The exposure window and Mosca's inequality

The four variables from Module 2 answer whether a specific piece of data is exposed at all. This lesson adds the timing question: given that exposure, how urgently does it need to be fixed? Michele Mosca's formulation gives a simple way to reason about that: act now if X + Y > Z.

X

Required secrecy lifetime — the confidentiality lifetime from Module 2, measured forward from today.

Y

Migration time — how long it will actually take to identify, replace, and verify the protecting cryptography for this data flow.

Z

Time until a cryptographically relevant quantum computer exists — unknown, and not something this course, or anyone, can measure.

1. 01

   Estimate X

   Take the confidentiality lifetime established in Module 2, measured from today, not from when the data was created.

2. 02

   Estimate Y

   Ask the team that would actually do the migration how long discovery, replacement, and verification would take for this specific flow — not a generic program estimate.

3. 03

   Treat Z as unknown

   No credible source can state a date for a cryptographically relevant quantum computer. Use Z as a labeled, contested planning input, never as a fact plugged into a spreadsheet.

4. 04

   Compare, and date the verdict

   If X + Y is large relative to any plausible Z, treat the flow as already late and prioritize it. Record the date of the verdict and the confidence behind X and Y, since either can change before Z ever resolves.

The practical effect is that X and Y do almost all of the work, because they are the only two variables anyone can actually estimate with any confidence. Long confidentiality lifetimes and slow migration paths are what turn a theoretical exposure into an urgent one, independent of whatever Z eventually turns out to be.