Post-quantum cryptography

Post-quantum cryptography, sometimes referred to as quantum-proof, quantum-safe, or quantum-resistant, is the development of cryptographic algorithms that are currently thought to be secure against a cryptanalytic attack by a quantum computer. Most widely used public-key algorithms rely on the difficulty of one of three mathematical problems: the integer factorization problem, the discrete logarithm problem or the elliptic-curve discrete logarithm problem. All of these problems could be easily solved on a sufficiently powerful quantum computer running Shor's algorithm or possibly alternatives.
As of 2026, quantum computers lack the processing power to break widely used cryptographic algorithms; however, because of the length of time required for migration to quantum-safe cryptography, cryptographers are already designing new algorithms to prepare for Y2Q or "Q-Day", the day when current algorithms will be vulnerable to quantum computing attacks. Mosca's theorem provides the risk analysis framework that helps organizations identify how quickly they need to start migrating.
Their work has gained attention from academics and industry through the PQCrypto conference series hosted since 2006, several workshops on Quantum Safe Cryptography hosted by the European Telecommunications Standards Institute, and the Institute for Quantum Computing. The rumoured existence of widespread harvest now, decrypt later programs has also been seen as a motivation for the early introduction of post-quantum algorithms, as data recorded now may still remain sensitive many years into the future.
In contrast to the threat quantum computing poses to current public-key algorithms, most current symmetric cryptographic algorithms and hash functions are considered to be relatively secure against attacks by quantum computers. While the quantum Grover's algorithm does speed up attacks against symmetric ciphers, doubling the key size can effectively counteract these attacks. Thus post-quantum symmetric cryptography does not need to differ significantly from current symmetric cryptography.
In 2024, the U.S. National Institute of Standards and Technology released final versions of its first three Post-Quantum Cryptography Standards.

Preparation

Digital infrastructures require robust cybersecurity. Cryptographic systems are vital to protect the confidentiality and authenticity of data. Quantum computing will be a threat to many of the classical cryptographic algorithms, which are used to achieve these protection goals but are only secure against classical computers. Data that is currently not quantum-safe, whether it is stores or transmitted, and that must remain confidential for a long time, may be compromised in the future by quantum computers. In addition, authenticity will also be jeopardised by quantum computers. The threat that quantum computing poses to cybersecurity can be countered by a timely, comprehensive and coordinated transition to post-quantum cryptography.

Algorithms

Post-quantum cryptography research is mostly focused on six different approaches:

Lattice-based cryptography

This approach includes cryptographic systems such as learning with errors, ring learning with errors, the ring learning with errors key exchange and the ring learning with errors signature, the older NTRU or GGH encryption schemes, and the newer NTRU signature and BLISS signatures. Some of these schemes like NTRU encryption have been studied for many years without anyone finding a feasible attack. Others like the ring-LWE algorithms have proofs that their security reduces to a worst-case problem. The Post-Quantum Cryptography Study Group sponsored by the European Commission suggested that the Stehle–Steinfeld variant of NTRU be studied for standardization rather than the NTRU algorithm. At that time, NTRU was still patented. Studies have indicated that NTRU may have more secure properties than other lattice based algorithms. Two lattice-based algorithms, CRYSTALS-Kyber and CRYSTALS-Dilithium were among the first post-quantum algorithms standardised by NIST.

Multivariate cryptography

This includes cryptographic systems such as the Rainbow scheme which is based on the difficulty of solving systems of multivariate equations. Various attempts to build secure multivariate equation encryption schemes have failed. However, multivariate signature schemes like Rainbow could provide the basis for a quantum secure digital signature. The Rainbow Signature Scheme is patented.

Hash-based cryptography

This includes cryptographic systems such as Lamport signatures, the Merkle signature scheme, the XMSS, the SPHINCS, the WOTS and the SPINCS⁺ schemes. Hash based digital signatures were invented in the late 1970s by Ralph Merkle and have been studied ever since as an interesting alternative to number-theoretic digital signatures like RSA and DSA. Their primary drawback is that for any hash-based public key, there is a limit on the number of signatures that can be signed using the corresponding set of private keys. This fact reduced interest in these signatures until interest was revived due to the desire for cryptography that was resistant to attack by quantum computers. There appear to be no patents on the Merkle signature scheme and there exist many non-patented hash functions that could be used with these schemes. The stateful hash-based signature scheme XMSS developed by a team of researchers under the direction of Johannes Buchmann is described in RFC 8391.
Note that all the above schemes are one-time or bounded-time signatures. Moni Naor and Moti Yung invented UOWHF hashing in 1989 and designed a signature based on hashing which can be unlimited-time in use.

Code-based cryptography

This includes cryptographic systems which rely on error-correcting codes, such as the McEliece and Niederreiter encryption algorithms and the related Courtois, Finiasz and Sendrier Signature scheme. The original McEliece signature using random Goppa codes has withstood scrutiny for over 40 years. However, many variants of the McEliece scheme, which seek to introduce more structure into the code used in order to reduce the size of the keys, have been shown to be insecure. The Post-Quantum Cryptography Study Group sponsored by the European Commission has recommended the McEliece public key encryption system as a candidate for long term protection against attacks by quantum computers. In 2025, NIST announced plans to standardize the code-based HQC encryption algorithm.

Isogeny-based cryptography

These cryptographic systems rely on the properties of isogeny graphs of elliptic curves over finite fields, in particular supersingular isogeny graphs, to create cryptographic systems. Among the more well-known representatives of this field are the Diffie–Hellman-like key exchange CSIDH, which can serve as a straightforward quantum-resistant replacement for the Diffie–Hellman and elliptic curve Diffie–Hellman key-exchange methods that are in widespread use today, and the signature scheme SQIsign which is based on the categorical equivalence between supersingular elliptic curves and maximal orders in particular types of quaternion algebras. Another widely noticed construction, SIDH/SIKE, was spectacularly broken in 2022. The attack is however specific to the SIDH/SIKE family of schemes and does not generalize to other isogeny-based constructions.

Symmetric key quantum resistance

Using sufficiently large key sizes, the symmetric key cryptographic systems like AES and SNOW 3G are already resistant to attack by a quantum computer. Further, key management systems and protocols that use symmetric key cryptography, instead of public key cryptography like Kerberos and the 3GPP Mobile Network Authentication Structure, are also inherently secure against attack by a quantum computer. Given its widespread deployment in the world, some researchers recommend expanded use of Kerberos-like symmetric key management as an efficient way to get post-quantum cryptography today.

Security reductions

In cryptography research, it is desirable to prove the equivalence of a cryptographic algorithm and a known hard mathematical problem. These proofs are often called "security reductions", and are used to demonstrate the difficulty of cracking the encryption algorithm. In other words, the security of a given cryptographic algorithm is reduced to the security of a known hard problem. Researchers are actively looking for security reductions in the prospects for post-quantum cryptography. Current results are given here:

Lattice-based cryptography – Ring-LWE Signature

In some versions of Ring-LWE there is a security reduction to the shortest-vector problem in a lattice as a lower bound on the security. The SVP is known to be NP-hard. Specific ring-LWE systems that have provable security reductions include a variant of Lyubashevsky's ring-LWE signatures defined in a paper by Güneysu, Lyubashevsky, and Pöppelmann. The GLYPH signature scheme is a variant of the Güneysu, Lyubashevsky, and Pöppelmann signature which takes into account research results that have come after the publication of the GLP signature in 2012. Another Ring-LWE signature is Ring-TESLA. There also exists a "derandomized variant" of LWE, called Learning with Rounding, which yields "improved speedup and bandwidth". While LWE uses the addition of a small error to conceal the lower bits, LWR uses rounding for the same purpose.

Lattice-based cryptography – NTRU, BLISS

The security of the NTRU encryption scheme and the BLISS signature is believed to be related to, but not provably reducible to, the closest vector problem in a lattice. The CVP is known to be NP-hard. The Post-Quantum Cryptography Study Group sponsored by the European Commission suggested that the Stehle–Steinfeld variant of NTRU, which does have a security reduction, be studied for long term use instead of the original NTRU algorithm.