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.

Write The Threshold Down

Do not say "parties do not collude" unless there is a control that makes that credible. Write the smallest collusion set that breaks privacy, then check whether the business arrangement, infrastructure, and incident response plan make that threshold realistic.

System	Collusion question
Secure aggregation	How many participants, coordinator components, or key servers must cooperate to reveal an update?
MPC	Which compute parties can share views before inputs are exposed?
Clean room	Can the platform operator and analyst combine logs, query history, and outputs?
Confidential computing	Can the cloud operator, workload owner, and attestation verifier bypass separation in practice?