We present a new algorithm for computing a^e where a in GF(2^k) and e is a positive integer. The proposed algorithm is more suitable for implementation in software, and relies on the Montgomery multiplication in GF(2^k). The speed of the exponentiation algorithm largely depends on the availability of a fast method for multiplying two polynomials of length w defined over GF(2). The theoretical analysis and our experiments indicate that the proposed exponentiation method is at least 6 times faster than the exponentiation method using the standard multiplication when w=8. Furthermore, the availability of a 32-bit GF(2) polynomial multiplication instruction on the underlying processor would make the new exponentiation algorithm up to 37 times faster.