Yep they are synonyms.
Merriam-Webster Collegiate® Dictionary
Function: transitive verb
Inflected Form: en·sured ; en·sur·ing
Etymology: Middle English, from Anglo-French ensurer, alteration of assurer ― more at ASSURE
: to make sure, certain, or safe : GUARANTEE
synonyms ENSURE, INSURE, ASSURE, SECURE mean to make a thing or person sure. ENSURE, INSURE and ASSURE are interchangeable in many contexts where they indicate the making certain or inevitable of an outcome, but ENSURE may imply a virtual guarantee <the government has ensured the safety of the refugees>, while INSURE sometimes stresses the taking of necessary measures beforehand <careful planning should insure the success of the party>, and ASSURE distinctively implies the removal of doubt and suspense from a person's mind <I assure you that no harm will be done>. SECURE implies action taken to guard against attack or loss <sent reinforcements to secure their position>.
© 2005 Merriam-Webster, Incorporated Merriam-Webster Collegiate® Thesaurus
to make something certain or sure ＜provisions ensuring that the rank and file have a voice in union policy-making＞
Synonyms: assure, cinch, insure, secure
Related Words: certify, guarantee, warrant; arrange, establish, provide, set out
© 2005 Merriam-Webster, Incorporated