Grover's algorithm
A quantum search algorithm that finds an item in an unstructured set of N in roughly √N steps. Against symmetric cryptography this halves effective key strength — a nuisance answered by doubling key sizes, not a break.
In practice
It reduces AES-256 to roughly 128 bits of effective security.
Related terms
A quantum algorithm that factors large integers and computes discrete logarithms in polynomial time. It is the specific reason RSA, Diffie-Hellman and elliptic-curve cryptography would fail against a sufficiently large quantum computer.
Cryptography where the same secret key both encrypts and decrypts. It is far less affected by quantum computing than public-key cryptography — Grover's algorithm only halves the effective key strength.
The standard symmetric block cipher used for bulk encryption. AES is not broken by quantum computers: Grover's algorithm gives only a square-root speedup, so AES-256 remains secure and AES-128 remains adequate for most uses.