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, 56 insertions, 0 deletions

diff --git a/crypto/dh/dh_backend.c b/crypto/dh/dh_backend.c
new file mode 100644
index 0000000000..bbeb096d55
--- /dev/null
+++ b/crypto/dh/dh_backend.c
@@ -0,0 +1,56 @@
+/*
+ * 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 "crypto/dh.h"
+
+/*
+ * The intention with the "backend" source file is to offer backend functions
+ * for legacy backends (EVP_PKEY_ASN1_METHOD and EVP_PKEY_METHOD) and provider
+ * implementations alike.
+ */
+
+int dh_key_fromdata(DH *dh, const OSSL_PARAM params[])
+{
+    const OSSL_PARAM *param_priv_key, *param_pub_key;
+    BIGNUM *priv_key = NULL, *pub_key = NULL;
+
+    if (dh == 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);
+
+    /*
+     * DH documentation says that a public key must be present if a
+     * private key is present.
+     * We want to have at least a public key either way, so we end up
+     * requiring it unconditionally.
+     */
+    if (param_priv_key != NULL && param_pub_key == NULL)
+        return 0;
+
+    if ((param_priv_key != NULL
+         && !OSSL_PARAM_get_BN(param_priv_key, &priv_key))
+        || (param_pub_key != NULL
+            && !OSSL_PARAM_get_BN(param_pub_key, &pub_key)))
+        goto err;
+
+    if (!DH_set0_key(dh, pub_key, priv_key))
+        goto err;
+
+    return 1;
+
+ err:
+    BN_clear_free(priv_key);
+    BN_free(pub_key);
+    return 0;
+}