text: import datetime
  import json
  
  from app import app
  from util.security.crypto import encrypt_string, decrypt_string
  
  # TTL (in seconds) for page tokens.
  _PAGE_TOKEN_TTL = datetime.timedelta(days=2).total_seconds()
  
  def decrypt_page_token(token_string):
    if token_string is None:
      return None
  
    unencrypted = decrypt_string(token_string, app.config['PAGE_TOKEN_KEY'], ttl=_PAGE_TOKEN_TTL)
    if unencrypted is None:
      return None
  
    try:
      return json.loads(unencrypted)
    except ValueError:
      return None
  
  
  def encrypt_page_token(page_token):
    return encrypt_string(json.dumps(page_token), app.config['PAGE_TOKEN_KEY'])
  
  