Mengenal_Blockchain.ipynb

{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Ilustrasi Cara Kerja Blockchain\n",
    "\n",
    "Blockchain adalah sebuah immutable linked list. Maksudnya struktur dari blockchain menyerupai linkedlist akan tetapi data dalam tiap block pada blockchain tidak dapat diubah. Bagaimana mekanismenya sehingga blockchain ini immutable akan terjawab jika kita memahami cara kerja dari blockchain."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Linked List\n",
    "\n",
    "Pertama kita harus paham dulu apa itu linked list. Kalau pernah belajar bahasa C, pasti familiar dengan linked list. Tidak seperti bahasa pemrograman lainnya, C hampir tidak punya fitur apa-apa jika dibandingkan dengan bahasa pemrograman modern high level seperti Java, Python, Ruby dll. C tidak punya vector atau array yang ukurannya dinamis (bisa di tambah panjang elemennya), tapi C punya struct dan malloc (memory allocation) untuk memungkinkan membuat yang mirip dengan itu. \n",
    "\n",
    "Jadi kita declare suatu struct, declare variabel yang ingin kita masukkan disitu dan terakhir variabel alamat memori yang menunjukkan element selanjutnya. Sampai disitu kita punya elemen pertama. Lalu ketika kita ingin membuat elemen kedua, kita pakai malloc untuk alokasi memori keperluan struct baru ini. Kita update variabel alamat memori di elemen pertama dengan alamat memori struct elemen kedua. Begitu seterusnya ketika membuat elemen ketiga, kita update variabel alamat memori elemen kedua dengan alamat memori struct elemen ketiga.\n",
    "\n",
    "![linkedlist](Linkedlist.png)\n",
    "\n"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Linked List Versi BlockChain\n",
    "\n",
    "Linked List Blockchain yang asli cukup kompleks dan bisa berbeda-beda sesuai kebutuhan akan sistem yang dibangun. Di sini akan dijelaskan bentuk blockchain yang umum dan disederhanakan tapi tetap dapat pointnya kenapa blockchain bersifat immutable.\n",
    "\n",
    "![Struktur_Blockchain](Blockchain-Structure.png)\n",
    "\n",
    "Dalam structnya secara umum terdiri dari hash dari block sebelumnya (seluruh data pada struct atau block sebelumnya di masukkan ke fungsi hash, biasanya yang digunakan adalah sha-256), Timestamp kapan block tersebut dibuat, data lainnya yang diperlukan untuk keperluan transaksi, dan terakhir tentu saja alamat memori atau pointer yang menunjukkan block selanjutnya.\n",
    "\n",
    "Hash inilah yang menjadi rahasia kenapa blockchain 1 dari 2 alasan kenapa blockchain immutable. <b>Blockchain bukannya tidak bisa dimanipulasi isi data didalamnya, tapi ketika isi datanya dimanipulasi kita tahu itu</b>. Dan kalau kita tahu bahwa suatu informasi itu palsu atau sudah dimanipulasi, kita bisa mengabaikan informasi yang tidak valid tersebut atau hal lainnya tergantung terhadap kepentingan blockchain digunakan. <b>Alasan kedua kenapa blockchain menjadi immutable adalah konsep Proof of Work (PoW)</b>. Kita akan bahas lebih lanjut lagi setelah hash.\n",
    "\n",
    "Untuk mempermudah memahami, ayo kita ilustrasikan. Ilustrasi disini kita memakai Python. Kita tidak akan menggunakan linked list, melainkan list akan tetapi esensi dari blockchain yang ingin disampaikan akan tetap sama. Jadi, misal kita mempunyai data murid yang ingin namanya dimasukkan dalam blockchain, sebut saja Andi, Budi, Dodi, Ani, dan Mike. Jika kita masukkan ini ke Blockchain sesuai urutannya, maka dalam blockchain akan tampak sebagai berikut."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "{'index': 1, 'timestamp': 1576601984.0738986, 'nama': 'kosong_5939365', 'previous_hash': 1}\n",
      "\n",
      "{'index': 2, 'timestamp': 1576601984.0739753, 'nama': 'kosong_1805286', 'previous_hash': '1a86df1e2bc3de51a7e442b8cecee1d5dbef19143ab9498c0e370804a2457cfa'}\n",
      "\n",
      "{'index': 3, 'timestamp': 1576601984.0740345, 'nama': 'kosong_6825513', 'previous_hash': 'bfb82e94f4c68ad72333d5a818c60f90e0588acf1e30f81da2c06f3bc13730ab'}\n",
      "\n",
      "{'index': 4, 'timestamp': 1576601984.0740786, 'nama': 'Andi', 'previous_hash': '3f803a47576faca495d30a6a964e7010d859871d18d26d28e311c0e04d558c0c'}\n",
      "\n",
      "{'index': 5, 'timestamp': 1576601984.07412, 'nama': 'Budi', 'previous_hash': '6194b19c1016cf658816601f9bd93316f1888c64cc15ad3d13d0311bf83c25fd'}\n",
      "\n",
      "{'index': 6, 'timestamp': 1576601984.0741613, 'nama': 'Dodi', 'previous_hash': 'ac991268f4d33306885916f5316a623b886da84903b62db857bc1563ca4a07c8'}\n",
      "\n",
      "{'index': 7, 'timestamp': 1576601984.0742023, 'nama': 'Ani', 'previous_hash': '68419bb46f9038325f13b18698e0f5e304b0f33417e467ae41783631510f2d9a'}\n",
      "\n",
      "{'index': 8, 'timestamp': 1576601984.0742424, 'nama': 'Mike', 'previous_hash': 'bffb551a79b83475598a031b624b8d252d32b4245e3de7ef159e4e6a69e9af97'}\n",
      "\n",
      "{'index': 9, 'timestamp': 1576601984.0742977, 'nama': 'kosong_3171842', 'previous_hash': '60e3c7fe95beb03923626f51affb5909ef8ffd1fc0b7c6027ffb87e365931f7d'}\n",
      "\n",
      "{'index': 10, 'timestamp': 1576601984.0743487, 'nama': 'kosong_8097977', 'previous_hash': '589ca29859d0fe992d5dc2dcfcb097e6c7c3b713bdbcc9a88d5cb57b9acf79f3'}\n",
      "\n",
      "{'index': 11, 'timestamp': 1576601984.0744023, 'nama': 'kosong_5496030', 'previous_hash': '2b28243091cd74233120c4897fda1fb2ca2921e5da2318c9e2b6144ba907a458'}\n",
      "\n"
     ]
    }
   ],
   "source": [
    "#import numpy as np\n",
    "import hashlib\n",
    "from time import time\n",
    "from random import randrange\n",
    "\n",
    "global_chain = []\n",
    "\n",
    "#setting\n",
    "#chain_len = 10\n",
    "\n",
    "def create_block(nama):\n",
    "    if len(global_chain) == 0:\n",
    "        previous_hash = 1\n",
    "    else:\n",
    "#        data_previous = '%s/%s/%s/%s'%(global_chain[-1]['index'], global_chain[-1]['timestamp'],\n",
    "#                                     global_chain[-1]['nama'],global_chain[-1]['previous_hash'])\n",
    "        previous_hash = gen_hash(global_chain[-1])\n",
    "    block = {\n",
    "            'index': len(global_chain) + 1,\n",
    "            'timestamp': time(),\n",
    "            'nama': nama,\n",
    "            'previous_hash': previous_hash\n",
    "            }\n",
    "    global_chain.append(block)\n",
    "\n",
    "def gen_hash(block):\n",
    "    data_str = '%s/%s/%s/%s'%(block['index'], block['timestamp'], block['nama'],block['previous_hash'])\n",
    "    data_hash = hashlib.sha256(data_str.encode('utf8')).hexdigest()\n",
    "    return data_hash\n",
    "    \n",
    "\n",
    "#print(len(global_chain))\n",
    "#print(hashlib.sha256('a'.encode('utf8')).hexdigest())\n",
    "create_block('kosong_%s'%(randrange(9999999)))\n",
    "create_block('kosong_%s'%(randrange(9999999)))\n",
    "create_block('kosong_%s'%(randrange(9999999)))\n",
    "create_block('Andi')\n",
    "create_block('Budi')\n",
    "create_block('Dodi')\n",
    "create_block('Ani')\n",
    "create_block('Mike')\n",
    "create_block('kosong_%s'%(randrange(9999999)))\n",
    "create_block('kosong_%s'%(randrange(9999999)))\n",
    "create_block('kosong_%s'%(randrange(9999999)))\n",
    "for i in range(len(global_chain)):\n",
    "    print(global_chain[i])\n",
    "    print()"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Nama Murid-murid tersebut pada blockchain tersimpan di block index 4 - 8. Dengan hashnya masing-masing berada pada block didepannya."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1 1\n",
      "\n",
      "2 1a86df1e2bc3de51a7e442b8cecee1d5dbef19143ab9498c0e370804a2457cfa\n",
      "\n",
      "3 bfb82e94f4c68ad72333d5a818c60f90e0588acf1e30f81da2c06f3bc13730ab\n",
      "\n",
      "4 3f803a47576faca495d30a6a964e7010d859871d18d26d28e311c0e04d558c0c\n",
      "\n",
      "5 6194b19c1016cf658816601f9bd93316f1888c64cc15ad3d13d0311bf83c25fd\n",
      "\n",
      "6 ac991268f4d33306885916f5316a623b886da84903b62db857bc1563ca4a07c8\n",
      "\n",
      "7 68419bb46f9038325f13b18698e0f5e304b0f33417e467ae41783631510f2d9a\n",
      "\n",
      "8 bffb551a79b83475598a031b624b8d252d32b4245e3de7ef159e4e6a69e9af97\n",
      "\n",
      "9 60e3c7fe95beb03923626f51affb5909ef8ffd1fc0b7c6027ffb87e365931f7d\n",
      "\n",
      "10 589ca29859d0fe992d5dc2dcfcb097e6c7c3b713bdbcc9a88d5cb57b9acf79f3\n",
      "\n",
      "11 2b28243091cd74233120c4897fda1fb2ca2921e5da2318c9e2b6144ba907a458\n",
      "\n"
     ]
    }
   ],
   "source": [
    "for i in range(len(global_chain)):\n",
    "    print(global_chain[i]['index'], global_chain[i]['previous_hash'])\n",
    "    print()"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Misal ada orang yang iseng ingin memalsukan blockchain dengan mengganti nama Budi menjadi Dedi. Maka yang terjadi adalah, ketika block data denga nama Dedi tersebut di hash, hasilnya hashnya akan berbeda dengan isi variabel block selanjutnya yang berisi previous_hash. Berikut ilustrasinya"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "percobaan manipulasi blockchain\n",
      "nama palsu: Dedi \n",
      "dengan hash -> 1694db221d5b808feec42943ed01f3e12d0e64349f943a359752236eeb924084\n",
      "hash seharusnya -> ac991268f4d33306885916f5316a623b886da84903b62db857bc1563ca4a07c8\n"
     ]
    }
   ],
   "source": [
    "global_chain_fake = global_chain.copy()\n",
    "global_chain_fake[4]['nama'] = 'Dedi'\n",
    "\n",
    "fake_hash = gen_hash(global_chain_fake[4])\n",
    "print('percobaan manipulasi blockchain')\n",
    "print('nama palsu: %s \\ndengan hash -> %s\\nhash seharusnya -> %s'%(global_chain_fake[4]['nama'],\n",
    "    fake_hash, global_chain_fake[5]['previous_hash']))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "blockchainnya menjadi tidak valid karena hash dari block index 5 berbeda dengan previous_hash pada block index 6. <b>Keluaran dari suatu hash function adalah unik untuk setiap kombinasi</b> walaupun masih ada peluang terjadinya collision, tapi itu benar-benar sangat sangat kecil. Ini artinya, ketika kita mengubah suatu blok, hash blok lainnya yang di depannya akan ikut berubah semua juga. Selain hash, PoW yang belum kita bahas juga harus berubah.\n",
    "\n",
    "Apa itu PoW? <b>PoW adalah variabel kedua yang menjadi validator bahwa suatu blokcian valid atau tidak</b>. PoW adalah suatu bilangan yang sifatnya sulit dicari tapi mudah untuk divalidasi. Misalnya, <b>carilah suatu bilangan X dimana ketika bilangan tersebut dianggap suatu string dan digabungkan dengan variabel previous_hash dengan format\n",
    "\n",
    "Xprevious_hash\n",
    "\n",
    "maka hash dari gabungan string ini akan menghasilkan hash yang ujungnya adalah 'a'</b>. Untuk ukuran PoW, Kasus bilangan ini sangat mudah dan cepat dicari komputer. Ini  dilakukan agar komputer yang dipakai ketika mempraktekkan tutorial ini tidak hang, toh ini cuma ilustrasi aja. Ok, mari kita ilustrasikan dengan menambah variabel PoW pada dict script blockchain kita dan menambah beberapa fungsi untuk mencari bilangan PoW dan validasinya.\n"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "{'index': 1, 'timestamp': 1576602026.3941288, 'PoW': 5, 'nama': 'kosong_9773681', 'previous_hash': 1}\n",
      "\n",
      "{'index': 2, 'timestamp': 1576602026.3947663, 'PoW': 81, 'nama': 'kosong_4830037', 'previous_hash': 'caaae0c4e4dc4681e91d43c563e9420674fa1112234c055a41081d60c3ac77b1'}\n",
      "\n",
      "{'index': 3, 'timestamp': 1576602026.3952703, 'PoW': 58, 'nama': 'kosong_3361986', 'previous_hash': 'cf9b2076f0ba204577e0d090aad31da06467db6e8a0d36045d0b9390d671af87'}\n",
      "\n",
      "{'index': 4, 'timestamp': 1576602026.3953836, 'PoW': 2, 'nama': 'Andi', 'previous_hash': '973bcfaff9f4feafb3088b384c6b243cf7107ca2617e38e1401341341f73f0b6'}\n",
      "\n",
      "{'index': 5, 'timestamp': 1576602026.3954916, 'PoW': 4, 'nama': 'Budi', 'previous_hash': '8f7954f79cecc081aa925c68d9092a6a9e02338088536c559cbb0f1d427a60e7'}\n",
      "\n",
      "{'index': 6, 'timestamp': 1576602026.3957129, 'PoW': 19, 'nama': 'Dodi', 'previous_hash': '6fbc240b76c97a5cdea31ffb9fa3b784ee48473f466a63fd998131e36d43dbb7'}\n",
      "\n",
      "{'index': 7, 'timestamp': 1576602026.3961165, 'PoW': 50, 'nama': 'Ani', 'previous_hash': '17658ce093205bc12d3ce3cfe6fb79c9e1d3da59f7a64fcf6f5df8e3a16cf160'}\n",
      "\n",
      "{'index': 8, 'timestamp': 1576602026.396743, 'PoW': 74, 'nama': 'Mike', 'previous_hash': 'c0c6a6554e55d06499307c998ca4a38607ac85a37bb16e95b7364a11b596f7af'}\n",
      "\n",
      "{'index': 9, 'timestamp': 1576602026.396992, 'PoW': 19, 'nama': 'kosong_3604790', 'previous_hash': '21df0d47e4073ea1454fc6b085069ae032771200c16852f40282071580254bf1'}\n",
      "\n",
      "{'index': 10, 'timestamp': 1576602026.3971822, 'PoW': 13, 'nama': 'kosong_6128023', 'previous_hash': '3c53409039f87cb39650e2f1cf37ab5f0e4f757e8440de7603cbb398f229fd6e'}\n",
      "\n",
      "{'index': 11, 'timestamp': 1576602026.3975403, 'PoW': 30, 'nama': 'kosong_9159104', 'previous_hash': '967ad01b04249b9ebd2870de62bc48c650dcf455037ce9ca8cdad5d94b418123'}\n",
      "\n"
     ]
    }
   ],
   "source": [
    "#import numpy as np\n",
    "import hashlib\n",
    "from time import time\n",
    "from random import randrange\n",
    "\n",
    "global_chain = []\n",
    "\n",
    "#setting\n",
    "chain_len = 10\n",
    "valid_proof_sign = 'a'\n",
    "\n",
    "def create_block(nama):\n",
    "    if len(global_chain) == 0:\n",
    "        previous_hash = 1\n",
    "    else:\n",
    "#        data_previous = '%s/%s/%s/%s'%(global_chain[-1]['index'], global_chain[-1]['timestamp'],\n",
    "#                                     global_chain[-1]['nama'],global_chain[-1]['previous_hash'])\n",
    "        previous_hash = gen_hash(global_chain[-1])\n",
    "    PoW = 0\n",
    "    while True:\n",
    "        proof = do_work(previous_hash, PoW)\n",
    "        if valid_proof(proof):\n",
    "            break\n",
    "        else:\n",
    "            PoW += 1\n",
    "    block = {\n",
    "            'index': len(global_chain) + 1,\n",
    "            'timestamp': time(),\n",
    "            'PoW': PoW,\n",
    "            'nama': nama,\n",
    "            'previous_hash': previous_hash\n",
    "            }\n",
    "    global_chain.append(block)\n",
    "\n",
    "def valid_proof(hash_proof_str):\n",
    "    if hash_proof_str[-len(valid_proof_sign):] == valid_proof_sign:\n",
    "        return True\n",
    "    else:\n",
    "        return False\n",
    "    \n",
    "def do_work(prev_hash_str, proof):\n",
    "    str_hash = '%s%s'%(proof, prev_hash_str)\n",
    "    work_res = hashlib.sha256(str_hash.encode('utf8')).hexdigest()\n",
    "    return work_res\n",
    "    \n",
    "def gen_hash(block):\n",
    "    data_str = '%s/%s/%s/%s/%s'%(block['index'], block['timestamp'], block['PoW'], block['nama'],block['previous_hash'])\n",
    "    data_hash = hashlib.sha256(data_str.encode('utf8')).hexdigest()\n",
    "    return data_hash\n",
    "    \n",
    "\n",
    "#print(len(global_chain))\n",
    "#print(hashlib.sha256('a'.encode('utf8')).hexdigest())\n",
    "create_block('kosong_%s'%(randrange(9999999)))\n",
    "create_block('kosong_%s'%(randrange(9999999)))\n",
    "create_block('kosong_%s'%(randrange(9999999)))\n",
    "create_block('Andi')\n",
    "create_block('Budi')\n",
    "create_block('Dodi')\n",
    "create_block('Ani')\n",
    "create_block('Mike')\n",
    "create_block('kosong_%s'%(randrange(9999999)))\n",
    "create_block('kosong_%s'%(randrange(9999999)))\n",
    "create_block('kosong_%s'%(randrange(9999999)))\n",
    "for i in range(len(global_chain)):\n",
    "    print(global_chain[i])\n",
    "    print()"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Pada prakteknya, blockchain digunakan untuk system peer to peer. Pada sistem ini, tidak ada yang namanya server pusat. Seluruh komputer (atau biasa disebut node) yang terlibat, menyimpan blockchain didalam memorinya. Ketika suatu node menambah block pada blockchain yang dimilikinya, dia akan broadcast blockchain miliknya dan node lain yang menerima informasi tersebut akan mencoba menvalidasi blockchain dan akan mengupdate blockchain dimemori masing-masing node tersebut jika blockchainnya valid dan jika rantai yang terbentuk dari blockchain yang dibroadcast tersebut mempunyai index yang lebih besar atau artinya blocknya lebih update. Ya, <b>bagian terpenting lainnya dari sistem blockchain selain blockchain itu sendiri adalah, seluruh node yang saling terhubung mempunyai list alamat node-node lainnya yang terhubung dengan mereka</b> agar mereka bisa saling berkomunikasi dan bekerja sama.\n",
    "\n",
    "Untuk itu, berikut adalah ilustrasi sederhana bagaimana suatu node menghandle blockchain yang di broadcast ke dirinya. Ini kondisi ketika blockchain yang dibroadcast ternyata valid dan lebih update (nilai indexnya lebih besar) daripada blockchain yang dimiliki node tersebut."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 18,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Blockchain telah diupdate menjadi lebih baru\n",
      "[{'index': 1, 'timestamp': 1576602026.3941288, 'PoW': 5, 'nama': 'kosong_9773681', 'previous_hash': 1}, {'index': 2, 'timestamp': 1576602026.3947663, 'PoW': 81, 'nama': 'kosong_4830037', 'previous_hash': 'caaae0c4e4dc4681e91d43c563e9420674fa1112234c055a41081d60c3ac77b1'}, {'index': 3, 'timestamp': 1576602026.3952703, 'PoW': 58, 'nama': 'kosong_3361986', 'previous_hash': 'cf9b2076f0ba204577e0d090aad31da06467db6e8a0d36045d0b9390d671af87'}, {'index': 4, 'timestamp': 1576602026.3953836, 'PoW': 2, 'nama': 'Andi', 'previous_hash': '973bcfaff9f4feafb3088b384c6b243cf7107ca2617e38e1401341341f73f0b6'}, {'index': 5, 'timestamp': 1576602026.3954916, 'PoW': 4, 'nama': 'Budi', 'previous_hash': '8f7954f79cecc081aa925c68d9092a6a9e02338088536c559cbb0f1d427a60e7'}, {'index': 6, 'timestamp': 1576602026.3957129, 'PoW': 19, 'nama': 'Dodi', 'previous_hash': '6fbc240b76c97a5cdea31ffb9fa3b784ee48473f466a63fd998131e36d43dbb7'}, {'index': 7, 'timestamp': 1576602026.3961165, 'PoW': 50, 'nama': 'Ani', 'previous_hash': '17658ce093205bc12d3ce3cfe6fb79c9e1d3da59f7a64fcf6f5df8e3a16cf160'}, {'index': 8, 'timestamp': 1576602026.396743, 'PoW': 74, 'nama': 'Mike', 'previous_hash': 'c0c6a6554e55d06499307c998ca4a38607ac85a37bb16e95b7364a11b596f7af'}, {'index': 9, 'timestamp': 1576602026.396992, 'PoW': 19, 'nama': 'kosong_3604790', 'previous_hash': '21df0d47e4073ea1454fc6b085069ae032771200c16852f40282071580254bf1'}, {'index': 10, 'timestamp': 1576602026.3971822, 'PoW': 13, 'nama': 'kosong_6128023', 'previous_hash': '3c53409039f87cb39650e2f1cf37ab5f0e4f757e8440de7603cbb398f229fd6e'}, {'index': 11, 'timestamp': 1576602026.3975403, 'PoW': 30, 'nama': 'kosong_9159104', 'previous_hash': '967ad01b04249b9ebd2870de62bc48c650dcf455037ce9ca8cdad5d94b418123'}]\n"
     ]
    }
   ],
   "source": [
    "def create_block(nama):\n",
    "    if len(global_chain) == 0:\n",
    "        previous_hash = 1\n",
    "    else:\n",
    "#        data_previous = '%s/%s/%s/%s'%(global_chain[-1]['index'], global_chain[-1]['timestamp'],\n",
    "#                                     global_chain[-1]['nama'],global_chain[-1]['previous_hash'])\n",
    "        previous_hash = gen_hash(global_chain[-1])\n",
    "    PoW = 0\n",
    "    while True:\n",
    "        proof = do_work(previous_hash, PoW)\n",
    "        if valid_proof(proof):\n",
    "            break\n",
    "        else:\n",
    "            PoW += 1\n",
    "    block = {\n",
    "            'index': len(global_chain) + 1,\n",
    "            'timestamp': time(),\n",
    "            'PoW': PoW,\n",
    "            'nama': nama,\n",
    "            'previous_hash': previous_hash\n",
    "            }\n",
    "    global_chain.append(block)\n",
    "\n",
    "def valid_proof(hash_proof_str):\n",
    "    if hash_proof_str[-len(valid_proof_sign):] == valid_proof_sign:\n",
    "        return True\n",
    "    else:\n",
    "        return False\n",
    "    \n",
    "def do_work(prev_hash_str, proof):\n",
    "    str_hash = '%s%s'%(proof, prev_hash_str)\n",
    "    work_res = hashlib.sha256(str_hash.encode('utf8')).hexdigest()\n",
    "    return work_res\n",
    "    \n",
    "def gen_hash(block):\n",
    "    data_str = '%s/%s/%s/%s/%s'%(block['index'], block['timestamp'], block['PoW'], block['nama'],block['previous_hash'])\n",
    "    data_hash = hashlib.sha256(data_str.encode('utf8')).hexdigest()\n",
    "    return data_hash\n",
    "\n",
    "\n",
    "def update_chain(current_blockchain, new_blockchain):\n",
    "    if current_blockchain[-1]['index'] > new_blockchain[-1]['index']:\n",
    "        print('Blockchain yang sekarang sudah update')\n",
    "        return current_blockchain\n",
    "    if valid_chain(current_blockchain, new_blockchain):\n",
    "        print('Blockchain telah diupdate menjadi lebih baru')\n",
    "        return new_blockchain\n",
    "    else:\n",
    "        print('Blockchain tidak diupdate karena blockchain baru rusak')\n",
    "        return current_blockchain\n",
    "\n",
    "def valid_chain(current_blockchain, new_blockchain):\n",
    "    current_index = current_blockchain[-1]['index']\n",
    "    comparable = False\n",
    "\n",
    "    #check validity of entire new_blockchain\n",
    "    #in real life blockchain, the first element is not always have 0 index\n",
    "    for i in range(len(new_blockchain)-1):\n",
    "        block = new_blockchain[i]\n",
    "        if block['index'] == current_index+1:\n",
    "            compared_block = new_blockchain[i]\n",
    "            comparable = True\n",
    "        if gen_hash(block) != new_blockchain[i+1]['previous_hash']:\n",
    "            print('hash tidak cocok')\n",
    "            return False\n",
    "        str_hash = '%s%s'%(block['PoW'], block['previous_hash'])\n",
    "        if not valid_proof(hashlib.sha256(str_hash.encode('utf8')).hexdigest()):\n",
    "            print('PoW tidak valid')\n",
    "            return False\n",
    "    \n",
    "    #check validity against current chain\n",
    "    if not comparable:\n",
    "        print('Blockchain tidak bisa divalidasi menggunakan blockchain dinode ini')\n",
    "        return False\n",
    "    if gen_hash(current_blockchain[-1]) != compared_block['previous_hash']:\n",
    "        print('hash tidak cocok')\n",
    "        return False\n",
    "\n",
    "    return True\n",
    "            \n",
    "\n",
    "broadcaster_blockchain = global_chain.copy()\n",
    "current_blockchain = global_chain[:7].copy()\n",
    "\n",
    "current_blockchain = update_chain(current_blockchain, broadcaster_blockchain)\n",
    "print(current_blockchain)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Ini kondisi ketika blockchain yang dibroadcast ternyata tidak valid tapi lebih update daripada blockchain yang dimiliki node tersebut."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 19,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "hash tidak cocok\n",
      "Blockchain tidak diupdate karena blockchain baru rusak\n",
      "[{'index': 1, 'timestamp': 1576602026.3941288, 'PoW': 5, 'nama': 'kosong_9773681', 'previous_hash': 1}, {'index': 2, 'timestamp': 1576602026.3947663, 'PoW': 81, 'nama': 'kosong_4830037', 'previous_hash': 'caaae0c4e4dc4681e91d43c563e9420674fa1112234c055a41081d60c3ac77b1'}, {'index': 3, 'timestamp': 1576602026.3952703, 'PoW': 58, 'nama': 'kosong_3361986', 'previous_hash': 'cf9b2076f0ba204577e0d090aad31da06467db6e8a0d36045d0b9390d671af87'}, {'index': 4, 'timestamp': 1576602026.3953836, 'PoW': 2, 'nama': 'Andi', 'previous_hash': '973bcfaff9f4feafb3088b384c6b243cf7107ca2617e38e1401341341f73f0b6'}, {'index': 5, 'timestamp': 1576602026.3954916, 'PoW': 4, 'nama': 'Budi', 'previous_hash': '8f7954f79cecc081aa925c68d9092a6a9e02338088536c559cbb0f1d427a60e7'}, {'index': 6, 'timestamp': 1576602026.3957129, 'PoW': 19, 'nama': 'Dodi', 'previous_hash': '6fbc240b76c97a5cdea31ffb9fa3b784ee48473f466a63fd998131e36d43dbb7'}, {'index': 7, 'timestamp': 1576602026.3961165, 'PoW': 50, 'nama': 'Ani', 'previous_hash': '17658ce093205bc12d3ce3cfe6fb79c9e1d3da59f7a64fcf6f5df8e3a16cf160'}]\n"
     ]
    }
   ],
   "source": [
    "def create_block(nama):\n",
    "    if len(global_chain) == 0:\n",
    "        previous_hash = 1\n",
    "    else:\n",
    "#        data_previous = '%s/%s/%s/%s'%(global_chain[-1]['index'], global_chain[-1]['timestamp'],\n",
    "#                                     global_chain[-1]['nama'],global_chain[-1]['previous_hash'])\n",
    "        previous_hash = gen_hash(global_chain[-1])\n",
    "    PoW = 0\n",
    "    while True:\n",
    "        proof = do_work(previous_hash, PoW)\n",
    "        if valid_proof(proof):\n",
    "            break\n",
    "        else:\n",
    "            PoW += 1\n",
    "    block = {\n",
    "            'index': len(global_chain) + 1,\n",
    "            'timestamp': time(),\n",
    "            'PoW': PoW,\n",
    "            'nama': nama,\n",
    "            'previous_hash': previous_hash\n",
    "            }\n",
    "    global_chain.append(block)\n",
    "\n",
    "def valid_proof(hash_proof_str):\n",
    "    if hash_proof_str[-len(valid_proof_sign):] == valid_proof_sign:\n",
    "        return True\n",
    "    else:\n",
    "        return False\n",
    "    \n",
    "def do_work(prev_hash_str, proof):\n",
    "    str_hash = '%s%s'%(proof, prev_hash_str)\n",
    "    work_res = hashlib.sha256(str_hash.encode('utf8')).hexdigest()\n",
    "    return work_res\n",
    "    \n",
    "def gen_hash(block):\n",
    "    data_str = '%s/%s/%s/%s/%s'%(block['index'], block['timestamp'], block['PoW'], block['nama'],block['previous_hash'])\n",
    "    data_hash = hashlib.sha256(data_str.encode('utf8')).hexdigest()\n",
    "    return data_hash\n",
    "\n",
    "\n",
    "def update_chain(current_blockchain, new_blockchain):\n",
    "    if current_blockchain[-1]['index'] > new_blockchain[-1]['index']:\n",
    "        print('Blockchain yang sekarang sudah update')\n",
    "        return current_blockchain\n",
    "    if valid_chain(current_blockchain, new_blockchain):\n",
    "        print('Blockchain telah diupdate menjadi lebih baru')\n",
    "        return new_blockchain\n",
    "    else:\n",
    "        print('Blockchain tidak diupdate karena blockchain baru rusak')\n",
    "        return current_blockchain\n",
    "\n",
    "def valid_chain(current_blockchain, new_blockchain):\n",
    "    current_index = current_blockchain[-1]['index']\n",
    "    comparable = False\n",
    "\n",
    "    #check validity of entire new_blockchain\n",
    "    #in real life blockchain, the first element is not always have 0 index\n",
    "    for i in range(len(new_blockchain)-1):\n",
    "        block = new_blockchain[i]\n",
    "        if block['index'] == current_index+1:\n",
    "            compared_block = new_blockchain[i]\n",
    "            comparable = True\n",
    "        if gen_hash(block) != new_blockchain[i+1]['previous_hash']:\n",
    "            print('hash tidak cocok')\n",
    "            return False\n",
    "        str_hash = '%s%s'%(block['PoW'], block['previous_hash'])\n",
    "        if not valid_proof(hashlib.sha256(str_hash.encode('utf8')).hexdigest()):\n",
    "            print('PoW tidak valid')\n",
    "            return False\n",
    "    \n",
    "    #check validity against current chain\n",
    "    if not comparable:\n",
    "        print('Blockchain tidak bisa divalidasi menggunakan blockchain dinode ini')\n",
    "        return False\n",
    "    if gen_hash(current_blockchain[-1]) != compared_block['previous_hash']:\n",
    "        print('hash tidak cocok')\n",
    "        return False\n",
    "\n",
    "    return True\n",
    "            \n",
    "\n",
    "broadcaster_blockchain = global_chain.copy()\n",
    "current_blockchain = global_chain[:7].copy()\n",
    "\n",
    "#make it not valid blockchain\n",
    "broadcaster_blockchain[8]['nama'] = 'Nina'\n",
    "\n",
    "current_blockchain = update_chain(current_blockchain, broadcaster_blockchain)\n",
    "print(current_blockchain)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Kalau diperhatikan dengan cermat, sebenarnya <b>blockchain tidak immutable pada block paling akhirnya</b>. Validasi PoW mungkin bisa dilakukan tapi validasi hash melibatkan block didepannnya. <b>Inilah yang dilakukan para miner bitcoin dan sejenisnya, membuat block. Dengan menimbun block transaksi yang sebenarnya dalam-dalam dengan block yang dibuat oleh para miner, block transaksi menjadi terlindungi dan immutable.</b> Selain itu, para miner ini juga memvalidai suatu transaksi pada blockchain. Atas jasa mereka tersebut, untuk setiap block yang mereka buat, mereka dihadiahi bitcoin atau cryptocurrency lainnya yang bersangkutan."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Referensi\n",
    "\n",
    "https://www.geeksforgeeks.org/data-structures/linked-list/, diakses pada 14 Desember 2019\n",
    "\n",
    "https://hackernoon.com/learn-blockchains-by-building-one-117428612f46, diakses pada 14 Desember 2019\n",
    "\n",
    "https://crypto.stackexchange.com/questions/47809/why-havent-any-sha-256-collisions-been-found-yet, diakses pada 17 Desember 2019\n",
    "\n",
    "https://en.wikipedia.org/wiki/Hashcash, diakses pada 18 Desember 2019\n",
    "\n",
    "Joshi, Archana & Han, Meng & Wang, Yan. (2018). A survey on security and privacy issues of blockchain technology. Mathematical Foundations of Computing. 1. 121-147. 10.3934/mfc.2018007. "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": []
  }
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