초·중·고 현대 암호학 교육과 국방 전술 폐쇄망 무결점 보안 터널(VPN): ZeliDesk의 온디바이스 순수 C# WireGuard 암호 프로토콜, Curve25519 고속 디피-헬만(ECDH), ChaCha20-Poly1305 인증 암호화, 및 10Gbps 제로-트러스트 전술 가상 사설망 아키텍처

using System;
using System.Buffers.Binary;
using System.Numerics;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

namespace ZeliDesk.Core.Security.WireGuard
{
    public static class ChaCha20Engine
    {
        private static readonly uint[] Sigma = { 0x61707865, 0x3320646e, 0x79622d32, 0x6b206574 }; // "expand 32-byte k"

        [MethodImpl(MethodImplOptions.AggressiveOptimization)]
        public static void EncryptBlock(
            ReadOnlySpan key32, 
            uint counter, 
            ReadOnlySpan nonce12, 
            ReadOnlySpan plaintext, 
            Span ciphertext)
        {
            Span state = stackalloc uint[16];
            state[0] = Sigma[0]; state[1] = Sigma[1]; state[2] = Sigma[2]; state[3] = Sigma[3];
            
            for (int i = 0; i < 8; i++)
                state[4 + i] = BinaryPrimitives.ReadUInt32LittleEndian(key32.Slice(i * 4, 4));

            state[12] = counter;
            state[13] = BinaryPrimitives.ReadUInt32LittleEndian(nonce12.Slice(0, 4));
            state[14] = BinaryPrimitives.ReadUInt32LittleEndian(nonce12.Slice(4, 4));
            state[15] = BinaryPrimitives.ReadUInt32LittleEndian(nonce12.Slice(8, 4));

            Span working = stackalloc uint[16];
            state.CopyTo(working);

            // 20 라운드 (10회 더블 라운드)
            for (int i = 0; i < 10; i++)
            {
                QuarterRound(ref working[0], ref working[4], ref working[8], ref working[12]);
                QuarterRound(ref working[1], ref working[5], ref working[9], ref working[13]);
                QuarterRound(ref working[2], ref working[6], ref working[10], ref working[14]);
                QuarterRound(ref working[3], ref working[7], ref working[11], ref working[15]);

                QuarterRound(ref working[0], ref working[5], ref working[10], ref working[15]);
                QuarterRound(ref working[1], ref working[6], ref working[11], ref working[12]);
                QuarterRound(ref working[2], ref working[7], ref working[8], ref working[13]);
                QuarterRound(ref working[3], ref working[4], ref working[9], ref working[14]);
            }

            Span keyStream = stackalloc byte[64];
            for (int i = 0; i < 16; i++)
                BinaryPrimitives.WriteUInt32LittleEndian(keyStream.Slice(i * 4, 4), working[i] + state[i]);

            int bytesToXor = Math.Min(plaintext.Length, 64);
            for (int i = 0; i < bytesToXor; i++)
                ciphertext[i] = (byte)(plaintext[i] ^ keyStream[i]);
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static void QuarterRound(ref uint a, ref uint b, ref uint c, ref uint d)
        {
            a += b; d = BitOperations.RotateLeft(d ^ a, 16);
            c += d; b = BitOperations.RotateLeft(b ^ c, 12);
            a += b; d = BitOperations.RotateLeft(d ^ a, 8);
            c += d; b = BitOperations.RotateLeft(b ^ c, 7);
        }
    }
}