I Spent a Weekend Implementing Post-Quantum Cryptography - Here's What I Learned

#security #cryptography #quantum #tutorial

Last month, our security auditor dropped a bomb during a routine review:

"What's your post-quantum cryptography migration plan?"

The room went silent. Twenty experienced developers and not one of us had a real answer.

That question sent me down a weekend rabbit hole that fundamentally changed how I think about encryption. If you're like I was vaguely aware that quantum computers might break encryption someday this post is for you.

The Wake-Up Call

Here's what made me panic: it's not about when quantum computers arrive. It's about how long your data needs to stay secret.

Current Year: 2025
Data Encryption Lifespan: 20 years
Risk Window: Starting NOW

Why? Because of Harvest Now, Decrypt Later (HNDL) attacks. Adversaries are already collecting encrypted data, storing it, and waiting for quantum computers powerful enough to decrypt it.

Your medical records from 2020? That API key you encrypted last year? They're all potentially exposed.

The Quantum Threat Timeline

According to research from the Global Risk Institute:

By 2034: 17-34% chance of breaking RSA-2048
By 2044: 79% chance

If you're encrypting data with 15–20 year lifespans, you’re already in the danger zone.

My Weekend Deep Dive

I stopped panicking and started coding. Here's what I learned:

1. The Standards Are Already Here

NIST released their first quantum-resistant algorithms in 2024:

ML-KEM (formerly CRYSTALS-Kyber) for key encapsulation
ML-DSA (formerly CRYSTALS-Dilithium) for digital signatures
SLH-DSA (formerly SPHINCS+) for hash-based signatures

2. Implementation Is Surprisingly Accessible

I expected complex math, but found usable libraries. Here’s a working example using Open Quantum Safe (OQS):

#include <oqs/oqs.h>

int main() {
    OQS_KEM *kem = OQS_KEM_new(OQS_KEM_alg_kyber_768);

    uint8_t *public_key = malloc(kem->length_public_key);
    uint8_t *secret_key = malloc(kem->length_secret_key);
    OQS_KEM_keypair(kem, public_key, secret_key);

    uint8_t *ciphertext = malloc(kem->length_ciphertext);
    uint8_t *shared_secret = malloc(kem->length_shared_secret);
    OQS_KEM_encaps(kem, ciphertext, shared_secret, public_key);

    printf("Generated %zu-byte quantum-safe shared secret\n", kem->length_shared_secret);

    OQS_KEM_free(kem);
    return 0;
}

3. Performance Surprised Me

I expected PQC to be slow it wasn’t. Here's what my benchmarks showed:

Algorithm Operation vs RSA-2048
Kyber-768 Key Gen 3× faster
Kyber-768 Encapsulation 2.5× faster
Dilithium-3 Signing 4× faster
Dilithium-3 Verification 2× faster

Yes, post-quantum crypto can be faster than legacy algorithms.

The Hybrid Approach (What You Should Do Now)
The smart strategy? Hybrid cryptography combine classical + post-quantum algorithms.

// Pseudo-code for hybrid key exchange
classical_secret = ECDH_key_exchange();
pq_secret = Kyber_key_exchange();

final_key = KDF(classical_secret || pq_secret);