[ot] hw/opentitan: ot_otp_eg: perform partition descrambling in DAI

Unscramble DAI reads from secret partitions, which are scrambled with a
scrambling key (configured per-partition as a netlist constant) via a
simple Present block cipher.

Co-authored-by: Alex Jones <[email protected]>
Signed-off-by: Luís Marques <[email protected]>
Signed-off-by: Alex Jones <[email protected]>

Author: luismarques

hw/opentitan/ot_otp_eg.c

@@ -1717,6 +1717,7 @@ static void ot_otp_eg_dai_read(OtOTPEgState *s)
     bool is_digest = ot_otp_eg_is_part_digest_offset(partition, address);
     bool is_readable = ot_otp_eg_is_readable(s, partition);
     bool is_wide = ot_otp_eg_is_wide_granule(partition, address);
+    bool is_secret = OtOTPPartDescs[partition].secret;
 
     /* "in all partitions, the digest itself is ALWAYS readable." */
     if (!is_digest && !is_readable) {
@@ -1770,6 +1771,18 @@ static void ot_otp_eg_dai_read(OtOTPEgState *s)
         }
     }
 
+    if (is_secret) {
+        const uint8_t *scrambling_key = s->otp_scramble_keys[partition];
+        g_assert(scrambling_key);
+        uint64_t data = ((uint64_t)data_hi << 32u) | data_lo;
+        OtPresentState *ps = ot_present_new();
+        ot_present_init(ps, scrambling_key);
+        ot_present_decrypt(ps, data, &data);
+        ot_present_free(ps);
+        data_lo = data;
+        data_hi = (data >> 32u);
+    }
+
     s->regs[R_DIRECT_ACCESS_RDATA_0] = data_lo;
     s->regs[R_DIRECT_ACCESS_RDATA_1] = data_hi;