Classical Cryptography vs Post-Quantum Cryptography

What is the difference?

In this article, “traditional asymmetric cryptography” means public-key algorithms based on integer factorization, finite-field discrete logarithms, elliptic-curve discrete logarithms, or related problems. “Post-quantum asymmetric cryptography” means an asymmetric algorithm intended to resist attacks from both classical and quantum computers. The term “classical” is often used interchangeably with “traditional” in this context, but it should not be read as a claim that every classical cryptographic algorithm is broken or obsolete today.1

The quantum concern is concentrated in public-key cryptography because a sufficiently capable cryptographically relevant quantum computer could threaten commonly used public-key key-establishment and signature schemes whose security depends on integer-factorization or discrete-logarithm problems. Traditional public-key algorithms therefore remain useful within their current security assumptions, but systems that need long-term confidentiality or durable authentication should account for the possibility of future quantum attacks and for recorded traffic being decrypted later.2345

Decision-useful comparison

The following comparison focuses on traditional asymmetric cryptography versus PQC. Exact sizes, latency, throughput, and security levels depend on the selected algorithm, parameter set, implementation, protocol, hardware, and operating environment. Consequently, PQC should not be evaluated using a single universal size or performance number.83

Traditional schemes often provide compact keys and signature or ciphertext values and have benefited from longer operational experience. PQC schemes commonly have larger public keys, ciphertexts, or signatures and can have different processing characteristics. Those differences can affect bandwidth, memory, certificate handling, hardware modules, latency, and message formats. PQC algorithms are also not necessarily drop-in replacements: traditional RSA and elliptic-curve systems can serve more than one role, while standardized PQC components may separate key establishment from signatures.83

Security assumptions and functions

Traditional asymmetric algorithms derive security from problems associated with factorization or discrete logarithms. The concern is not that a quantum computer automatically defeats all cryptography; rather, known quantum algorithms create a different threat model for certain public-key assumptions. PQC algorithms are selected or designed around assumptions intended to withstand both classical and quantum attacks, although “post-quantum” describes a security goal, not an unconditional guarantee. A future classical attack, quantum attack, implementation weakness, or misuse can still compromise a PQC deployment.2318

The functions must also be compared separately. Key establishment may use traditional Diffie–Hellman or RSA-style exchanges, whereas PQC commonly uses KEMs such as ML-KEM. Authentication may use traditional signatures or PQC signature standards such as ML-DSA and SLH-DSA. Because these functions and interfaces differ, replacing a key-establishment mechanism does not automatically replace a signature mechanism, and replacing one does not automatically address every cryptographic use in an application.132

At the time covered by the cited NIST project evidence, NIST expected ML-KEM, ML-DSA, and SLH-DSA to provide the foundation for most deployments and stated that they could and should be put into use. The same evidence says that Falcon and HQC were selected for ongoing standardization and that additional work continued. This makes the migration status more mature than an experiment, but not static or complete.9

Why symmetric algorithms and hashes are different

Symmetric encryption and hash functions do not have the same known quantum-risk profile as public-key cryptography. NIST’s transition draft states that existing NIST standards for symmetric cryptography—including hash functions, XOFs, block ciphers, KDFs, and DRBGs—are significantly less vulnerable to known quantum attacks than the referenced public-key standards. It further states that NIST-approved symmetric primitives providing at least 128 bits of classical security are believed to meet at least category 1 security requirements.4

This distinction does not mean that symmetric cryptography and hashes require no review. Systems still need to assess key sizes, security levels, algorithm usage, implementation quality, protocol composition, and lifecycle requirements. The practical PQC transition is principally a public-key transition, but a complete cryptographic inventory should identify where symmetric encryption, hashing, KDFs, signatures, certificates, and key establishment are each used. Calling all classical cryptography broken would therefore be technically inaccurate.43

Sizes, performance, and implementation maturity

PQC can impose resource costs that are visible at system boundaries. ETSI reports that post-quantum algorithms typically have larger public keys, ciphertexts, or signature values than traditional algorithms, with consequences for protocol bandwidth. NIST’s transition material also identifies larger key sizes and different computational requirements as issues for cryptographic hardware, including hardware security modules and trusted platform modules. Engineers should measure message expansion, certificate and key storage, CPU time, memory, latency, and failure behavior in the target environment rather than relying on a generic benchmark.834

Maturity has more than one dimension. Standardization and validation provide an important baseline, but newer algorithms may not have received the same depth of study as RSA or finite-field and elliptic-curve Diffie–Hellman. ETSI notes concerns about cryptanalysis maturity and the difficulty of detecting implementation mistakes, including side-channel protection challenges. A deployment decision should therefore consider the selected standard, parameter set, implementation provenance, validation needs, independent review, secure coding, side-channel defenses, and operational monitoring.185

Standards permit implementations in software, firmware, hardware, or combinations of these, and validation is concerned with conforming behavior. NIST guidance also emphasizes managing cryptographic data during execution and at rest, including intermediate values and decapsulation keys. Moving to PQC does not remove ordinary requirements for protecting secrets, checking inputs, destroying sensitive data appropriately, or preventing leakage.6710

Protocol impact and hybrid deployment

PQC migration is not necessarily a one-for-one algorithm replacement. Protocols and infrastructure may need redesign because of changed key sizes, ciphertext and signature sizes, resource use, interfaces, and algorithm roles. Applications, libraries, certificates, hardware modules, firmware, and user-facing systems may all require changes. Developers may need to refactor code, test interoperability, optimize implementations, and accommodate updated protocols and libraries.3

A hybrid scheme combines traditional and post-quantum components. It may be used to preserve compatibility with clients that do not yet support PQC, to retain security if a PQC algorithm or implementation is later weakened, or to satisfy protocol or validation constraints. The intended hybrid security property is generally that compromising one component alone is insufficient to break the combined construction, but this property depends on correct binding and composition.8

Hybrid deployment is not automatically safer. Poorly designed combinations can create signing or decapsulation-oracle problems, invalidate security assumptions, increase protocol and key-management complexity, or permit downgrade attacks during algorithm negotiation. ETSI also notes that hybrid protocols can reduce some bandwidth, computation, and latency overheads in particular pairings, but the requirements depend on the protocol and use case. A hybrid design needs a documented security analysis, explicit component-failure behavior, protected negotiation, interoperability tests, and independent review.

Migration status and a practical decision process

The evidence describes migration as a staged transition in which upgraded and non-upgraded agents may need to coexist and communicate. Public-key cryptography is often used between organizations or endpoints that have not previously interacted, so coordination across organizations, products, ecosystems, and protocol communities is unusually important. The transition should be treated as a program of discovery, prioritization, engineering, deployment, and retirement—not as a library upgrade performed once.34

1. Inventory public-key uses, including key establishment, signatures, certificates, stored keys, hardware modules, firmware, protocols, applications, and external dependencies.
2. Prioritize information and communications whose confidentiality or authenticity must remain protected for many years, including recorded traffic that could be targeted by “harvest now, decrypt later” attacks.
3. Separate decisions for key establishment, digital signatures, symmetric encryption, hashes, KDFs, and other cryptographic functions; do not assume one replacement covers all uses.
4. Select standardized and appropriately parameterized algorithms for the use case, then benchmark sizes, CPU, memory, latency, bandwidth, error handling, and hardware support in representative environments.
5. Plan interoperability and staged deployment. Where justified, evaluate a carefully specified hybrid mode, including downgrade resistance, component binding, validation, and the consequences if either component fails.
6. Test implementations and protocols, review side-channel and key-management controls, monitor operational behavior, and retain crypto-agility so algorithms can be changed as standards and evidence evolve.
7. Define controlled legacy use and retirement criteria rather than leaving traditional algorithms enabled indefinitely without an explicit risk decision.

Urgency should be risk-based. NIST’s transition material identifies harvest-now-decrypt-later risk as a reason that some applications may need quantum-resistant key establishment before classical schemes are generally disallowed. Organizations should therefore compare data lifetime, exposure, migration lead time, interoperability constraints, and the consequences of authentication failure or retrospective decryption. A system with short-lived, low-value data may have a different schedule from one protecting information whose value persists for many years.45

Conclusion

Traditional asymmetric cryptography and PQC solve similar public-key functions under different security assumptions. PQC addresses the prospective quantum threat to many traditional public-key schemes, but it brings new algorithms, interfaces, sizes, performance characteristics, implementation considerations, and interoperability work. Symmetric algorithms and hashes have a less severe known quantum-risk profile and should not be treated as equivalent to quantum-vulnerable public-key systems. A defensible transition combines inventory and prioritization with measured implementation, protocol-aware migration, carefully analyzed hybrids where appropriate, and continuing attention to ordinary cryptographic and operational risks.2384