Self-custody and post-quantum backups

Short answer: Self-custody depends on keeping private keys safe, and today's public-key cryptography is expected to be breakable by a large enough quantum computer. The practical concern is "harvest now, decrypt later": data and public keys captured today could be attacked years from now, and a backup is a long-lived exposure. No such quantum computer exists yet, but long-lived keys and backups are worth planning for now, starting with knowing what cryptography they rely on.

The Nature of the Quantum Threat

The primary concern for self-custody is the vulnerability of public-key cryptography to quantum algorithms. Traditional blockchain security and digital signatures rely on mathematical problems that are difficult for classical computers but potentially solvable by sufficiently powerful quantum computers. Researchers have proposed that quantum computers could theoretically use public blockchain data to derive private keys in certain cases. If this capability is realized, it would allow an attacker to bypass standard wallet backups and access funds stored on vulnerable addresses. This theoretical risk makes the transition to post-quantum cryptography essential for maintaining the integrity of self-custody. The threat is not merely about breaking encryption in real-time during a transaction; it is about the ability to retroactively compromise the security of assets held over long periods. Therefore, the definition of a "quantum-ready" device or system is one built to run the upgrades of the future, specifically designed to withstand the specific types of attacks that quantum computers could theoretically execute against current cryptographic standards.

Harvest Now, Decrypt Later

Post-quantum cryptography is often misunderstood as preparation for a quantum computer accessing a data center in the immediate future. In reality, it addresses a different temporal dynamic: protecting long-lived data from adversaries who capture it today and wait for the technology to decrypt it later. This logic underpins the "harvest now, decrypt later" cyberattack strategy. An attacker does not need quantum computing power right now; they only need it later. By capturing encrypted information or observing public data today, they position themselves to decrypt that data once quantum capabilities are available. This changes the nature of the risk from a speculative future event to a present-day planning decision. For self-custody, where keys may need to remain valid for decades, this strategy is particularly relevant. Backup systems, which are promises made to one’s future self, must be designed with the assumption that data captured today could be exposed tomorrow. Thus, the timeline for implementing quantum-resistant cryptography is compressed, requiring immediate consideration of migration strategies rather than waiting for quantum hardware to become operationally dominant.

Implications for Backup and Custody

The intersection of self-custody and post-quantum readiness highlights the fragility of traditional backup assumptions. If an attacker can access encrypted information today and store it, the security of a backup depends not only on the strength of the encryption at the time of creation but also on the longevity of that encryption against future computational advances. For self-custody, this means that the act of creating a backup is not a one-time security event but the beginning of a long-term exposure window. Systems must be designed to validate and restore operations even if the underlying cryptographic assumptions change. The move to post-quantum cryptography is therefore not just a technical upgrade but a fundamental re-evaluation of how long-lived data is protected. By adopting hybrid implementations and prioritizing the migration of critical custody systems, stakeholders can mitigate the risk of having their self-custody assets compromised by future quantum capabilities. The goal is to ensure that the promise of backup remains intact, regardless of the computational landscape in which it is eventually invoked.

FAQ

Why does quantum computing matter for self-custody?

Digital signatures and key schemes rely on mathematical problems that a sufficiently powerful quantum computer could solve. In some cases, public data could then be used to derive private keys.

What does "harvest now, decrypt later" mean?

An attacker collects encrypted data or public information today and waits for future quantum capability to attack it. It matters most for data and keys that must stay secure for decades.

Why are backups affected?

Creating a backup starts a long exposure window. Its security depends on how long the encryption stays strong against future advances, not only on its strength on the day it was made.

What does "quantum-ready" mean for a device?

According to the Trezor guide, it describes a device built so it can run the upgrades needed for post-quantum cryptography as they arrive.

Are quantum computers able to attack wallets today?

No. This is a planning risk for the future, not a current attack, and nothing here is a prediction of when it will happen.