Quantum computers get a lot of hype, and most of it's justified once you understand what they can actually break. RSA, the encryption used all over the internet including email authentication like DKIM, relies on one simple fact: multiplying two huge prime numbers is easy, but factoring the result back into those two primes is hard. Really hard. Hard enough that on a regular computer, a big enough key would take longer than the age of the universe to crack.
A quantum computer changes that math. There's an algorithm called Shor's algorithm, built specifically to factor huge numbers fast on a quantum computer. If someone eventually builds one big enough (we're talking tens of millions of stable qubits; today's largest are around 1,000), RSA doesn't get "harder to crack." It breaks.
Here's the part that surprises people: not all cryptography has this problem. Quantum-resistant crypto doesn't rely on factoring numbers at all. It's built on different math problems, things like navigating an extremely high-dimensional lattice, that nobody has found a fast way to solve, quantum computer or not. Shor's algorithm doesn't touch it, because Shor's algorithm only knows how to attack one specific kind of problem, and lattice problems aren't it.
Why care now, when a real code-breaking quantum computer might be a decade or more out? Because of "harvest now, decrypt later." Someone can record your encrypted traffic today and just sit on it until a quantum computer exists to crack it. If what you're protecting needs to stay secret for years, waiting until quantum computers show up before switching algorithms is too late. The data was already taken.
That's the reason NIST has been standardizing post-quantum algorithms, and why I've been digging into what it means for something like DKIM. RSA isn't dead yet, but it's got an expiration date.
If you want to see this played out in code rather than just talked about, I put together quantum-dkim-email-factoring, which actually runs Shor's algorithm factoring toy RSA keys in a quantum simulator.