Quantifying Attacker Payoff in Harvest Now, Decrypt Later Attacks: A Persistence-Based Framework for Trade Secret Risk

Harvest-now, decrypt-later (HNDL) assumes that encrypted secrets stolen today will remain worth decrypting when quantum computers eventually make that possible. But what if they do not? This paper models HNDL from the attacker’s perspective by asking a different question: how much will a stolen secret still be worth when it can finally be decrypted? We develop a conditional retained-value framework combining probabilistic Q-day timing, information depreciation, and adversary-access delay. The results reveal a sharp divide. Rapidly depreciating secrets can lose much of their strategic value before quantum decryption becomes useful, while persistent secrets can retain substantial attacker payoff even under relatively early postquantum cryptography (PQC) migration. A stylised aerospace defence analogue demonstrates the effect, while an exploratory citation-based persistence analysis examines whether long horizon information persistence can be observed empirically. The findings expose a blind spot in calendar-based PQC planning: the same quantum timeline does not create the same HNDL risk for every secret. HNDL exposure depends not only on when quantum decryption arrives, but on what remains worth stealing when it does.