Section 18. In the discretion of the authority such bonds shall be secured by a trust agreement by and between the authority and a corporate trustee, which may be any trust company or bank having the powers of a trust company within the commonwealth. Either the resolution providing for the issue of bonds or such trust agreement may contain such provisions for protecting and enforcing the rights and remedies of the bondholders as may be reasonable and proper and not in violation of law, including covenants setting forth the duties of the authority in relation to the acquisition, improvement, maintenance, operation, repair and insurance of property, and the custody, safeguarding and application of all moneys and may pledge or assign the revenues to be received, but shall not convey or mortgage any property.