fix(mbedtls): Fix incorrect assert for H/W MPI operations

- Closes https://github.com/espressif/esp-idf/issues/11850

components/mbedtls/port/bignum/esp_bignum.c

@@ -632,12 +632,23 @@ static int mpi_mult_mpi_failover_mod_mult( mbedtls_mpi *Z, const mbedtls_mpi *X,
 
     Z->MBEDTLS_PRIVATE(s) = X->MBEDTLS_PRIVATE(s) * Y->MBEDTLS_PRIVATE(s);
     /*
-     * If this condition fails then most likely hardware peripheral
+     * Relevant: https://github.com/espressif/esp-idf/issues/11850
+     * If the first condition fails then most likely hardware peripheral
      * has produced an incorrect result for MPI operation. This can
      * happen if data fed to the peripheral register was incorrect.
-     * Relevant: https://github.com/espressif/esp-idf/issues/8710#issuecomment-1249178698
+     *
+     * z_words is calculated as the worst-case possible size of the result
+     * MPI Z. The difference between z_words and the actual words taken by
+     * the MPI result (mpi_words(Z)) can be a maximum of 1 word.
+     * The value z_bits (actual bits taken by the MPI result) is calculated
+     * as x_bits + y_bits bits, however, in some cases, z_bits can be
+     * x_bits + y_bits - 1 bits (see example below).
+     * 0b1111 * 0b1111 = 0b11100001 -> 8 bits
+     * 0b1000 * 0b1000 = 0b01000000 -> 7 bits.
+     * The code rounds up to the nearest word size, so the maximum difference
+     * could be of only 1 word. The second condition handles this.
      */
-    assert(mpi_words(Z) == z_words);
+    assert((z_words >= mpi_words(Z)) && (z_words - mpi_words(Z) <= (size_t)1));
 cleanup:
     esp_mpi_disable_hardware_hw_op();
     return ret;

components/mbedtls/test_apps/main/test_mbedtls_mpi.c

@@ -1,6 +1,6 @@
 /* mbedTLS bignum (MPI) self-tests as unit tests
  *
- * SPDX-FileCopyrightText: 2021-2022 Espressif Systems (Shanghai) CO LTD
+ * SPDX-FileCopyrightText: 2021-2023 Espressif Systems (Shanghai) CO LTD
  *
  * SPDX-License-Identifier: Apache-2.0
  */
@@ -151,6 +151,11 @@ TEST_CASE("test MPI multiplication", "[bignum]")
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
                         0);
 
+    /* Multiply two specific MPI values to reproduce issue mentioned in https://github.com/espressif/esp-idf/issues/11850 */
+    test_bignum_mult("454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545454545",
+                     "03",
+                     "CFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCFCF",
+                     0);
 }
 
 static bool test_bignum_modexp(const char *z_str, const char *x_str, const char *y_str, const char *m_str, int ret_error)