Quantum cryptanalysis
Attacking cryptography with quantum algorithms, which demolishes today's public-key systems but mostly just chafes the symmetric ones.
The field was born in 1994 when Shor's algorithm showed that factoring and discrete logarithms run in polynomial time on a quantum computer, taking RSA and elliptic curves down with them. Grover's algorithm offers only a quadratic speed-up over brute force, so symmetric keys survive by doubling in length.
The genuinely awkward result came from Simon's algorithm. Kuwakado and Morii used it in 2010 to break a three-round Feistel cipher, and in 2016 Kaplan, Leurent, Leverrier and Naya-Plasencia showed that CBC-MAC, GMAC and OCB collapse if the attacker can submit queries in superposition. That model is contested, since few real systems answer quantum queries. But it proved that doubling the key length is not enough when the mode of operation itself leaks a period.