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Solve the following problem using Python, implementing the functions described below, one line at a time
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<USER_TASK:>
Description:
 def security(policy, app_secret):
 """ Creates a valid signature and policy based on provided app secret and parameters ```python from filestack import Client, security # a policy requires at least an expiry policy = {'expiry': 56589012, 'call': ['read', 'store', 'pick']} sec = security(policy, 'APP_SECRET') client = Client('API_KEY', security=sec) ``` """

    validate(policy)
    policy_enc = base64.urlsafe_b64encode(json.dumps(policy).encode('utf-8'))

    signature = hmac.new(app_secret.encode('utf-8'),
                         policy_enc,
                         hashlib.sha256).hexdigest()

    return {'policy': policy_enc, 'signature': signature}