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_plugin.c

@@ -52,6 +52,7 @@
 #include "openssl_pkcs12.h"
 #include "openssl_rng.h"
 #include "openssl_hmac.h"
+#include "openssl_kdf.h"
 #include "openssl_aead.h"
 #include "openssl_x_diffie_hellman.h"
 #include "openssl_ed_public_key.h"
@@ -654,6 +655,11 @@ METHOD(plugin_t, get_features, int,
 			PLUGIN_PROVIDE(SIGNER, AUTH_HMAC_SHA2_512_256),
 			PLUGIN_PROVIDE(SIGNER, AUTH_HMAC_SHA2_512_512),
 #endif
+#if OPENSSL_VERSION_NUMBER >= 0x10101000L
+		/* HKDF is available since 1.1.0, expand-only mode only since 1.1.1 */
+		PLUGIN_REGISTER(KDF, openssl_kdf_create),
+			PLUGIN_PROVIDE(KDF, KDF_PRF_PLUS),
+#endif
 #endif /* OPENSSL_NO_HMAC */
 #if (OPENSSL_VERSION_NUMBER >= 0x1000100fL && !defined(OPENSSL_NO_AES)) || \
 	(OPENSSL_VERSION_NUMBER >= 0x1010000fL && !defined(OPENSSL_NO_CHACHA))