Abstract: Linear complexity and k-error linear complexity of periodic sequences are two important security indices of stream cipher systems. The k-error linear complexity of 3n-periodic sequences over the finite field F3 was discussed, and some of its properties and error sequences were obtained. By means of the properties, a result was presented, showing how the computation of the k-error linear complexity of a sequences with period 3n can be reduced to the same computation for sequences with period 3n－1, for any positive integer n.

Key words: stream cipher, periodic sequence, linear complexity, k-error linear complexity, error sequence