new_contents: import hashlib
  import json
  
  # Helper functions
  def hash(block):
      block_string = json.dumps(block, sort_keys=True).encode()
      return hashlib.sha256(block_string).hexdigest()
  
  def proof_of_work(last_proof):
      proof = 0
      while valid_proof(last_proof, proof) is False:
          proof += 1
      return proof
  
  def valid_proof(last_proof, proof):
      guess = '{0}{1}'.format(last_proof, proof).encode()
      guess_hash = hashlib.sha256(guess).hexdigest()
      return guess_hash[:4] == "0000"
  
  def valid_chain(chain):
      last_block = chain[0]
      for current_index in range(1, len(chain)):
          block = chain[current_index]
          
          if block['previous_hash'] != hash(last_block): return False
          if not valid_proof(last_block['proof'], block['proof']): return False
  
          last_block = block
  
      return True
  
subject: Create core_utils.py