Collusion

Collusion occurs when parties that are treated as separate in the design combine information.

Practical Example

Two MPC compute parties share their views, violating the threshold assumption that protected the inputs.

Questions To Ask

• Which parties can collude before privacy breaks?
• Is the platform operator separate from the analyst?
• Can participants compare outputs across repeated queries?
• Are legal and technical separation assumptions aligned?

PET Implications

Secure aggregation, MPC, clean rooms, and TEEs all need explicit collusion assumptions.