diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index 3ba1777..eb8446e 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -186,6 +186,15 @@
     #endif /* !MBEDTLS_NO_UDBL_DIVISION */
 #endif /* !MBEDTLS_HAVE_INT64 */
 
+/*
+ * Sanity check that exactly one of MBEDTLS_HAVE_INT32 or MBEDTLS_HAVE_INT64 is defined,
+ * so that code elsewhere doesn't have to check.
+ */
+#if (!(defined(MBEDTLS_HAVE_INT32) || defined(MBEDTLS_HAVE_INT64))) || \
+    (defined(MBEDTLS_HAVE_INT32) && defined(MBEDTLS_HAVE_INT64))
+#error "Only 32-bit or 64-bit limbs are supported in bignum"
+#endif
+
 /** \typedef mbedtls_mpi_uint
  * \brief The type of machine digits in a bignum, called _limbs_.
  *
diff --git a/library/bignum_core.c b/library/bignum_core.c
index 48b640b..dbf6d1d 100644
--- a/library/bignum_core.c
+++ b/library/bignum_core.c
@@ -77,38 +77,17 @@
     return 0;
 }
 
-/* Convert a big-endian byte array aligned to the size of mbedtls_mpi_uint
- * into the storage form used by mbedtls_mpi. */
-static mbedtls_mpi_uint mpi_bigendian_to_host_c(mbedtls_mpi_uint a)
-{
-    uint8_t i;
-    unsigned char *a_ptr;
-    mbedtls_mpi_uint tmp = 0;
-
-    for (i = 0, a_ptr = (unsigned char *) &a; i < ciL; i++, a_ptr++) {
-        tmp <<= CHAR_BIT;
-        tmp |= (mbedtls_mpi_uint) *a_ptr;
-    }
-
-    return tmp;
-}
-
 static mbedtls_mpi_uint mpi_bigendian_to_host(mbedtls_mpi_uint a)
 {
     if (MBEDTLS_IS_BIG_ENDIAN) {
         /* Nothing to do on bigendian systems. */
         return a;
     } else {
-        switch (sizeof(mbedtls_mpi_uint)) {
-            case 4:
-                return (mbedtls_mpi_uint) MBEDTLS_BSWAP32((uint32_t) a);
-            case 8:
-                return (mbedtls_mpi_uint) MBEDTLS_BSWAP64((uint64_t) a);
-        }
-
-        /* Fall back to C-based reordering if we don't know the byte order
-         * or we couldn't use a compiler-specific builtin. */
-        return mpi_bigendian_to_host_c(a);
+#if defined(MBEDTLS_HAVE_INT32)
+        return (mbedtls_mpi_uint) MBEDTLS_BSWAP32(a);
+#elif defined(MBEDTLS_HAVE_INT64)
+        return (mbedtls_mpi_uint) MBEDTLS_BSWAP64(a);
+#endif
     }
 }
 
diff --git a/library/x509_crt.c b/library/x509_crt.c
index 2cbced2..8d07694 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -2754,8 +2754,8 @@
             p++;
         }
         if (num_digits != 0) {
-            addr[nonzero_groups++] = MBEDTLS_IS_BIG_ENDIAN ? group :
-                                     (group << 8) | (group >> 8);
+            MBEDTLS_PUT_UINT16_BE(group, addr, nonzero_groups);
+            nonzero_groups++;
             if (*p == '\0') {
                 break;
             } else if (*p == '.') {