diff --git a/components/mbedtls/port/esp_bignum.c b/components/mbedtls/port/esp_bignum.c
index 4dac2b510..644277ebe 100644
--- a/components/mbedtls/port/esp_bignum.c
+++ b/components/mbedtls/port/esp_bignum.c
@@ -361,6 +361,18 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi* Z, const mbedtls_mpi* X, const mbedtls_mpi
     mbedtls_mpi *Rinv;    /* points to _Rinv (if not NULL) othwerwise &RR_new */
     mbedtls_mpi_uint Mprime;
 
+    if (mbedtls_mpi_cmp_int(M, 0) <= 0 || (M->p[0] & 1) == 0) {
+        return MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
+    }
+
+    if (mbedtls_mpi_cmp_int(Y, 0) < 0) {
+        return MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
+    }
+
+    if (mbedtls_mpi_cmp_int(Y, 0) == 0) {
+        return mbedtls_mpi_lset(Z, 1);
+    }
+
     if (hw_words * 32 > 4096) {
         return MBEDTLS_ERR_MPI_NOT_ACCEPTABLE;
     }
@@ -394,13 +406,24 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi* Z, const mbedtls_mpi* X, const mbedtls_mpi
     start_op(RSA_START_MODEXP_REG);
 
     /* X ^ Y may actually be shorter than M, but unlikely when used for crypto */
-    MBEDTLS_MPI_CHK( mbedtls_mpi_grow(Z, m_words) );
+    if ((ret = mbedtls_mpi_grow(Z, m_words)) != 0) {
+        esp_mpi_release_hardware();
+        goto cleanup;
+    }
 
     wait_op_complete(RSA_START_MODEXP_REG);
 
     mem_block_to_mpi(Z, RSA_MEM_Z_BLOCK_BASE, m_words);
     esp_mpi_release_hardware();
 
+    // Compensate for negative X
+    if (X->s == -1 && (Y->p[0] & 1) != 0) {
+        Z->s = -1;
+        MBEDTLS_MPI_CHK(mbedtls_mpi_add_mpi(Z, M, Z));
+    } else {
+        Z->s = 1;
+    }
+
  cleanup:
     if (_Rinv == NULL) {
         mbedtls_mpi_free(&Rinv_new);
diff --git a/components/mbedtls/test/test_mbedtls_mpi.c b/components/mbedtls/test/test_mbedtls_mpi.c
index ade0fdb6b..084fe3484 100644
--- a/components/mbedtls/test/test_mbedtls_mpi.c
+++ b/components/mbedtls/test/test_mbedtls_mpi.c
@@ -11,6 +11,7 @@
 #include "unity.h"
 #include "sdkconfig.h"
 
+#define MBEDTLS_OK 0
 
 /* Debugging function to print an MPI number to stdout. Happens to
    print output that can be copy-pasted directly into a Python shell.
@@ -116,11 +117,14 @@ TEST_CASE("test MPI multiplication", "[bignum]")
                      4096);
 }
 
-static void test_bignum_modexp(const char *z_str, const char *x_str, const char *y_str, const char *m_str)
+static bool test_bignum_modexp(const char *z_str, const char *x_str, const char *y_str, const char *m_str, int ret_error)
 {
     mbedtls_mpi Z, X, Y, M;
     char z_buf[400] = { 0 };
     size_t z_buf_len = 0;
+    bool fail = false;
+
+    printf("%s = (%s ^ %s) mod %s  ret=%d ... ", z_str, x_str, y_str, m_str, ret_error);
 
     mbedtls_mpi_init(&Z);
     mbedtls_mpi_init(&X);
@@ -136,38 +140,72 @@ static void test_bignum_modexp(const char *z_str, const char *x_str, const char
     //mbedtls_mpi_printf("M", &M);
 
     /* Z = (X ^ Y) mod M */
-    TEST_ASSERT_FALSE(mbedtls_mpi_exp_mod(&Z, &X, &Y, &M, NULL));
-
-    mbedtls_mpi_write_string(&Z, 16, z_buf, sizeof(z_buf)-1, &z_buf_len);
-    TEST_ASSERT_EQUAL_STRING_MESSAGE(z_str, z_buf, "mbedtls_mpi_exp_mod incorrect");
+    if (ret_error != mbedtls_mpi_exp_mod(&Z, &X, &Y, &M, NULL)) {
+        fail = true;
+    }
     
+    if (ret_error == MBEDTLS_OK) {
+        mbedtls_mpi_write_string(&Z, 16, z_buf, sizeof(z_buf)-1, &z_buf_len);
+        if (memcmp(z_str, z_buf, strlen(z_str)) != 0) {
+            printf("\n Expected '%s' Was '%s' \n", z_str, z_buf);
+            fail = true;
+        }
+    }
+
     mbedtls_mpi_free(&Z);
     mbedtls_mpi_free(&X);
     mbedtls_mpi_free(&Y);
     mbedtls_mpi_free(&M);
+
+    if (fail == true) {
+        printf(" FAIL\n");
+    } else {
+        printf(" PASS\n");
+    }
+    return fail;
 }
 
 TEST_CASE("test MPI modexp", "[bignum]")
 {
-    test_bignum_modexp("01000000", "1000", "2", "FFFFFFFF");
-    test_bignum_modexp("014B5A90", "1234", "2", "FFFFFFF");
-    test_bignum_modexp("01234321", "1111", "2", "FFFFFFFF");
-    test_bignum_modexp("02", "5", "1", "3");
-    test_bignum_modexp("22", "55", "1", "33");
-    test_bignum_modexp("0222", "555", "1", "333");
-    test_bignum_modexp("2222", "5555", "1", "3333");
-    test_bignum_modexp("11", "5555", "1", "33");
+    bool test_error = false;
+    printf("Z = (X ^ Y) mod M \n");
+    // test_bignum_modexp(Z, X, Y, M, ret_error);
+    test_error |= test_bignum_modexp("01000000", "1000", "2", "FFFFFFFF", MBEDTLS_OK);
+    test_error |= test_bignum_modexp("014B5A90", "1234", "2", "FFFFFFF", MBEDTLS_OK);
+    test_error |= test_bignum_modexp("01234321", "1111", "2", "FFFFFFFF", MBEDTLS_OK);
+    test_error |= test_bignum_modexp("02", "5", "1", "3", MBEDTLS_OK);
+    test_error |= test_bignum_modexp("22", "55", "1", "33", MBEDTLS_OK);
+    test_error |= test_bignum_modexp("0222", "555", "1", "333", MBEDTLS_OK);
+    test_error |= test_bignum_modexp("2222", "5555", "1", "3333", MBEDTLS_OK);
+    test_error |= test_bignum_modexp("11", "5555", "1", "33", MBEDTLS_OK);
+    test_error |= test_bignum_modexp("55", "1111", "1", "77", MBEDTLS_OK);
+    test_error |= test_bignum_modexp("88", "1111", "2", "BB", MBEDTLS_OK);
+    test_error |= test_bignum_modexp("01000000", "2", "128", "FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF", MBEDTLS_OK);
+    test_error |= test_bignum_modexp("0ABCDEF12345", "ABCDEF12345", "1", "FFFFFFFFFFFF", MBEDTLS_OK);
+    test_error |= test_bignum_modexp("0ABCDE", "ABCDE", "1", "FFFFF", MBEDTLS_OK);
 
-    test_bignum_modexp("55", "1111", "1", "77");
-    test_bignum_modexp("88", "1111", "2", "BB");
+    test_error |= test_bignum_modexp("04",  "2",  "2",  "9",  MBEDTLS_OK);
+    test_error |= test_bignum_modexp("04",  "2", "-2",  "9",  MBEDTLS_ERR_MPI_BAD_INPUT_DATA);
+    test_error |= test_bignum_modexp("04",  "2",  "2", "-9",  MBEDTLS_ERR_MPI_BAD_INPUT_DATA);
+    test_error |= test_bignum_modexp("04",  "2", "-2", "-9",  MBEDTLS_ERR_MPI_BAD_INPUT_DATA);
 
-    test_bignum_modexp("01000000", "2", "128",
-                       "FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF");
+    test_error |= test_bignum_modexp("01",  "2",  "0",  "9",  MBEDTLS_OK);
+    test_error |= test_bignum_modexp("04",  "2",  "0",  "0",  MBEDTLS_ERR_MPI_BAD_INPUT_DATA);
+    test_error |= test_bignum_modexp("04",  "2",  "2",  "0",  MBEDTLS_ERR_MPI_BAD_INPUT_DATA);
+    test_error |= test_bignum_modexp("00",  "0",  "2",  "9",  MBEDTLS_OK);
+    test_error |= test_bignum_modexp("01",  "0",  "0",  "9",  MBEDTLS_OK);
 
-    /* failures below here... */
-    test_bignum_modexp("0ABCDEF12345", "ABCDEF12345", "1", "FFFFFFFFFFFF");
-    test_bignum_modexp("0ABCDE", "ABCDE", "1", "FFFFF");
+    test_error |= test_bignum_modexp("04", "-2",  "2",  "9",  MBEDTLS_OK);
+    test_error |= test_bignum_modexp("01", "-2",  "0",  "9",  MBEDTLS_OK);
+    test_error |= test_bignum_modexp("07", "-2",  "7",  "9",  MBEDTLS_OK);
+    test_error |= test_bignum_modexp("07", "-2",  "1",  "9",  MBEDTLS_OK);
+    test_error |= test_bignum_modexp("02",  "2",  "1",  "9",  MBEDTLS_OK);
+    test_error |= test_bignum_modexp("01",  "2",  "0",  "9",  MBEDTLS_OK);
 
-    test_bignum_modexp("04", "2", "2", "9");
+    test_error |= test_bignum_modexp("05",  "5",  "7",  "7",  MBEDTLS_OK);
+    test_error |= test_bignum_modexp("02", "-5",  "7",  "7",  MBEDTLS_OK);
+    test_error |= test_bignum_modexp("01", "-5",  "7",  "3",  MBEDTLS_OK);
+
+    TEST_ASSERT_FALSE_MESSAGE(test_error, "mbedtls_mpi_exp_mod incorrect for some tests\n");
 }