diff --git a/wolfcrypt/src/sp_int.c b/wolfcrypt/src/sp_int.c
index 14d985ae2..2b4ec7542 100644
--- a/wolfcrypt/src/sp_int.c
+++ b/wolfcrypt/src/sp_int.c
@@ -2178,6 +2178,7 @@ static int _sp_mont_red(sp_int* a, sp_int* m, sp_int_digit mp);
 static void _sp_zero(sp_int* a)
 {
     a->used = 0;
+    a->dp[0] = 0;
 #ifdef WOLFSSL_SP_INT_NEGATIVE
     a->sign = MP_ZPOS;
 #endif
@@ -3236,9 +3237,12 @@ int sp_sub_d(sp_int* a, sp_int_digit d, sp_int* r)
  * @param  [in]   n  Number (SP digit) to multiply by.
  * @param  [out]  r  SP integer result.
  * @param  [in]   o  Number of digits to move result up by.
+ * @return  MP_OKAY on success.
+ * @return  MP_VAL when result is too large for sp_int.
  */
-static void _sp_mul_d(sp_int* a, sp_int_digit n, sp_int* r, int o)
+static int _sp_mul_d(sp_int* a, sp_int_digit n, sp_int* r, int o)
 {
+    int err = MP_OKAY;
     int i;
     sp_int_word t = 0;
 
@@ -3257,9 +3261,18 @@ static void _sp_mul_d(sp_int* a, sp_int_digit n, sp_int* r, int o)
         t >>= SP_WORD_SIZE;
     }
 
-    r->dp[o++] = (sp_int_digit)t;
+    if (t > 0) {
+        if (o == r->size) {
+            err = MP_VAL;
+        }
+        else {
+            r->dp[o++] = (sp_int_digit)t;
+        }
+    }
     r->used = o;
     sp_clamp(r);
+
+    return err;
 }
 #endif /* (WOLFSSL_SP_MATH_ALL && !WOLFSSL_RSA_VERIFY_ONLY) ||
         *  WOLFSSL_SP_SMALL || (WOLFSSL_KEY_GEN && !NO_RSA) */
@@ -3287,7 +3300,7 @@ int sp_mul_d(sp_int* a, sp_int_digit d, sp_int* r)
     }
 
     if (err == MP_OKAY) {
-        _sp_mul_d(a, d, r, 0);
+        err = _sp_mul_d(a, d, r, 0);
     #ifdef WOLFSSL_SP_INT_NEGATIVE
         if (d == 0) {
             r->sign = MP_ZPOS;
@@ -3705,6 +3718,9 @@ int sp_div_2_mod_ct(sp_int* a, sp_int* m, sp_int* r)
     if ((a == NULL) || (m == NULL) || (r == NULL)) {
         err = MP_VAL;
     }
+    if ((err == MP_OKAY) && (r->size < m->used + 1)) {
+        err = MP_VAL;
+    }
 
     if (err == MP_OKAY) {
         sp_int_word  w = 0;
@@ -4208,37 +4224,44 @@ static int sp_cmp_mag_ct(sp_int* a, sp_int* b, int len)
  */
 int sp_addmod_ct(sp_int* a, sp_int* b, sp_int* m, sp_int* r)
 {
+    int err = MP_OKAY;
     sp_int_word  w = 0;
     sp_int_digit mask;
     int i;
 
-    if (0) {
-        sp_print(a, "a");
-        sp_print(b, "b");
-        sp_print(m, "m");
+    if ((r->size < m->used + 1) || (m->used == m->size)) {
+        err = MP_VAL;
     }
 
-    _sp_add_off(a, b, r, 0);
-    mask = 0 - (sp_cmp_mag_ct(r, m, m->used + 1) != MP_LT);
-    for (i = 0; i < m->used; i++) {
-        sp_int_digit mask_r = 0 - (i < r->used);
-        w        += m->dp[i] & mask;
-        w         = (r->dp[i] & mask_r) - w;
-        r->dp[i]  = (sp_int_digit)w;
-        w         = (w >> DIGIT_BIT) & 1;
-    }
-    r->dp[i] = 0;
-    r->used = i;
-#ifdef WOLFSSL_SP_INT_NEGATIVE
-    r->sign = a->sign;
-#endif /* WOLFSSL_SP_INT_NEGATIVE */
-    sp_clamp(r);
+    if (err == MP_OKAY) {
+        if (0) {
+            sp_print(a, "a");
+            sp_print(b, "b");
+            sp_print(m, "m");
+        }
 
-    if (0) {
-        sp_print(r, "rma");
+        _sp_add_off(a, b, r, 0);
+        mask = 0 - (sp_cmp_mag_ct(r, m, m->used + 1) != MP_LT);
+        for (i = 0; i < m->used; i++) {
+            sp_int_digit mask_r = 0 - (i < r->used);
+            w        += m->dp[i] & mask;
+            w         = (r->dp[i] & mask_r) - w;
+            r->dp[i]  = (sp_int_digit)w;
+            w         = (w >> DIGIT_BIT) & 1;
+        }
+        r->dp[i] = 0;
+        r->used = i;
+    #ifdef WOLFSSL_SP_INT_NEGATIVE
+        r->sign = a->sign;
+    #endif /* WOLFSSL_SP_INT_NEGATIVE */
+        sp_clamp(r);
+
+        if (0) {
+            sp_print(r, "rma");
+        }
     }
 
-    return MP_OKAY;
+    return err;
 }
 #endif /* WOLFSSL_SP_MATH_ALL && HAVE_ECC */
 
@@ -4259,39 +4282,46 @@ int sp_addmod_ct(sp_int* a, sp_int* b, sp_int* m, sp_int* r)
  */
 int sp_submod_ct(sp_int* a, sp_int* b, sp_int* m, sp_int* r)
 {
+    int err = MP_OKAY;
     sp_int_word  w = 0;
     sp_int_digit mask;
     int i;
 
-    if (0) {
-        sp_print(a, "a");
-        sp_print(b, "b");
-        sp_print(m, "m");
+    if (r->size < m->used + 1) {
+        err = MP_VAL;
     }
 
-    mask = 0 - (sp_cmp_mag_ct(a, b, m->used + 1) == MP_LT);
-    for (i = 0; i < m->used + 1; i++) {
-        sp_int_digit mask_a = 0 - (i < a->used);
-        sp_int_digit mask_m = 0 - (i < m->used);
+    if (err == MP_OKAY) {
+        if (0) {
+            sp_print(a, "a");
+            sp_print(b, "b");
+            sp_print(m, "m");
+        }
 
-        w         += m->dp[i] & mask_m & mask;
-        w         += a->dp[i] & mask_a;
-        r->dp[i]   = (sp_int_digit)w;
-        w        >>= DIGIT_BIT;
-    }
-    r->dp[i] = (sp_int_digit)w;
-    r->used = i + 1;
-#ifdef WOLFSSL_SP_INT_NEGATIVE
-    r->sign = MP_ZPOS;
-#endif /* WOLFSSL_SP_INT_NEGATIVE */
-    sp_clamp(r);
-    _sp_sub_off(r, b, r, 0);
+        mask = 0 - (sp_cmp_mag_ct(a, b, m->used) == MP_LT);
+        for (i = 0; i < m->used; i++) {
+            sp_int_digit mask_a = 0 - (i < a->used);
+            sp_int_digit mask_m = 0 - (i < m->used);
 
-    if (0) {
-        sp_print(r, "rms");
+            w         += m->dp[i] & mask_m & mask;
+            w         += a->dp[i] & mask_a;
+            r->dp[i]   = (sp_int_digit)w;
+            w        >>= DIGIT_BIT;
+        }
+        r->dp[i] = (sp_int_digit)w;
+        r->used = i + 1;
+    #ifdef WOLFSSL_SP_INT_NEGATIVE
+        r->sign = MP_ZPOS;
+    #endif /* WOLFSSL_SP_INT_NEGATIVE */
+        sp_clamp(r);
+        _sp_sub_off(r, b, r, 0);
+
+        if (0) {
+            sp_print(r, "rms");
+        }
     }
 
-    return MP_OKAY;
+    return err;
 }
 #endif /* WOLFSSL_SP_MATH_ALL && HAVE_ECC */
 
@@ -4628,7 +4658,10 @@ int sp_div(sp_int* a, sp_int* d, sp_int* r, sp_int* rem)
 
 #ifdef WOLFSSL_SP_SMALL
             do {
-                _sp_mul_d(d, t, trial, i - d->used);
+                err = _sp_mul_d(d, t, trial, i - d->used);
+                if (err != MP_OKAY) {
+                    break;
+                }
                 c = _sp_cmp_abs(trial, sa);
                 if (c == MP_GT) {
                     t--;
@@ -4636,6 +4669,10 @@ int sp_div(sp_int* a, sp_int* d, sp_int* r, sp_int* rem)
             }
             while (c == MP_GT);
 
+            if (err != MP_OKAY) {
+                break;
+            }
+
             _sp_sub_off(sa, trial, sa, 0);
             tr->dp[i - d->used] += t;
             if (tr->dp[i - d->used] < t) {
@@ -4676,7 +4713,7 @@ int sp_div(sp_int* a, sp_int* d, sp_int* r, sp_int* rem)
         }
         sa->used = i + 1;
 
-        if (rem != NULL) {
+        if ((err == MP_OKAY) && (rem != NULL)) {
 #ifdef WOLFSSL_SP_INT_NEGATIVE
             sa->sign = (sa->used == 0) ? MP_ZPOS : aSign;
 #endif /* WOLFSSL_SP_INT_NEGATIVE */
@@ -4691,7 +4728,7 @@ int sp_div(sp_int* a, sp_int* d, sp_int* r, sp_int* rem)
             }
 #endif
         }
-        if (r != NULL) {
+        if ((err == MP_OKAY) && (r != NULL)) {
             sp_copy(tr, r);
             sp_clamp(r);
 #ifdef WOLFSSL_SP_INT_NEGATIVE
@@ -12467,11 +12504,10 @@ static int _sp_read_radix_10(sp_int* a, const char* in)
             err = MP_VAL;
             break;
         }
-        if (a->used + 1 > a->size) {
-            err = MP_VAL;
+        err = _sp_mul_d(a, 10, a, 0);
+        if (err != MP_OKAY) {
             break;
         }
-        _sp_mul_d(a, 10, a, 0);
         (void)_sp_add_d(a, ch, a);
     }
 #ifdef WOLFSSL_SP_INT_NEGATIVE
diff --git a/wolfssl/wolfcrypt/sp_int.h b/wolfssl/wolfcrypt/sp_int.h
index 7e9710161..fd5ec9b9b 100644
--- a/wolfssl/wolfcrypt/sp_int.h
+++ b/wolfssl/wolfcrypt/sp_int.h
@@ -648,9 +648,12 @@ typedef struct sp_ecc_ctx {
 #define CheckFastMathSettings()   (SP_WORD_SIZE == CheckRunTimeFastMath())
 
 
-/* The number of bytes to a sp_int with 'cnt' digits. */
+/* The number of bytes to a sp_int with 'cnt' digits.
+ * Must have at least one digit.
+ */
 #define MP_INT_SIZEOF(cnt) \
-    (sizeof(sp_int) - (SP_INT_DIGITS - (cnt)) * sizeof(sp_int_digit))
+    (sizeof(sp_int) - (SP_INT_DIGITS - (((cnt) == 0) ? 1 : (cnt))) * \
+     sizeof(sp_int_digit))
 /* The address of the next sp_int after one with 'cnt' digits. */
 #define MP_INT_NEXT(t, cnt) \
         (sp_int*)(((byte*)(t)) + MP_INT_SIZEOF(cnt))