Add the Arc4 PRNG draft implementation

Chaos/Cipher/Arc4.hpp

@@ -1,9 +1,112 @@
 #ifndef CHAOS_CIPHER_ARC4_HPP
 #define CHAOS_CIPHER_ARC4_HPP
 
+#include <array>
+#include <cstdint>
+#include <vector>
+
+#include "Service/ChaosException.hpp"
+
 namespace Chaos::Cipher::Arc4
 {
 
+class Arc4Gen
+{
+public:
+    Arc4Gen()
+        : IsInitialized_(false)
+    { }
+
+    template<typename InputIt>
+    Arc4Gen(InputIt keyBegin, InputIt keyEnd)
+    {
+        RekeyImpl(keyBegin, keyEnd);
+    }
+
+    template<typename InputIt>
+    void Rekey(InputIt keyBegin, InputIt keyEnd)
+    {
+        RekeyImpl(keyBegin, keyEnd);
+    }
+
+    template<typename OutputIt>
+    void Generate(OutputIt out, uint64_t bytesCount)
+    {
+        EnsureInitialized();
+
+        for (uint64_t cnt = 0; cnt < bytesCount; ++cnt)
+        {
+            Step();
+            *out++ = Lookup_[static_cast<uint8_t>(Lookup_[I_] + Lookup_[J_])];
+        }
+    }
+
+    void Drop(uint64_t bytesCount)
+    {
+        EnsureInitialized();
+
+        for (uint64_t cnt = 0; cnt < bytesCount; ++cnt)
+        {
+            Step();
+        }
+    }
+
+private:
+    bool IsInitialized_;
+
+    uint8_t I_;
+    uint8_t J_;
+    std::array<uint8_t, 256> Lookup_;
+
+    void EnsureInitialized() const
+    {
+        if (!IsInitialized_)
+        {
+            throw Service::ChaosException("Arc4Gen: not initialized");
+        }
+    }
+
+    template<typename InputIt>
+    void RekeyImpl(InputIt keyBegin, InputIt keyEnd)
+    {
+        I_ = 0;
+        J_ = 0;
+
+        for (uint64_t idx = 0; idx < Lookup_.size(); ++idx)
+        {
+            Lookup_[idx] = static_cast<uint8_t>(idx);
+        }
+
+        std::vector<uint8_t> key(keyBegin, keyEnd);
+
+        if (key.size() < 5)
+        {
+            throw Service::ChaosException("Arc4Gen: key is too small");
+        }
+
+        uint8_t a = 0;
+        uint8_t b = 0;
+
+        for (uint64_t idx = 0; idx < Lookup_.size(); ++idx)
+        {
+            a = static_cast<uint8_t>(idx);
+            b = b + Lookup_[a] + key[a % key.size()];
+
+            std::swap(Lookup_[a], Lookup_[b]);
+        }
+
+        IsInitialized_ = true;
+    }
+
+    void Step()
+    {
+        I_ = I_ + 1;
+        J_ = J_ + Lookup_[I_];
+
+        std::swap(Lookup_[I_], Lookup_[J_]);
+    }
+};
+
 } // namespace Chaos::Cipher::Arc4
 
 #endif