Ibex implements a set of extra features (when the SecureIbex parameter is set) to support security-critical applications. All features are runtime configurable via bits in the cpuctrl custom CSR.
When enabled (via the data_ind_timing control bit in the cpuctrl register), execution times and power consumption of all instructions shall be independent of input data. This makes it more difficult for an external observer to infer secret data by observing power lines or exploiting timing side-channels.
In Ibex, most instructions already execute independent of their input operands. When data-independent timing is enabled: * Branches execute identically regardless of their taken/not-taken status * Early completion of multiplication by zero/one is removed * Early completion of divide by zero is removed
When enabled (via the dummy_instr_en control bit in the cpuctrl register), dummy instructions will be inserted at random intervals into the execution pipeline. The dummy instructions have no functional impact on processor state, but add some difficult-to-predict timing and power disruption to executed code. This disruption makes it more difficult for an attacker to infer what is being executed, and also makes it more difficult to execute precisely timed fault injection attacks.
The frequency of injected instructions can be tuned via the dummy_instr_mask bits in the cpuctrl register.
| dummy_instr_mask | Interval |
| 000 | Dummy instruction every 0 - 4 real instructions |
| 001 | Dummy instruction every 0 - 8 real instructions |
| 011 | Dummy instruction every 0 - 16 real instructions |
| 111 | Dummy instruction every 0 - 32 real instructions |
Other values of dummy_instr_mask are legal, but will have a less predictable impact.
The interval between instruction insertion is randomized in the core using an LFSR. Sofware can periodically re-seed this LFSR with true random numbers (if available) via the secureseed CSR. This will make the insertion interval of dummy instructions much harder for an attacker to predict.