(Quantum computing poses a threat to which cryptographic algorithm?

Large-scale quantum computers threaten many widely deployed public-key algorithms because quantum algorithms can solve their underlying hard math problems efficiently. In particular, Shor’s algorithm can factor large integers and compute discrete logarithms in polynomial time, which directly breaks the security assumptions of RSA (factoring) and traditional Diffie–Hellman/ECC (discrete log). That makes RSA the clearest target among the options. By comparison, symmetric algorithms like AES are affected mainly by Grover’s algorithm, which provides a quadratic speedup for brute force; this can be mitigated by using larger keys (e.g., AES-256). Hash functions like SHA-256 also face Grover-style speedups for preimage search, again mitigated by output length/security margin, and collision resistance is impacted differently. ECB is a mode of operation, not a standalone algorithm, and its issues are classical (pattern leakage), not specifically quantum. Therefore, the algorithm most directly threatened by quantum computing among the choices is RSA.