An simple implementation of blockchain using python

blockchain.ipynb

{
 "cells": [
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [],
   "source": [
    "from hashlib import sha256\n",
    "import json"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Hashes, the digital fingerprint"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824\n",
      "936a185caaa266bb9cbe981e9e05cb78cd732b0b3280eb944412bb6f8f8f07af\n"
     ]
    }
   ],
   "source": [
    "print(sha256(b\"hello\").hexdigest())\n",
    "print(sha256(b\"helloworld\").hexdigest())"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Let's define an arbitary condition for the hashes, let's say the hash value has to start with 000. This will define what a singned block signifies"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [],
   "source": [
    "def is_signed_block(hash_value: str):\n",
    "    return hash_value.startswith(\"000\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "now to mine the block, we should find a nonce with the data whose hash will start with 000. As that's when we can declare a block signed."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {},
   "outputs": [],
   "source": [
    "def mine(block_data: dict):\n",
    "    for possible_nonce in range(0, 10000000000000):\n",
    "        data_to_hash = {**block_data, \"nonce\": possible_nonce}\n",
    "        hash_value = sha256(json.dumps(data_to_hash).encode()).hexdigest()\n",
    "        if is_signed_block(hash_value):\n",
    "            return possible_nonce\n",
    "    raise Exception(\"could not mine block!\")"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Block:\n",
    "    data: dict\n",
    "    prev_block_hash: str\n",
    "    nonce: int\n",
    "    curr_block_hash: str\n",
    "\n",
    "    def __init__(self, data: dict, prev_block_hash: str) -> None:\n",
    "        self.data = data\n",
    "        self.prev_block_hash = prev_block_hash\n",
    "        self.nonce = mine({\n",
    "            \"prev_block_hash\": prev_block_hash,\n",
    "            \"data\": data,\n",
    "        })\n",
    "        self.curr_block_hash = sha256(json.dumps({\n",
    "            \"prev_block_hash\": prev_block_hash,\n",
    "            \"data\": data,\n",
    "            \"nonce\": self.nonce\n",
    "        }).encode()).hexdigest()\n",
    "\n",
    "    def display(self):\n",
    "        return json.dumps(self.__dict__, indent=4)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {},
   "outputs": [],
   "source": [
    "INCEPTION_HASH = \"0000000000000000000000000000000000000000000000000000000000000000\"\n",
    "INCEPTION_BLOCK: Block = Block(\n",
    "    data={}, \n",
    "    prev_block_hash=INCEPTION_HASH,\n",
    ")"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Blockchain:\n",
    "    block_map: dict[str, Block] = {}\n",
    "    latest_block_hash = None\n",
    "\n",
    "    def __init__(self) -> None:\n",
    "        self.block_map[INCEPTION_HASH] = INCEPTION_BLOCK\n",
    "        self.latest_block_hash = INCEPTION_HASH\n",
    "\n",
    "    def add_block(self, block_data: dict):\n",
    "        block = Block(data=block_data, prev_block_hash=self.latest_block_hash)\n",
    "        self.block_map[block.curr_block_hash] = block\n",
    "        self.latest_block_hash = block.curr_block_hash\n",
    "\n",
    "    def show(self):\n",
    "        pointer = self.latest_block_hash\n",
    "        while True:\n",
    "            pointed_block = self.block_map[pointer]\n",
    "            print(pointed_block.display())\n",
    "            pointer=pointed_block.prev_block_hash\n",
    "            if pointer == INCEPTION_BLOCK.prev_block_hash:\n",
    "                break\n"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "{\n",
      "    \"data\": {\n",
      "        \"userId\": \"immutabledata\"\n",
      "    },\n",
      "    \"prev_block_hash\": \"0002e24d15509dbf2d533175050b5b7f03a760bec0ae38d84d646853a1fb4531\",\n",
      "    \"nonce\": 10628,\n",
      "    \"curr_block_hash\": \"0003ed123bbbbd1ae2f0141f9389da14cec9230f81be414c1cca2c8f3f2e4511\"\n",
      "}\n",
      "{\n",
      "    \"data\": {\n",
      "        \"userId\": \"bharatkalluri\"\n",
      "    },\n",
      "    \"prev_block_hash\": \"0000000000000000000000000000000000000000000000000000000000000000\",\n",
      "    \"nonce\": 1353,\n",
      "    \"curr_block_hash\": \"0002e24d15509dbf2d533175050b5b7f03a760bec0ae38d84d646853a1fb4531\"\n",
      "}\n"
     ]
    }
   ],
   "source": [
    "blockchain = Blockchain()\n",
    "blockchain.add_block({\"userId\": \"bharatkalluri\"})\n",
    "blockchain.add_block({\"userId\": \"immutabledata\"})\n",
    "blockchain.show()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {},
   "outputs": [],
   "source": [
    "def verify_blockchain(blocks: list[dict]):\n",
    "    # assumes the blocks are LIFO\n",
    "    blocks = blocks[::-1]\n",
    "    for block in blocks:\n",
    "        curr_block_hash = block['curr_block_hash']\n",
    "        block_data = {\n",
    "            'prev_block_hash': block['prev_block_hash'],\n",
    "            \"data\": block['data'],\n",
    "            'nonce': block['nonce']\n",
    "        }\n",
    "        is_block_untampered = sha256(json.dumps(block_data).encode()).hexdigest() == curr_block_hash\n",
    "        if is_block_untampered:\n",
    "            print(\"verified block: \", block)\n",
    "        else:\n",
    "            raise Exception(\"data has been tampered with!\")"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "verified block:  {'data': {'userId': 'bharatkalluri'}, 'prev_block_hash': '0000000000000000000000000000000000000000000000000000000000000000', 'nonce': 1353, 'curr_block_hash': '0002e24d15509dbf2d533175050b5b7f03a760bec0ae38d84d646853a1fb4531'}\n",
      "verified block:  {'data': {'userId': 'immutabledata'}, 'prev_block_hash': '0002e24d15509dbf2d533175050b5b7f03a760bec0ae38d84d646853a1fb4531', 'nonce': 10628, 'curr_block_hash': '0003ed123bbbbd1ae2f0141f9389da14cec9230f81be414c1cca2c8f3f2e4511'}\n"
     ]
    }
   ],
   "source": [
    "verify_blockchain(\n",
    "    [\n",
    "        {\n",
    "            'data': {'userId': 'immutabledata'},\n",
    "            'prev_block_hash': '0002e24d15509dbf2d533175050b5b7f03a760bec0ae38d84d646853a1fb4531',\n",
    "            'nonce': 10628,\n",
    "            'curr_block_hash': '0003ed123bbbbd1ae2f0141f9389da14cec9230f81be414c1cca2c8f3f2e4511',\n",
    "        },\n",
    "        {\n",
    "            'data': {'userId': 'bharatkalluri'},\n",
    "            'prev_block_hash': '0000000000000000000000000000000000000000000000000000000000000000',\n",
    "            'nonce': 1353,\n",
    "            'curr_block_hash': '0002e24d15509dbf2d533175050b5b7f03a760bec0ae38d84d646853a1fb4531',\n",
    "        },\n",
    "    ]\n",
    ")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "But if any data is tampered with, then we'll see an error since hash's will be mismatching"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 12,
   "metadata": {},
   "outputs": [
    {
     "ename": "Exception",
     "evalue": "data has been tampered with!",
     "output_type": "error",
     "traceback": [
      "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[0;31mException\u001b[0m                                 Traceback (most recent call last)",
      "Cell \u001b[0;32mIn [12], line 1\u001b[0m\n\u001b[0;32m----> 1\u001b[0m verify_blockchain(\n\u001b[1;32m      2\u001b[0m     [\n\u001b[1;32m      3\u001b[0m         {\n\u001b[1;32m      4\u001b[0m             \u001b[39m'\u001b[39;49m\u001b[39mdata\u001b[39;49m\u001b[39m'\u001b[39;49m: {\u001b[39m'\u001b[39;49m\u001b[39muserId\u001b[39;49m\u001b[39m'\u001b[39;49m: \u001b[39m'\u001b[39;49m\u001b[39mimmutabledata\u001b[39;49m\u001b[39m'\u001b[39;49m},\n\u001b[1;32m      5\u001b[0m             \u001b[39m'\u001b[39;49m\u001b[39mprev_block_hash\u001b[39;49m\u001b[39m'\u001b[39;49m: \u001b[39m'\u001b[39;49m\u001b[39m0002e24d15509dbf2d533175050b5b7f03a760bec0ae38d84d646853a1fb4531\u001b[39;49m\u001b[39m'\u001b[39;49m,\n\u001b[1;32m      6\u001b[0m             \u001b[39m'\u001b[39;49m\u001b[39mnonce\u001b[39;49m\u001b[39m'\u001b[39;49m: \u001b[39m10628\u001b[39;49m,\n\u001b[1;32m      7\u001b[0m             \u001b[39m'\u001b[39;49m\u001b[39mcurr_block_hash\u001b[39;49m\u001b[39m'\u001b[39;49m: \u001b[39m'\u001b[39;49m\u001b[39m0003ed123bbbbd1ae2f0141f9389da14cec9230f81be414c1cca2c8f3f2e4511\u001b[39;49m\u001b[39m'\u001b[39;49m,\n\u001b[1;32m      8\u001b[0m         },\n\u001b[1;32m      9\u001b[0m         {\n\u001b[1;32m     10\u001b[0m             \u001b[39m'\u001b[39;49m\u001b[39mdata\u001b[39;49m\u001b[39m'\u001b[39;49m: {\u001b[39m'\u001b[39;49m\u001b[39muserId\u001b[39;49m\u001b[39m'\u001b[39;49m: \u001b[39m'\u001b[39;49m\u001b[39mharatkalluri\u001b[39;49m\u001b[39m'\u001b[39;49m},\n\u001b[1;32m     11\u001b[0m             \u001b[39m'\u001b[39;49m\u001b[39mprev_block_hash\u001b[39;49m\u001b[39m'\u001b[39;49m: \u001b[39m'\u001b[39;49m\u001b[39m0000000000000000000000000000000000000000000000000000000000000000\u001b[39;49m\u001b[39m'\u001b[39;49m,\n\u001b[1;32m     12\u001b[0m             \u001b[39m'\u001b[39;49m\u001b[39mnonce\u001b[39;49m\u001b[39m'\u001b[39;49m: \u001b[39m1353\u001b[39;49m,\n\u001b[1;32m     13\u001b[0m             \u001b[39m'\u001b[39;49m\u001b[39mcurr_block_hash\u001b[39;49m\u001b[39m'\u001b[39;49m: \u001b[39m'\u001b[39;49m\u001b[39m0002e24d15509dbf2d533175050b5b7f03a760bec0ae38d84d646853a1fb4531\u001b[39;49m\u001b[39m'\u001b[39;49m,\n\u001b[1;32m     14\u001b[0m         },\n\u001b[1;32m     15\u001b[0m     ]\n\u001b[1;32m     16\u001b[0m )\n",
      "Cell \u001b[0;32mIn [10], line 15\u001b[0m, in \u001b[0;36mverify_blockchain\u001b[0;34m(blocks)\u001b[0m\n\u001b[1;32m     13\u001b[0m     \u001b[39mprint\u001b[39m(\u001b[39m\"\u001b[39m\u001b[39mverified block: \u001b[39m\u001b[39m\"\u001b[39m, block)\n\u001b[1;32m     14\u001b[0m \u001b[39melse\u001b[39;00m:\n\u001b[0;32m---> 15\u001b[0m     \u001b[39mraise\u001b[39;00m \u001b[39mException\u001b[39;00m(\u001b[39m\"\u001b[39m\u001b[39mdata has been tampered with!\u001b[39m\u001b[39m\"\u001b[39m)\n",
      "\u001b[0;31mException\u001b[0m: data has been tampered with!"
     ]
    }
   ],
   "source": [
    "verify_blockchain(\n",
    "    [\n",
    "        {\n",
    "            'data': {'userId': 'immutabledata'},\n",
    "            'prev_block_hash': '0002e24d15509dbf2d533175050b5b7f03a760bec0ae38d84d646853a1fb4531',\n",
    "            'nonce': 10628,\n",
    "            'curr_block_hash': '0003ed123bbbbd1ae2f0141f9389da14cec9230f81be414c1cca2c8f3f2e4511',\n",
    "        },\n",
    "        {\n",
    "            'data': {'userId': 'haratkalluri'},\n",
    "            'prev_block_hash': '0000000000000000000000000000000000000000000000000000000000000000',\n",
    "            'nonce': 1353,\n",
    "            'curr_block_hash': '0002e24d15509dbf2d533175050b5b7f03a760bec0ae38d84d646853a1fb4531',\n",
    "        },\n",
    "    ]\n",
    ")"
   ]
  }
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