Update rom_ctrl scrambling logic
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rivos-eblot
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Dec 17, 2024
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For Earlgrey, the number of PRINCE cipher half rounds was increased from 2 to 3 (5 effective rounds to 7 effective rounds, including the substitution perm rounds). This was added for improved security, and requires a simple constant change.
In Earlgrey, the S&P layer has been removed from the data scrambling logic, which affects the scrambling of the generated ROM. The functionality is removed entirely from the ROM Control to improve error detection guarantees, interactions with ECC, and timing. This commit removes the now-redundant logic related to the S&P layer, instead directly unscrambling words by XORing the keystream and the `scr`/`in` data.
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This PR updates the ROM Control to match Earlgrey's latest functionality. See the commit messages for more details.
This PR also contains a commit to update
pyot.py, to allow for the use of aliases in test-specific configuration values. Please let me know if there's already a way to do this - it didn't seem to be working for me, and I've found it a useful feature for e.g. using a specific ROM for a specific test.This has been tested against the test
kmac_app_rom_test_fpga_cw310_rom_with_fake_keys, using the builtmask_rom_fpga_cw310.39.scr.vmemproduced by the test, which was failing previously, but now passes. The test and mask ROM were built from master on OpenTitan.