openssl: Add a prf+ implementation based on OpenSSL's HKDF implementation

The HKDF-Expand() function defined in RFC 5869 is basically the same as
IKEv2's prf+(), so we can use the former to implement the latter.
However, we can only support HMAC-based PRFs this way, which should be
fine as others are rarely used.

src/libstrongswan/plugins/openssl/openssl_kdf.h

@@ -0,0 +1,45 @@
+/*
+ * Copyright (C) 2022 Tobias Brunner, codelabs GmbH
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+/**
+ * Implements key derivation functions (KDF) via OpenSSL, in particular prf+,
+ * which is implemented via OpenSSL's HKDF implementation.
+ *
+ * @defgroup openssl_kdf openssl_kdf
+ * @{ @ingroup openssl_p
+ */
+
+#ifndef OPENSSL_KDF_H_
+#define OPENSSL_KDF_H_
+
+#include <crypto/kdfs/kdf.h>
+
+/**
+ * Creates a new kdf_t object.
+ *
+ * @param algo		algorithm to instantiate
+ * @param args		algorithm-specific arguments
+ * @return			kdf_t object, NULL if not supported
+ */
+kdf_t *openssl_kdf_create(key_derivation_function_t algo, va_list args);
+
+#endif /** OPENSSL_KDF_H_ @}*/