Key Takeaways on Post-Quantum Cryptography
- Post-quantum cryptography refers to algorithms designed to withstand attacks from both quantum and classical computers.
- Future quantum computers could compromise electronic signature and certification mechanisms based on RSA and elliptic curves.
- Electronic signatures, digital seals, timestamps, and digital certificates will need to evolve to maintain their security guarantees.
- NIST published the first post-quantum standards in 2024. ANSSI recommends a gradual transition, particularly through the use of hybrid mechanisms.
- Documents stored for the long term require special attention to ensure that their origin, integrity, and date remain verifiable.
- For businesses and trusted service providers, crypto-agility—that is, the ability to adapt cryptographic mechanisms—is becoming a key factor in ensuring the long-term viability of digital evidence.
What is post-quantum cryptography?
Cryptography is used to protect digital communications, verify the identity of senders, and detect any alterations to a document.
Some of these mechanisms are based on mathematical problems that are extremely difficult for today's computers to solve. This is particularly true of the RSA and ECC algorithms, which are widely used in certificate and digital signature infrastructures.
Quantum computing could undermine this security. Using a mathematical algorithm known asShor’s algorithm, a sufficiently powerful and reliable quantum computer would be able to solve some of these problems much more efficiently than a classical computer.
Post-quantum cryptography ( PQC) aims to replace vulnerable mechanisms with new algorithms designed to withstand these computational capabilities.
Unlike quantum cryptography, it runs on classical computing infrastructure and does not require a quantum computer to be used.
Q-Day: When will quantum computers be able to compromise current cryptography?
Q-Day refers to the point in time when the computational capabilities of quantum computers will make it possible to break certain asymmetric cryptographic algorithms, notably RSA and ECC, which are currently widely used to secure data exchanges and electronic signatures.
No specific date can be given. Estimates vary depending on expected technological advances, particularly in error correction and the computational capacity of quantum computers.
For organisations, however, the risk is not limited to this future deadline. Some encrypted confidential data may already be intercepted and stored by attackers in the hope of being able to decrypt it at a later date. This strategy is known as “Harvest Now, Decrypt Later.”
When it comes to digital evidence, the challenge is different: the goal is to preserve the reliability of signatures, certificates, and timestamps issued today, so that they can still be verified after Q-Day.
It is precisely to prepare for this deadline that cybersecurity authorities are advocating for a gradual transition to post-quantum cryptography.
Why does quantum computing threaten the evidentiary value of documents?
Trust in an electronic document relies, in particular, on the ability to verify its origin, integrity, and, when necessary, its date.
However, the cryptographic mechanisms used to provide these guarantees may become vulnerable as technologies evolve.
Electronic Signatures and Stamps: A Risk of Tampering
A cryptographic signature verifies that a document was signed using a specific private key and that it has not been altered since it was signed.
If the algorithm used becomes vulnerable, an attacker with the necessary capabilities could potentially reconstruct that key and generate new fraudulent signatures.
The same issue applies to electronic seals, which are used to certify the origin and integrity of documents issued by legal entities.
This development would not automatically invalidate all prior signatures. However, it could complicate their verification and evidentiary assessment, particularly in the absence of evidence establishing that they were generated before the cryptographic mechanisms were compromised.
Electronic timestamping: How can proof of the date be preserved?
The electronic timestamp allows data to be linked to a date and time. When it is qualified under the eIDAS Regulation, it is presumed to accurately reflect the specified date and time, as well as to ensure the integrity of the data to which it is linked.
Timestamp tokens, however, rely on cryptographic signatures. If these signatures were to become vulnerable to quantum attacks, their verification could be compromised.
For qualified trust service providers (QTSPs), the challenge is therefore to update the cryptographic mechanisms used while ensuring the continuity and reliability of the services provided.
Electronic Archiving: Evidence Must Remain Verifiable Over Time
Commercial contracts, intellectual property files, or regulatory documents may need to be retained for many years.
Simply storing them in physical form is not enough to guarantee their evidential value. It must also be possible to establish the reliability of the mechanisms used to sign, time-stamp, or authenticate them.
Electronic archiving systems must therefore include mechanisms for long-term preservation, such as the renewal of cryptographic proofs, the retention of validation data, and the use of up-to-date algorithms.
The goal is to maintain the ability to verify the authenticity and integrity of documents despite technological advancements.
Post-quantum cryptography: What standards and what transition timeline?
The transition to post-quantum cryptography is now underway on an international scale. In August 2024, NIST, the U.S. standards organization, published its first three standards: FIPS 203 (ML-KEM) for key establishment, and FIPS 204 (ML-DSA) and FIPS 205 (SLH-DSA) for digital signatures.
In France,ANSSI recommends a phased approach based on hybridization: combining proven classical algorithms with post-quantum mechanisms to maintain system security during the transition.
The agency also aims to introduce post-quantum requirements for product certification starting in 2027.
At the European level, the roadmap published in June 2025 calls for member states to begin their transition by the end of 2026, with priority given to the most sensitive infrastructure, which is scheduled to be migrated by the end of 2030 at the latest.
Key Milestones in the Post-Quantum Transition
| Year | Stage | Description |
|---|---|---|
| 2024 | First NIST standards | Publication of the first three post-quantum cryptographic standards. |
| 2026 | Start of the European transition | Member states are encouraged to begin their transition before the end of the year. |
| 2027 | Changes to ANSSI qualifications | Introduction of post-quantum requirements for product certification. |
| 2030 | European Target for Critical Infrastructure | Deadline set for the migration of priority infrastructure. |
Sources: NIST, ANSSI, and the European Commission. Not all of these milestones have the same regulatory impact.
Is blockchain also threatened by quantum computing?
Blockchain is sometimes presented as a way to permanently guarantee the integrity and timestamping of digital information. However, distributed ledgers also rely on cryptographic mechanisms.
Some signatures used in blockchains could be compromised by sufficiently powerful quantum computers. Their future security will therefore depend on the algorithms used and the ability of the relevant networks to adapt them.
From a legal standpoint, the eIDAS Regulation V2 establishes a framework for qualified electronic registers, which are presumed to have records that are intact and in chronological order.
However, simply using a blockchain does not automatically provide these legal safeguards.
How can businesses prepare for the post-quantum transition?
For organizations that produce or retain electronic evidence, the priority is to anticipate technological changes without compromising the safeguards currently in place.
Several steps can be taken:
- Mapping cryptographic uses: identifying the signatures, certificates,timestamping services, and archiving systems in use.
- Assess retention periods: distinguish between documents with short retention periods and evidence that must remain verifiable for several decades.
- Consult trusted service providers: verify the existence of a post-quantum migration roadmap and mechanisms for preserving evidence.
- Prioritize crypto-agility: ensure that algorithms and keys can evolve without compromising service continuity.
These preparations are particularly relevant to legal departments, CISO, and compliance officers, whose responsibilities center on the protection and preservation of information.
Conclusion: Preserving Digital Trust Beyond Technological Advances
Quantum computing poses a new challenge for digital evidence: the security of a cryptographic mechanism may change long after the document to which it is associated has been created.
Post-quantum cryptography provides a technical solution to this challenge, but its adoption requires a gradual transformation of infrastructure, repositories, and retention practices.
For both companies and trusted service providers, the challenge lies in preserving the verifiability of evidence over time by anticipating the renewal of the cryptographic mechanisms that currently underpin their reliability.
Sources
- ANSSI, Post-Quantum Cryptography (PQC): https://cyber.gouv.fr/enjeux-technologiques/cryptographie-post-quantique/
- ANSSI, FAQ on Post-Quantum Cryptography: https://cyber.gouv.fr/enjeux-technologiques/cryptographie-post-quantique/faq-pqc/
- NIST, Announcing Approval of Three Federal Information Processing Standards for Post-Quantum Cryptography, August 13, 2024: https://www.nist.gov/news-events/news/2024/08/announcing-approval-three-federal-information-processing-standards-fips
- European Commission, “EU Reinforces Its Cybersecurity with Post-Quantum Cryptography,” June 23, 2025: https://digital-strategy.ec.europa.eu/en/news/eu-reinforces-its-cybersecurity-post-quantum-cryptography
- European Union, eIDAS Regulation No. 910/2014, as amended in particular by Regulation (EU) 2024/1183: https://eur-lex.europa.eu/legal-content/FR/TXT/?uri=CELEX:02014R0910-20241018
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