See comment below. More generally what I'm seeking is something in the card form factor which is suitable to store cryptographic secrets (i.e., a smartcard).

See comment below. More generally what I'm seeking is something in the card form factor which is suitable to store cryptographic secrets (i.e., a smartcard). Separate IC is a disadvantage here since it creates a vulnerable security boundary and makes it infeasible to integrate the chip into a thin card.