[otbn,doc] Update Internal SecWipe description
OTBN does not overwrite the state with zeros after recent changes
in internal secure wiping. This commit updates the documentation to
match with the current behaviour. Also adds a note to OTBN developers
initialise the registers before using them to avoid ISS mismatches.
Signed-off-by: Canberk Topal <ctopal@lowrisc.org>
diff --git a/hw/ip/otbn/doc/_index.md b/hw/ip/otbn/doc/_index.md
index a6e81ec..69c0c39 100644
--- a/hw/ip/otbn/doc/_index.md
+++ b/hw/ip/otbn/doc/_index.md
@@ -1063,8 +1063,11 @@
* The modulus (accessible through the MOD0 to MOD7 CSRs and the MOD WSR)
The wiping procedure is a two-step process:
-* Overwrite the state with randomness from URND.
-* Overwrite the state with zeros.
+* Overwrite the state with randomness from URND and request a reseed of URND.
+* Overwrite the state with randomness from reseeded URND.
+
+Note that after internal secure wipe, the state of registers is undefined.
+In order to prevent mismatches between ISS and RTL, software needs to initialise a register with a full-word write before using its value.
Loop and call stack pointers are reset.
