Affordable security or big guy vs small guy: Does the depth of your pockets impact your protocols?

When we design a security protocol we assume that the humans (or organizations) playing Alice and Bob do not make a difference. In particular, their financial capacity seems to be irrelevant. In the latest trend to guarantee that secure multi-party computation protocols are fair and not vulnerable to malicious aborts, a slate of protocols has been proposed based on penalty mechanisms. We look at two well-known penalty mechanisms, and show that the so-called see-saw mechanism (Kumaresan et al., CCS 15), is only fit for people with deep pockets, well beyond the stake in the multi-party computation itself. Depending on the scheme, fairness is not affordable by everyone which has several policy implications on protocol design. To explicitly capture the above issues, we introduce a new property called financial fairness.