[crypto] Check LOAD_CHECKSUM when writing into DMEM

OTBN offers the feature to read back the checksum when writing
data into memory.

This commit extends the `otbn_dmem_write` function such that
we compare the checksum computed by OTBN after writing to its
memory to an expected checksum we compute on the fly.

Signed-off-by: Pascal Nasahl <[email protected]>
(cherry picked from commit 98c92ec)

Author: nasahlpa

sw/device/lib/crypto/drivers/otbn.c

@@ -130,7 +130,6 @@ status_t otbn_dmem_write(size_t num_words, const uint32_t *src,
   random_order_init(&order, num_words);
 
   size_t count = 0;
-  size_t expected_count = random_order_len(&order);
 
   for (; launderw(count) < num_words; count = launderw(count) + 1) {
     // The value obtained from `advance()` is laundered, to prevent
@@ -143,7 +142,8 @@ status_t otbn_dmem_write(size_t num_words, const uint32_t *src,
     barrierw(idx);
 
     // Perform the write.
-    abs_mmio_write32(kBase + OTBN_DMEM_REG_OFFSET + dest + idx_word, src[idx]);
+    abs_mmio_write32(kBase + OTBN_DMEM_REG_OFFSET + dest + idx_word,
+                     src[idx]);
 
     // Update the CRC. According to the OTBN documentation, each CRC update
     // consists of 48-bit: {imem, idx, wdata}
@@ -163,7 +163,8 @@ status_t otbn_dmem_write(size_t num_words, const uint32_t *src,
   // Get the computed (expected) checksum, fetch the checksum from the OTBN
   // LOAD_CHECKSUM register, and compare both registers.
   uint32_t checksum_expected = crc32_finish(&ctx);
-  uint32_t checksum = abs_mmio_read32(kBase + OTBN_LOAD_CHECKSUM_REG_OFFSET);
+  uint32_t checksum =
+      abs_mmio_read32(kBase + OTBN_LOAD_CHECKSUM_REG_OFFSET);
   HARDENED_CHECK_EQ(checksum, checksum_expected);
 
   return OTCRYPTO_OK;