Quantum computing does not threaten every algorithm protecting internet traffic today. A lot of the broader discussion about the quantum threat tends to blur that distinction, and when treated broadly, encryption is considered a monolithic category vulnerable to attack. In fact, one class of problems, such as factoring large numbers or solving discrete logarithms, is focused on the threat, while everything else falls relatively safely behind. That is the practical heart of post-quantum cryptography, knowing exactly what working algorithms need to be replaced when and in which exact manner.
That particular substitution is discussed in PQC for replacing vulnerable encryption algorithms, which describes how new algorithms are being designed to fill the roles that quantum-vulnerable ones can no longer securely occupy.
Why are some algorithms vulnerable but not others?
Encryption as a whole uses advanced math, which affects its vulnerability. RSA, Diffie-Hellman, or elliptic curve cryptography all rely on mathematical problems that classical computers can not easily break; a sufficiently large quantum computer could solve them relatively efficiently with the right quantum algorithm. You are symmetric encryption, the kind securing data-at-rest using a shared secret key, and therefore facing far less quantum risk given that known attacks on symmetric ciphers show only marginal speedup from quantum algorithms (e.g., quadratically with Grover, then simply not replacing your cipher but increasing the key length).
This classification is crucial for the priority attached to migration and how it ultimately unfolds. Symmetric algorithms mainly require parameter adjustments, while public-key algorithms are indeed in dire need of replacement. It causes organizations to either panic about systems that only need minor tuning or worse, complacent about those that are in dire need of a reset and fail to see the collective scope/amount of work ahead.
What Actually Gets Replaced
New algorithms are required for two fundamental mathematical functions that are compromised by quantum computers. Traditionally, key exchange, where two parties agree to a known secret over an insecure channel, has either been based on RSA or variants of Diffie-Hellman. The same frail mathematics lies underneath digital signatures, which authenticate identity and can detect tampering. These two functions require replacements for dissimilar quantum-resistant problems, because increasing a vulnerable algorithm’s key size does nothing to defend against the vulnerabilities at its heart.
The recently finalized federal standards are specifically of these two types. An ephemeral key encapsulation mechanism is a federal standard, based on the presumed security of something called “lattice-based mathematics,” and is thought to be secure against both quantum computers and classical attacks to replace RSA and Diffie-Hellman as the shared secret establishment method that came together in 1977. Companion standards encompass digital signatures with similar lattice-based and hash-based approaches, providing organizations a holistic, government-vetted toolkit at both major replacement tiers.
Why This Isn’t a Simple Swap
A detailed technical resource for engineers dealing with this transition, IETF guidance for engineers on PQC, states a simple but important point about this shift: unlike past cryptographic transitions (such as hash function competitions), this change is different—specifically, is not the same approach that was taken when transitioning to newer hash functions after older ones were compromised. Since post-quantum algorithms often have fundamental differences with the legacy algorithms they replace, this means that not all protocols can be simply drop-in replacements.
For example, key and signature sizes perfectly illustrate this. In fact, post-quantum algorithms typically generate larger keys, signatures and ciphertexts than their traditional counterparts — often to a great extent. The big size increase can also impact network protocols that were never designed to deal with cryptographic material of this scale, and may require changing how messages are sent and fragmented on the wire (or perhaps even which algorithm is used).
Why Hybrid Approaches Are Ideal During Transition
Because these replacement algorithms are new compared to centuries-old classical cryptography, many organizations are choosing not to make the leap all at once. Hybrid schemes are a combination of traditional and post-quantum algorithms, ensuring that as long as at least one of the two is not broken, security is guaranteed. In the near term, if someone other than you discovers a flaw in a newly adopted post-quantum algorithm, your traditional algorithm continues to protect the traffic. Should a quantum computer successfully break that traditional algorithm at some point, the post-quantum component carries on holding its own.
This layering adds some complexity and overhead, but it solves a genuine problem: post-quantum algorithms haven’t had nearly the decades of analysis that RSA and ECC have. The alternative, where organizations view the transition as a reversible step rather than an irreversible one-off swap, gives them an important margin of safety whilst confidence in the new algorithms continues to ramp up.
Getting the Sequencing Right
Not all of the quantum-vulnerable algorithms need to be replaced on the same schedule. Because of the harvest now, decrypt-later risk key exchange takes precedence, data encrypted today by public key algorithms vulnerable to quantum computers remains vulnerable after they are captured and then decrypted at a later date within a matter of hours when sufficiently capable quantum computers become available. Digital signatures deal with another, though no less severe, pressure, specifically for long-lived roots of trust (e.g., root certificates or firmware signing keys) that need to remain trustworthy years after their creation.
Understanding this difference prevents organizations from attempting to turn the dial on all fronts at once, a strategy that tends to lead to rushed decisions and increased mistakes. A sequencing replacement that focuses on what risk applies to what system is a far more manageable and defensible migration plan.
Frequently Asked Questions
Post-quantum security, does the whole system need the algorithm replaced?
No. The only algorithms that need to be completely replaced are those based on integer factorization or discrete logarithm problems, which mainly include key exchange and digital signatures. Symmetric encryption usually requires parameter changes than a wholly new algorithm.
Can you just drop in post-quantum algorithms?
Not always. Since many post-quantum algorithms produce larger keys and signatures than their classical counterparts, they are not just drop-in replacements for some protocols if those protocols have assumptions about message size built into their design.
Do we need to leave algorithms behind totally tomorrow?
Not for most organizations. Widely proposed hybrid approaches (which combine classical and post-quantum algorithms) during the transition phase, create a fall-back while confidence in newer algorithms continues to grow from empirical experimentation.
