+ if (priv_key != NULL) {
+ size_t sz;
+ int ecbits;
+ int ecdh_cofactor_mode;
+
+ /*
+ * Key import/export should never leak the bit length of the secret
+ * scalar in the key.
+ *
+ * For this reason, on export we use padded BIGNUMs with fixed length.
+ *
+ * When importing we also should make sure that, even if short lived,
+ * the newly created BIGNUM is marked with the BN_FLG_CONSTTIME flag as
+ * soon as possible, so that any processing of this BIGNUM might opt for
+ * constant time implementations in the backend.
+ *
+ * Setting the BN_FLG_CONSTTIME flag alone is never enough, we also have
+ * to preallocate the BIGNUM internal buffer to a fixed public size big
+ * enough that operations performed during the processing never trigger
+ * a realloc which would leak the size of the scalar through memory
+ * accesses.
+ *
+ * Fixed Length
+ * ------------
+ *
+ * The order of the large prime subgroup of the curve is our choice for
+ * a fixed public size, as that is generally the upper bound for
+ * generating a private key in EC cryptosystems and should fit all valid
+ * secret scalars.
+ *
+ * For padding on export we just use the bit length of the order
+ * converted to bytes (rounding up).
+ *
+ * For preallocating the BIGNUM storage we look at the number of "words"
+ * required for the internal representation of the order, and we
+ * preallocate 2 extra "words" in case any of the subsequent processing
+ * might temporarily overflow the order length.
+ */
+ ecbits = EC_GROUP_order_bits(ecg);
+ if (ecbits <= 0)
+ goto err;
+
+ sz = (ecbits + 7 ) / 8;
+ if (!OSSL_PARAM_BLD_push_BN_pad(tmpl,
+ OSSL_PKEY_PARAM_PRIV_KEY,
+ priv_key, sz))
+ goto err;
+ selection |= OSSL_KEYMGMT_SELECT_PRIVATE_KEY;
+
+ /*
+ * The ECDH Cofactor Mode is defined only if the EC_KEY actually
+ * contains a private key, so we check for the flag and export it only
+ * in this case.
+ */
+ ecdh_cofactor_mode =
+ (EC_KEY_get_flags(eckey) & EC_FLAG_COFACTOR_ECDH) ? 1 : 0;
+
+ /* Export the ECDH_COFACTOR_MODE parameter */
+ if (!OSSL_PARAM_BLD_push_int(tmpl,
+ OSSL_PKEY_PARAM_USE_COFACTOR_ECDH,
+ ecdh_cofactor_mode))
+ goto err;
+ selection |= OSSL_KEYMGMT_SELECT_OTHER_PARAMETERS;
+ }
+
+ params = OSSL_PARAM_BLD_to_param(tmpl);
+
+ /* We export, the provider imports */
+ rv = evp_keymgmt_import(to_keymgmt, to_keydata, selection, params);