In [HLS04a], Han et al. presented a nice overview of some side channel attacks (SCA), and some classical countermeasures. However, their proposed countermeasures against SCA are so inefficient that the efficiency of XTR with SCA countermeasures is at least 129 times slower than that of XTR without them. Thus they remained the construction of the efficient countermeasures against SCA as an open question. In this paper, we show that XTR can be also attacked by the modified refined power analysis (MRPA) and the modified zero-value attack (MZVA). To show validity of MRPA and MZVA on XTR, we give some numerical data of them. We propose a novel efficient countermeasure (XTR-RSE) against "SCAs": SPA, Data-bit DPA, Address-bit DPA, Doubling attack, MRPA, and MZVA. We show that XTR-RSE itself without other countermeasures is secure against all "SCAs". From our implementation results, if we compare XTR with ECC with countermeasures against "SCAs", we think XTR is as suitable to smart-cards as ECC due to the efficiency of the proposed XTR-RSE.