Manuscript received November 9;
1997;
revised October 15;
1998. This work was supported by the Russian Foundation of Basis Research under Grant 99-01-00586. The authors are with SibSATIS;
Novosibirsk;
630102;
Russia. Communicated by D. Stinson;
Associate Editor for Complexity and Cryptography. Publisher Item Identifier S 0018-9448(99)05864-2;