LibCrypto: Add support for the POSIX cksum algorithm

Userland/Libraries/LibCrypto/Checksum/cksum.h

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) 2024, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Span.h>
+#include <AK/Types.h>
+#include <LibCrypto/Checksum/ChecksumFunction.h>
+
+namespace Crypto::Checksum {
+
+class cksum : public ChecksumFunction<u32> {
+public:
+    cksum() = default;
+    cksum(ReadonlyBytes data)
+    {
+        update(data);
+    }
+
+    virtual void update(ReadonlyBytes data) override;
+    virtual u32 digest() override;
+
+private:
+    u32 m_state { 0 };
+    off_t m_size { 0 };
+};
+
+}