/* read/writes from s->s2->mac_data using length for encrypt and
* decrypt. It sets the s->s2->padding, s->[rw]length and
* s->s2->pad_data ptr if we are encrypting */
/* read/writes from s->s2->mac_data using length for encrypt and
* decrypt. It sets the s->s2->padding, s->[rw]length and
* s->s2->pad_data ptr if we are encrypting */