EVP: Implement support for key downgrading in backends

Downgrading EVP_PKEYs from containing provider side internal keys to
containing legacy keys demands support in the EVP_PKEY_ASN1_METHOD.

This became a bit elaborate because the code would be almost exactly
the same as the import functions int EVP_KEYMGMT.  Therefore, we end
up moving most of the code to common backend support files that can be
used both by legacy backend code and by our providers.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11375)

1 files changed, 229 insertions, 0 deletions

diff --git a/crypto/ec/ec_backend.c b/crypto/ec/ec_backend.c
new file mode 100644
index 0000000000..b4520a7c60
--- /dev/null
+++ b/crypto/ec/ec_backend.c
@@ -0,0 +1,229 @@
+/*
+ * Copyright 2020 The OpenSSL Project Authors. All Rights Reserved.
+ *
+ * Licensed under the Apache License 2.0 (the "License").  You may not use
+ * this file except in compliance with the License.  You can obtain a copy
+ * in the file LICENSE in the source distribution or at
+ * https://www.openssl.org/source/license.html
+ */
+
+#include <openssl/core_names.h>
+#include <openssl/objects.h>
+#include <openssl/params.h>
+#include "crypto/bn.h"
+#include "crypto/ec.h"
+
+/*
+ * The intention with the "backend" source file is to offer backend support
+ * for legacy backends (EVP_PKEY_ASN1_METHOD and EVP_PKEY_METHOD) and provider
+ * implementations alike.
+ */
+
+int ec_set_param_ecdh_cofactor_mode(EC_KEY *ec, const OSSL_PARAM *p)
+{
+    const EC_GROUP *ecg = EC_KEY_get0_group(ec);
+    const BIGNUM *cofactor;
+    int mode;
+
+    if (!OSSL_PARAM_get_int(p, &mode))
+        return 0;
+
+    /*
+     * mode can be only 0 for disable, or 1 for enable here.
+     *
+     * This is in contrast with the same parameter on an ECDH EVP_PKEY_CTX that
+     * also supports mode == -1 with the meaning of "reset to the default for
+     * the associated key".
+     */
+    if (mode < 0 || mode > 1)
+        return 0;
+
+    if ((cofactor = EC_GROUP_get0_cofactor(ecg)) == NULL )
+        return 0;
+
+    /* ECDH cofactor mode has no effect if cofactor is 1 */
+    if (BN_is_one(cofactor))
+        return 1;
+
+    if (mode == 1)
+        EC_KEY_set_flags(ec, EC_FLAG_COFACTOR_ECDH);
+    else if (mode == 0)
+        EC_KEY_clear_flags(ec, EC_FLAG_COFACTOR_ECDH);
+
+    return 1;
+}
+
+/*
+ * Callers of ec_key_fromdata MUST make sure that ec_key_params_fromdata has
+ * been called before!
+ *
+ * This function only gets the bare keypair, domain parameters and other
+ * parameters are treated separately, and domain parameters are required to
+ * define a keypair.
+ */
+int ec_key_fromdata(EC_KEY *ec, const OSSL_PARAM params[], int include_private)
+{
+    const OSSL_PARAM *param_priv_key, *param_pub_key;
+    BN_CTX *ctx = NULL;
+    BIGNUM *priv_key = NULL;
+    unsigned char *pub_key = NULL;
+    size_t pub_key_len;
+    const EC_GROUP *ecg = NULL;
+    EC_POINT *pub_point = NULL;
+    int ok = 0;
+
+    ecg = EC_KEY_get0_group(ec);
+    if (ecg == NULL)
+        return 0;
+
+    param_priv_key =
+        OSSL_PARAM_locate_const(params, OSSL_PKEY_PARAM_PRIV_KEY);
+    param_pub_key =
+        OSSL_PARAM_locate_const(params, OSSL_PKEY_PARAM_PUB_KEY);
+
+    ctx = BN_CTX_new_ex(ec_key_get_libctx(ec));
+    if (ctx == NULL)
+        goto err;
+    /*
+     * We want to have at least a public key either way, so we end up
+     * requiring it unconditionally.
+     */
+    if (param_pub_key == NULL
+            || !OSSL_PARAM_get_octet_string(param_pub_key,
+                                            (void **)&pub_key, 0, &pub_key_len)
+            || (pub_point = EC_POINT_new(ecg)) == NULL
+            || !EC_POINT_oct2point(ecg, pub_point,
+                                   pub_key, pub_key_len, ctx))
+        goto err;
+
+    if (param_priv_key != NULL && include_private) {
+        int fixed_words;
+        const BIGNUM *order;
+
+        /*
+         * 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.
+         */
+        order = EC_GROUP_get0_order(ecg);
+        if (order == NULL || BN_is_zero(order))
+            goto err;
+
+        fixed_words = bn_get_top(order) + 2;
+
+        if ((priv_key = BN_secure_new()) == NULL)
+            goto err;
+        if (bn_wexpand(priv_key, fixed_words) == NULL)
+            goto err;
+        BN_set_flags(priv_key, BN_FLG_CONSTTIME);
+
+        if (!OSSL_PARAM_get_BN(param_priv_key, &priv_key))
+            goto err;
+    }
+
+    if (priv_key != NULL
+            && !EC_KEY_set_private_key(ec, priv_key))
+        goto err;
+
+    if (!EC_KEY_set_public_key(ec, pub_point))
+        goto err;
+
+    ok = 1;
+
+ err:
+    BN_CTX_free(ctx);
+    BN_clear_free(priv_key);
+    OPENSSL_free(pub_key);
+    EC_POINT_free(pub_point);
+    return ok;
+}
+
+int ec_key_domparams_fromdata(EC_KEY *ec, const OSSL_PARAM params[])
+{
+    const OSSL_PARAM *param_ec_name;
+    EC_GROUP *ecg = NULL;
+    char *curve_name = NULL;
+    int ok = 0;
+
+    if (ec == NULL)
+        return 0;
+
+    param_ec_name = OSSL_PARAM_locate_const(params, OSSL_PKEY_PARAM_EC_NAME);
+    if (param_ec_name == NULL) {
+        /* explicit parameters */
+
+        /*
+         * TODO(3.0): should we support explicit parameters curves?
+         */
+        return 0;
+    } else {
+        /* named curve */
+        int curve_nid;
+
+        if (!OSSL_PARAM_get_utf8_string(param_ec_name, &curve_name, 0)
+                || curve_name == NULL
+                || (curve_nid = ec_curve_name2nid(curve_name)) == NID_undef)
+            goto err;
+
+        if ((ecg = EC_GROUP_new_by_curve_name_ex(ec_key_get_libctx(ec),
+                                                 curve_nid)) == NULL)
+            goto err;
+    }
+
+    if (!EC_KEY_set_group(ec, ecg))
+        goto err;
+
+    /*
+     * TODO(3.0): if the group has changed, should we invalidate the private and
+     * public key?
+     */
+
+    ok = 1;
+
+ err:
+    OPENSSL_free(curve_name);
+    EC_GROUP_free(ecg);
+    return ok;
+}
+
+int ec_key_otherparams_fromdata(EC_KEY *ec, const OSSL_PARAM params[])
+{
+    const OSSL_PARAM *p;
+
+    if (ec == NULL)
+        return 0;
+
+    p = OSSL_PARAM_locate_const(params, OSSL_PKEY_PARAM_USE_COFACTOR_ECDH);
+    if (p != NULL && !ec_set_param_ecdh_cofactor_mode(ec, p))
+        return 0;
+
+    return 1;
+}