tree 208d0377f70ea375e87a5b3e1d6b095c0f1c2c83
parent a01a0cc9621247b40b2a149154496f5cf7377e4d
author Pirmin Vogel <vogelpi@lowrisc.org> 1649070281 +0200
committer Pirmin Vogel <vogelpi@lowrisc.org> 1649145831 +0200

[kmac] Ignore entropy refresh requests when running in SW entropy mode

When running in EDN entropy mode, software can manually request the
entropy to be refreshed from EDN.

If instead running in SW entropy mode, SW provides the entropy via CSRs.
SW is still able to request the entropy to be refreshed. Prior to this
commit, the design would then go into the StRandEdn state and request
fresh entropy from EDN, but the entropy received from EDN would be
ignored. Instead, the LFSR would be reseeded using the value provided by
SW via CSRs. If SW "forgot" to update the CSRs, the LFSR
would be reseeded using the same value. This is a bit counter-intuitive
and can be exploitet for SCA. It's safer to just ignore these requests
when running in SW entropy mode.

Signed-off-by: Pirmin Vogel <vogelpi@lowrisc.org>