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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _GenerateAssertion(self):
 """Generates the signed assertion that will be used in the request. Returns: string, signed Json Web Token (JWT) assertion. """

    now = int(time.time())
    payload = {
        'aud': RpcHelper.TOKEN_ENDPOINT,
        'scope': 'https://www.googleapis.com/auth/identitytoolkit',
        'iat': now,
        'exp': now + RpcHelper.MAX_TOKEN_LIFETIME_SECS,
        'iss': self.service_account_email
    }
    return crypt.make_signed_jwt(
        crypt.Signer.from_string(self.service_account_key),
        payload)