openssl: Fixes for DH with OpenSSL 3.0

While we could assign the DH object to a EVP_PKEY object, this won't work
with BoringSSL as it doesn't seem to support EVP_PKEY_derive() for DH.

src/libstrongswan/plugins/openssl/openssl_util.c

@@ -22,6 +22,11 @@
 #include <openssl/evp.h>
 #include <openssl/x509.h>
 
+#if OPENSSL_VERSION_NUMBER >= 0x30000000L
+/* for EVP_PKEY_CTX_set_dh_pad */
+#include <openssl/dh.h>
+#endif
+
 /* these were added with 1.1.0 when ASN1_OBJECT was made opaque */
 #if OPENSSL_VERSION_NUMBER < 0x10100000L
 #define OBJ_get0_data(o) ((o)->data)
@@ -48,6 +53,14 @@ bool openssl_compute_shared_key(EVP_PKEY *priv, EVP_PKEY *pub, chunk_t *shared)
 		goto error;
 	}
 
+#if OPENSSL_VERSION_NUMBER >= 0x30000000L
+	if (EVP_PKEY_base_id(priv) == EVP_PKEY_DH &&
+		EVP_PKEY_CTX_set_dh_pad(ctx, 1) <= 0)
+	{
+		goto error;
+	}
+#endif
+
 	if (EVP_PKEY_derive_set_peer(ctx, pub) <= 0)
 	{
 		goto error;