“…Recently, it was discovered that FAPKC0, FAPKC1 and FAPKC93 are insecure with respect to digital signatures [17] as well as insecure with respect to encryption [6,13,17] In general, FAPKC2 is insecure against attacks [6,13,17], but Bao and Igarashi's key-generating scheme of FAPKC2 [6] is insecure because it cannot resist linear R a , R b attack [18]. FAPKC3's key generator selects automata in the user's key by means of linear and/or non-linear R a , R b transformation and their reverse transformation, and sieves out such a key in which the compound automaton's weak inverse can be found out by the linear R a , R b transformation method [5,18]. So FAPKC3 can resist linear R a , R b attack.…”