A computer system and associated methods are disclosed for mitigating side-channel attacks using a shared cache. The computer system includes a main memory, a shared cache and a cache controller for the shared cache including a scrambling function that scrambles addresses of memory accesses according to the respective scrambling keys selected for a sequence of time periods. Different cache tiers may implement different scrambling functions optimized to the architecture of each cache tier. Scrambling keys may be updated to reduce predictability of shared cache to memory address mappings. These updates may occur opportunistically, on demand or on specified schedule. Multiple scrambling keys may be simultaneously active during transitions between active time periods.