From daf50ed88521dc6e61cd80e57dc00f6a3d3d13bf Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?kleines=20Filmr=C3=B6llchen?= <filmroellchen@serenityos.org>
Date: Thu, 18 May 2023 15:57:19 +0200
Subject: [PATCH] LibCrypto: Add generic 8-bit CRC

The implementation of this is naive enough so it can handle all 8-bit
CRC polynomials, of which there are quite a few. The table generation
and update procedure is MSB first, which is backwards from the LSB first
method of CRC32.
---
 Userland/Libraries/LibCrypto/Checksum/CRC8.h | 76 ++++++++++++++++++++
 1 file changed, 76 insertions(+)
 create mode 100644 Userland/Libraries/LibCrypto/Checksum/CRC8.h

diff --git a/Userland/Libraries/LibCrypto/Checksum/CRC8.h b/Userland/Libraries/LibCrypto/Checksum/CRC8.h
new file mode 100644
index 0000000000..8850162c97
--- /dev/null
+++ b/Userland/Libraries/LibCrypto/Checksum/CRC8.h
@@ -0,0 +1,76 @@
+/*
+ * Copyright (c) 2023, kleines Filmröllchen <filmroellchen@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Types.h>
+#include <LibCrypto/Checksum/ChecksumFunction.h>
+
+namespace Crypto::Checksum {
+
+// A generic 8-bit Cyclic Redundancy Check.
+// Note that as opposed to CRC32, this class operates with MSB first, so the polynomial must not be reversed.
+// For example, the polynomial x⁸ + x² + x + 1 is represented as 0x07 and not 0xE0.
+template<u8 polynomial>
+class CRC8 : public ChecksumFunction<u8> {
+public:
+    // This is a big endian table, while CRC-32 uses a little endian table.
+    static constexpr auto generate_table()
+    {
+        Array<u8, 256> data {};
+        u8 value = 0x80;
+        auto i = 1u;
+        do {
+            if ((value & 0x80) != 0) {
+                value = polynomial ^ (value << 1);
+            } else {
+                value = value << 1;
+            }
+
+            for (auto j = 0u; j < i; ++j) {
+                data[i + j] = value ^ data[j];
+            }
+            i <<= 1;
+        } while (i < 256);
+        return data;
+    }
+
+    static constexpr auto table = generate_table();
+
+    virtual ~CRC8() = default;
+
+    CRC8() = default;
+    CRC8(ReadonlyBytes data)
+    {
+        update(data);
+    }
+
+    CRC8(u8 initial_state, ReadonlyBytes data)
+        : m_state(initial_state)
+    {
+        update(data);
+    }
+
+    // FIXME: This implementation is naive and slow.
+    //        Figure out how to adopt the slicing-by-8 algorithm (see CRC32) for 8 bit polynomials.
+    virtual void update(ReadonlyBytes data) override
+    {
+        for (size_t i = 0; i < data.size(); i++) {
+            size_t table_index = (m_state ^ data.at(i)) & 0xFF;
+            m_state = (table[table_index] ^ (static_cast<u32>(m_state) << 8)) & 0xFF;
+        }
+    }
+
+    virtual u8 digest() override
+    {
+        return m_state;
+    }
+
+private:
+    u8 m_state { 0 };
+};
+
+}