diff --git a/library/bignum.c b/library/bignum.c
index 391ff87..0e7b3ce 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -58,8 +58,24 @@
 #define mbedtls_free       free
 #endif
 
+#define MPI_VALIDATE_RET( cond )                                       \
+    MBEDTLS_INTERNAL_VALIDATE_RET( cond, MBEDTLS_ERR_MPI_BAD_INPUT_DATA )
+#define MPI_VALIDATE( cond )                                           \
+    MBEDTLS_INTERNAL_VALIDATE( cond )
+
+#define ciL    (sizeof(mbedtls_mpi_uint))         /* chars in limb  */
+#define biL    (ciL << 3)               /* bits  in limb  */
+#define biH    (ciL << 2)               /* half limb size */
+
 #define MPI_SIZE_T_MAX  ( (size_t) -1 ) /* SIZE_T_MAX is not standard */
 
+/*
+ * Convert between bits/chars and number of limbs
+ * Divide first in order to avoid potential overflows
+ */
+#define BITS_TO_LIMBS(i)  ( (i) / biL + ( (i) % biL != 0 ) )
+#define CHARS_TO_LIMBS(i) ( (i) / ciL + ( (i) % ciL != 0 ) )
+
 /* Implementation that should never be optimized out by the compiler */
 static void mbedtls_mpi_zeroize( mbedtls_mpi_uint *v, size_t n )
 {