refactor: aggregate arc4 into libtransmission (#3684)

libtransmission/tr-arc4.h

@@ -0,0 +1,80 @@
+// This file Copyright © 2021-2022 Mike Gelfand
+// It may be used under the 3-clause BSD (SPDX: BSD-3-Clause).
+// License text can be found in the licenses/ folder.
+
+#pragma once
+
+#include <array>
+#include <cstddef> // size_t
+#include <cstdint> // uint8_t
+
+/**
+ * This is a tiny and reusable implementation of alleged RC4 cipher.
+ * https://en.wikipedia.org/wiki/RC4
+ *
+ * The use of RC4 is declining due to security concerns.
+ * Popular cryptographic libraries have deprecated or removed it:
+ *
+ *  - OpenSSL: disabled by default in 1.1, moved to "legacy" in 3.0
+ *  - WolfSSL (CyaSSL): disabled by default in 3.4.6
+ *  - MbedTLS (PolarSSL): removed in 3.0
+ *
+ * Nonetheless it's still used in BitTorrent Protocol Encryption
+ * https://en.wikipedia.org/wiki/BitTorrent_protocol_encryption,
+ * so this header file provides an implementation.
+ */
+class tr_arc4
+{
+public:
+    constexpr tr_arc4(void const* key, size_t key_length)
+    {
+        for (size_t i = 0; i < 256; ++i)
+        {
+            s_[i] = static_cast<uint8_t>(i);
+        }
+
+        for (size_t i = 0, j = 0; i < 256; ++i)
+        {
+            j = static_cast<uint8_t>(j + s_[i] + ((uint8_t const*)key)[i % key_length]);
+            arc4_swap(i, j);
+        }
+    }
+
+    constexpr void process(void const* src_data, void* dst_data, size_t data_length)
+    {
+        for (size_t i = 0; i < data_length; ++i)
+        {
+            ((uint8_t*)dst_data)[i] = ((uint8_t const*)src_data)[i] ^ arc4_next();
+        }
+    }
+
+    constexpr void discard(size_t length)
+    {
+        while (length-- > 0)
+        {
+            arc4_next();
+        }
+    }
+
+private:
+    constexpr void arc4_swap(size_t i, size_t j)
+    {
+        auto const tmp = s_[i];
+        s_[i] = s_[j];
+        s_[j] = tmp;
+    }
+
+    constexpr uint8_t arc4_next()
+    {
+        i_ += 1;
+        j_ += s_[i_];
+
+        arc4_swap(i_, j_);
+
+        return s_[static_cast<uint8_t>(s_[i_] + s_[j_])];
+    }
+
+    uint8_t i_ = 0;
+    uint8_t j_ = 0;
+    std::array<uint8_t, 256> s_ = {};
+};