Cybersecurity
Post-Quantum Cryptography Explained
Published 18 June 2026 · 6 min read · LLM Advocates Quantum Practice
Post-quantum cryptography, often shortened to PQC, refers to classical cryptographic algorithms designed to resist attack from both classical and quantum computers. It is easy to conflate PQC with quantum cryptography or quantum key distribution, but the two are different disciplines solving related problems in different ways.
PQC vs. quantum key distribution
Quantum key distribution uses the physical properties of photons to detect eavesdropping on a communication channel — it is a hardware-dependent, physics-based approach usually deployed over dedicated fibre or satellite links. Post-quantum cryptography, by contrast, is software: new mathematical problems (such as those based on structured lattices, hash functions or codes) believed to be hard for both classical and quantum computers to solve, implemented as drop-in replacements for today's RSA and elliptic-curve algorithms. For most organisations, PQC migration — not quantum hardware — is the near-term compliance obligation.
What "migration" actually involves
Migrating to post-quantum cryptography is rarely a simple software patch. It typically requires a cryptographic inventory (identifying every system, certificate and protocol that relies on vulnerable algorithms), a risk-prioritisation exercise (which systems protect data with the longest shelf life and therefore the greatest 'harvest now, decrypt later' exposure), vendor and supply-chain engagement (since many organisations do not control every cryptographic component in their stack), and a phased rollout that preserves interoperability with partners who may migrate on a different timeline.
In India, this is no longer a hypothetical exercise. Government guidance directed at critical information infrastructure sectors — including defence, power, telecommunications and core government systems — has set staged expectations for post-quantum cryptography implementation, with earlier deadlines for critical infrastructure and later ones for the broader economy. Organisations that are not formally designated as critical infrastructure would still be well served treating this as their own compliance runway rather than waiting for a mandate that names them directly.
The legal function's role is to translate this technical migration into governance: documenting the inventory and risk assessment defensibly, updating vendor and cloud contracts to require PQC-readiness, and building a crypto-agility clause into new agreements so that the next transition — because there will be one — does not require renegotiating from scratch.