Understanding Post-Quantum

Entrust has taken a leading role in preparing for post-quantum cryptography by collaborating with other organizations to propose new IETF X.509 certificate formats that place traditional encryption methods like RSA and ECC side-by-side with new PQ algorithms.

For example, we’re closely following the work of organizations like the National Institute of Standards and Technology (NIST), which has a project underway to develop algorithms that are resistant to quantum computing and eventually standardize them. We want to help companies sustain their IT ecosystem to reduce replacements, maintain system uptime, and avoid costly changes caused by a lack of preparation.

Entrust has been actively leading the discussions in IETF Forums, where solutions can be considered within the PQ community. Our public propositions are published in the IETF standards forum:

Composite Keys and Signatures for Use in Internet PKI
The widespread adoption of post-quantum cryptography will bring the need for an entity to possess more than one public key for multiple cryptographic algorithms. Since the trustworthiness of individual post-quantum algorithms is in question, a multi-key cryptographic operation will need to be performed so that breaking it requires cracking each component algorithm individually. This requires defining new structures for holding composite public keys and composite signature data.

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Multiple Public-Key Algorithm X.509 Certificates
This document describes a method of embedding alternative sets of cryptographic materials into X.509v3 digital certificates, X.509v2 Certificate Revocation Lists (CRLs), and PKCS #10 Certificate Signing Requests (CSRs).

The embedded alternative cryptographic materials allow a public key infrastructure to use multiple cryptographic algorithms in a single object. Moreover, it enables it to transition to the new cryptographic schemes while maintaining backward compatibility with systems using the existing algorithms. Three X.509 extensions and three PKCS #10 attributes are defined, and the signing and verification procedures for the alternative cryptographic material contained in the extensions and attributes are detailed.

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Problem Statement for Post-Quantum Multi-Algorithm PKI
The post-quantum community (for example, surrounding the NIST PQC competition) is pushing for "hybridized" crypto that combines RSA/ECC with new primitives to hedge our bets against both quantum adversaries. It’s also advocating for algorithmic/mathematical breaks of the new primitives. After two stalled submissions, Entrust submitted a draft that acts as a semi-formal problem statement and an overview of the three main solution categories.

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How Post-Quantum Computing Will Affect Cryptography
Properly designed digital signature schemes used for authentication will remain secure until the day a suitable quantum computer actually comes online. Today’s quantum computers are limited in size and, therefore, pose no threat to present-day cryptography. And several significant engineering obstacles must be overcome before the threat becomes real.

Nevertheless, experts think these obstacles will fade in time. Many predict that a quantum computer capable of breaking today’s standard public-key algorithms will be available within the planned life of systems currently in development.

Today’s public-key algorithms are deployed for authentication, digital signature, data encryption, and key establishment purposes. Once quantum computers of sufficient size become a reality, we’ll need to replace cryptographic schemes for each of these functions.

Data encryption and key-agreement algorithms are susceptible to a recorded-cipher-text attack, in which an adversary today records exchanges protected by pre-quantum algorithms and stores the cipher text for analysis in the future. This is what’s known as a “harvest now, decrypt later” strategy. Once a viable quantum computer is created, hackers will be able to recover the plaintext. Depending on the required algorithm security lifetime, pre-quantum cryptography will become vulnerable sooner for these key purposes.

Once a suitable quantum computer exists, a signer could repudiate signatures created earlier, claiming that they were forged using a private key broken later by a quantum computer.

Post-Quantum and Classical Hybrid Cryptography
There are different approaches on how to prepare for secure cryptographical communications in a post-quantum age. Using a hybrid approach is one of the more popular methods being proposed as a way of transitioning to the as yet undefined PQ algorithms.

The hybrid approach suggests that rather than trust one algorithm, it places traditional algorithms like RSA and ECC alongside new PQ algorithms. This is helpful for current use cases while pre-quantum is an acceptable method for authentication and to test IT ecosystems against PQ algorithms.