diff --git a/blockchain/proof_of_work.py b/blockchain/proof_of_work.py
new file mode 100644
index 000000000000..eeae6d8c55ec
--- /dev/null
+++ b/blockchain/proof_of_work.py
@@ -0,0 +1,267 @@
+from __future__ import annotations
+
+import hashlib
+import json
+from time import time
+
+
+class Block:
+    """
+    A single block in a proof-of-work blockchain.
+
+    Each block contains data along with a cryptographic hash linking it to the
+    previous block. The block's hash must satisfy a difficulty target (start
+    with a certain number of zero bits) which is found through mining.
+
+    Attributes:
+        index: Position of the block in the chain
+        timestamp: Unix time when the block was created
+        data: Payload stored in the block
+        previous_hash: Hash of the previous block (hex digest)
+        nonce: Number used to satisfy the proof-of-work requirement
+        hash: SHA-256 hash of this block (hex digest)
+
+    >>> Block(0, 1000.0, "genesis", "0", 0, "0000abc")
+    Block(0, 1000.0, 'genesis', '0', 0, hash=0000abc)
+
+    Blocks with the same content are equal:
+    >>> Block(0, 0.0, "a", "0", 1, "ab") == Block(0, 0.0, "a", "0", 1, "ab")
+    True
+    >>> Block(0, 0.0, "a", "0", 1, "ab") == Block(0, 0.0, "b", "0", 1, "ab")
+    False
+    """
+
+    def __init__(
+        self,
+        index: int,
+        timestamp: float,
+        data: str,
+        previous_hash: str,
+        nonce: int,
+        hash_value: str,
+    ) -> None:
+        self.index = index
+        self.timestamp = timestamp
+        self.data = data
+        self.previous_hash = previous_hash
+        self.nonce = nonce
+        self.hash = hash_value
+
+    def __repr__(self) -> str:
+        return (
+            f"Block({self.index}, {self.timestamp}, "
+            f"{self.data!r}, {self.previous_hash!r}, {self.nonce}, "
+            f"hash={self.hash})"
+        )
+
+    def __eq__(self, other: object) -> bool:
+        if not isinstance(other, Block):
+            return NotImplemented
+        return (
+            self.index == other.index
+            and self.timestamp == other.timestamp
+            and self.data == other.data
+            and self.previous_hash == other.previous_hash
+            and self.nonce == other.nonce
+            and self.hash == other.hash
+        )
+
+    def to_dict(self) -> dict:
+        """Serialise the block to a JSON-compatible dictionary.
+
+        >>> Block(0, 0.0, "a", "0", 0, "ab").to_dict()["data"]
+        'a'
+        """
+        return {
+            "index": self.index,
+            "timestamp": self.timestamp,
+            "data": self.data,
+            "previous_hash": self.previous_hash,
+            "nonce": self.nonce,
+            "hash": self.hash,
+        }
+
+
+def calculate_hash(
+    index: int,
+    timestamp: float,
+    data: str,
+    previous_hash: str,
+    nonce: int,
+) -> str:
+    """Compute the SHA-256 hash of a block's contents.
+
+    The hash is taken over the concatenation of index, timestamp, data,
+    previous_hash, and nonce, serialised as a JSON string.
+
+    >>> h = calculate_hash(1, 0.0, "tx", "0000", 42)
+    >>> isinstance(h, str) and len(h) == 64
+    True
+
+    Same inputs always produce the same hash:
+    >>> a = calculate_hash(1, 0.0, "tx", "0000", 42)
+    >>> a == calculate_hash(1, 0.0, "tx", "0000", 42)
+    True
+    """
+    payload = json.dumps(
+        [index, timestamp, data, previous_hash, nonce],
+        sort_keys=True,
+        separators=(",", ":"),
+    )
+    return hashlib.sha256(payload.encode()).hexdigest()
+
+
+def mine_block(
+    index: int,
+    timestamp: float,
+    data: str,
+    previous_hash: str,
+    difficulty: int,
+) -> Block:
+    """Mine a new block by finding a nonce that satisfies the difficulty target.
+
+    The block's hash must start with ``difficulty`` consecutive zero characters.
+    This is a brute-force search that increments the nonce until a valid hash
+    is found. Higher difficulty values require exponentially more work.
+
+    Difficulty must be a non-negative integer:
+    Mined blocks always satisfy the proof-of-work requirement:
+    >>> b = mine_block(1, 0.0, "hello", "abcd", 3)
+    >>> b.hash[:3]
+    '000'
+
+    >>> b = mine_block(2, 0.0, "world", b.hash, 4)
+    >>> b.hash[:4]
+    '0000'
+
+    Different nonces produce different hashes for the same block data:
+    >>> a = calculate_hash(5, 0.0, "same", "prev", 0)
+    >>> b = calculate_hash(5, 0.0, "same", "prev", 1)
+    >>> a != b
+    True
+    """
+    prefix = "0" * difficulty
+    nonce = 0
+    hash_value = calculate_hash(index, timestamp, data, previous_hash, nonce)
+    while not hash_value.startswith(prefix):
+        nonce += 1
+        hash_value = calculate_hash(index, timestamp, data, previous_hash, nonce)
+    return Block(index, timestamp, data, previous_hash, nonce, hash_value)
+
+
+class Blockchain:
+    """
+    A simple proof-of-work blockchain.
+
+    Maintains a chain of blocks where each block references the hash of the
+    previous block. New blocks are mined with an adjustable difficulty level,
+    and the integrity of the chain can be verified at any time.
+
+    >>> chain = Blockchain(difficulty=2)
+    >>> _ = chain.add_block("Payment: Alice -> Bob: 10 BTC")
+    >>> _ = chain.add_block("Payment: Bob -> Charlie: 5 BTC")
+    >>> len(chain)
+    3
+    >>> chain.is_valid()
+    True
+
+    Tampering with block data invalidates the chain:
+    >>> chain.chain[1].data = "Tampered!"
+    >>> chain.is_valid()
+    False
+    """
+
+    def __init__(self, difficulty: int = 2) -> None:
+        self.difficulty = difficulty
+        self.chain: list[Block] = []
+        self._create_genesis_block()
+
+    def _create_genesis_block(self) -> None:
+        """Create the first block of the chain (the genesis block).
+
+        The genesis block has index 0, arbitrary data, and previous_hash "0".
+        """
+        genesis = mine_block(0, time(), "Genesis Block", "0", self.difficulty)
+        self.chain.append(genesis)
+
+    def add_block(self, data: str) -> Block:
+        """Add a new block with the given data to the chain.
+
+        The block is mined with the chain's configured difficulty level.
+
+        >>> chain = Blockchain(difficulty=1)
+        >>> block = chain.add_block("test")
+        >>> block.index
+        1
+        >>> block.data
+        'test'
+        >>> chain.chain[-1].hash == block.hash
+        True
+        """
+        previous_block = self.chain[-1]
+        new_block = mine_block(
+            len(self.chain),
+            time(),
+            data,
+            previous_block.hash,
+            self.difficulty,
+        )
+        self.chain.append(new_block)
+        return new_block
+
+    def is_valid(self) -> bool:
+        """Verify the integrity of the entire blockchain.
+
+        Checks that:
+        1. Every block's hash matches its recomputed hash
+        2. Every block's previous_hash matches the hash of the preceding block
+        3. The genesis block has index 0 and previous_hash "0"
+
+        >>> chain = Blockchain(difficulty=1)
+        >>> chain.is_valid()
+        True
+        """
+        for i in range(1, len(self.chain)):
+            current = self.chain[i]
+            previous = self.chain[i - 1]
+
+            if current.hash != calculate_hash(
+                current.index,
+                current.timestamp,
+                current.data,
+                current.previous_hash,
+                current.nonce,
+            ):
+                return False
+            if current.previous_hash != previous.hash:
+                return False
+        return True
+
+    def __len__(self) -> int:
+        return len(self.chain)
+
+    def to_json(self) -> str:
+        """Serialise the entire blockchain to a JSON string.
+
+        >>> chain = Blockchain(difficulty=1)
+        >>> _ = chain.add_block("data")
+        >>> isinstance(chain.to_json(), str)
+        True
+        """
+        return json.dumps([block.to_dict() for block in self.chain], indent=2)
+
+
+if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()
+
+    print("Mining a simple blockchain (difficulty=3)...")
+    chain = Blockchain(difficulty=3)
+    chain.add_block("Transaction 1: Alice sends 10 BTC to Bob")
+    chain.add_block("Transaction 2: Bob sends 5 BTC to Charlie")
+    chain.add_block("Transaction 3: Charlie sends 2 BTC to Dave")
+
+    print(f"\nBlockchain valid: {chain.is_valid()}")
+    print(f"Total blocks: {len(chain)}")
+    print(f"\nFull chain:\n{chain.to_json()}")