >= 1, -1 for failure
+ * @return guaranteed number of supported homomorphic operations >= 1,
+ * or desired_ops, in case that is lower,
+ * or -1 if less than one homomorphic operation is possible
*/
int
GNUNET_CRYPTO_paillier_encrypt (const struct GNUNET_CRYPTO_PaillierPublicKey *public_key,
const gcry_mpi_t m,
+ int desired_ops,
struct GNUNET_CRYPTO_PaillierCiphertext *ciphertext)
{
int possible_opts;
GNUNET_assert (NULL != (tmp1 = gcry_mpi_set_ui (NULL, 1)));
GNUNET_assert (NULL != (tmp2 = gcry_mpi_set_ui (NULL, 2)));
gcry_mpi_mul_2exp (tmp1, tmp1, GNUNET_CRYPTO_PAILLIER_BITS);
-
+
+ // count number of possible operations
+ // this would be nicer with gcry_mpi_get_nbits, however it does not return
+ // the BITLENGTH of the given MPI's value, but the bits required
+ // to represent the number as MPI.
for (possible_opts = -2; gcry_mpi_cmp (tmp1, m) > 0; possible_opts++) {
gcry_mpi_div (tmp1, NULL, tmp1, tmp2, 0);
}
gcry_mpi_release (tmp1);
gcry_mpi_release (tmp2);
+
if (possible_opts < 1)
- return -1;
- else
- // reduce by one to guarantee the final homomorphic operation
- ciphertext->remaining_ops = htonl (possible_opts);
+ possible_opts = 0;
+ //soft-cap by caller
+ possible_opts = (desired_ops < possible_opts)? desired_ops : possible_opts;
+
+ ciphertext->remaining_ops = htonl (possible_opts);
GNUNET_assert (0 != (n_square = gcry_mpi_new (0)));
GNUNET_assert (0 != (r = gcry_mpi_new (0)));
GNUNET_CRYPTO_mpi_scan_unsigned (&lambda, private_key->lambda, sizeof private_key->lambda);
GNUNET_CRYPTO_mpi_scan_unsigned (&mu, private_key->mu, sizeof private_key->mu);
GNUNET_CRYPTO_mpi_scan_unsigned (&n, public_key, sizeof *public_key);
- GNUNET_CRYPTO_mpi_scan_unsigned (&c, ciphertext, sizeof *ciphertext);
+ GNUNET_CRYPTO_mpi_scan_unsigned (&c, ciphertext->bits, sizeof ciphertext->bits);
gcry_mpi_mul (n_square, n, n);
// m = c^lambda mod n^2