Harvest Now, Decrypt Later

Harvest now, decrypt later is a cyberattack strategy in which an adversary collects encrypted information today and stores it until future capabilities make decryption possible. It is particularly associated with the prospect of sufficiently powerful quantum computers breaking certain public-key cryptographic systems, exposing information that was confidential when collected.

How the strategy works

The attacker separates data collection from decryption. Information need not be readable immediately to have intelligence or commercial value later.

• In the harvesting stage, an adversary intercepts encrypted communications or obtains encrypted files and retains the ciphertext and relevant cryptographic information.
• In the later decryption stage, the adversary uses advances in computing or cryptanalysis to defeat the protection and recover readable information.
• The quantum-related concern is that a sufficiently capable quantum computer could break widely used public-key systems such as RSA and elliptic-curve cryptography.
• Where vulnerable public-key cryptography protects the establishment or exchange of encryption keys, recovering those keys could expose the stored communications, even when the data itself was encrypted using a symmetric cipher.

Why it is a security concern

The central risk is the mismatch between how long information must remain secret and how long its cryptographic protection will remain effective. A future breakthrough can therefore create a present-day confidentiality risk.

• Information requiring long-term secrecy, such as sensitive government communications, personal records and commercial secrets, is especially relevant because it may remain valuable when decryption becomes feasible.
• The attack does not require an adversary to possess a cryptographically capable quantum computer at the time of collection.
• A later security upgrade cannot retroactively protect copies of ciphertext already harvested by an adversary.
• The threat does not mean that quantum computing breaks all encryption equally; the principal concern discussed here is vulnerable public-key cryptography.

How organisations should respond

Defence requires preparation before vulnerable cryptography can be broken, rather than waiting for a practical attack.

• Organisations should create a cryptographic inventory identifying where cryptography is used, which algorithms protect information, and how long that information must remain confidential.
• Migration should prioritise sensitive, long-lived information and systems that require substantial time to upgrade.
• Post-quantum cryptography uses algorithms designed to resist attacks by both classical and quantum computers and can operate on conventional computing systems.
• Crypto-agility, the ability to replace cryptographic algorithms and implementations, helps organisations manage migration and respond to future vulnerabilities.