diff --git a/Userland/Libraries/LibCrypto/Forward.h b/Userland/Libraries/LibCrypto/Forward.h
new file mode 100644
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+++ b/Userland/Libraries/LibCrypto/Forward.h
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+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Concepts.h>
+
+namespace Crypto {
+
+class SignedBigInteger;
+class UnsignedBigInteger;
+
+template<typename T>
+concept BigInteger = IsSame<T, SignedBigInteger> || IsSame<T, UnsignedBigInteger>;
+
+}