Mosca's Inequality

Set how long data must stay secret, how long the migration takes, and when a quantum computer arrives. The timeline shows the years an attacker who recorded the data can read it.

RiskMigrationdemo

Objectives

Apply x + y > z to decide whether recorded data is exposed, and compute for how many years.
Explain why key exchange must move before signatures.
Explain why shortening the migration is the one choice that does not depend on z.

50%

Set how long data must stay secret, how long the migration takes, and when a quantum computer arrives.

if x + y > z, recorded data is exposed for x + y − z years

Three numbers decide whether you are late

x is how many years the data must stay secret. y is how many years the move to post-quantum cryptography takes. z is how many years until a cryptographically relevant quantum computer (CRQC) exists. Data sent with classical key exchange just before the migration ends must stay secret until year y + x. If that is later than year z, an attacker who recorded it can read it while it still matters. The buttons load round-number illustrations, not forecasts.

x: years the data must stay secret

y: years the migration takes

z: years until a CRQC (an assumption)

x + y

x + y − z

migration done

2031

x + y = 15 is greater than z = 10. From 2036 to 2041, recorded data that should still be secret can be decrypted: x + y − z = 5 years.

Nobody knows z

x and y are yours to estimate. z is not. In the Global Risk Institute's 2025 report, published 03/09/2026, 26 experts put the chance of a CRQC within 10 years at 28–49%, and within 15 years at 51–70%. These are opinions, not measurements.

with x = 10 and y = 5, you are safe only if

z ≥ 15 years, that is, no CRQC before 2041

The move that does not depend on z is to shorten y: start the migration now. Key exchange comes first, because recorded traffic is already at risk. Hybrid key exchange, such as X25519MLKEM768 in TLS, protects new connections today.

step 1/2

CPE 645 / CS 685

Mosca's Inequality

Grover's Search