mppe_rc4_encrypt
mppe_rc4_encrypt(_this, &_this->recv, len - 2, pktp, pktp);
mppe_rc4_encrypt(_this, &_this->send, 2, (u_char *)&proto, outp);
mppe_rc4_encrypt(_this, &_this->send, len, pktp, outp + 2);
static void mppe_rc4_encrypt(mppe *, mppe_rc4_t *, int, u_char *, u_char *);