diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index 8367cd34e6..d2280c3c6f 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -99,6 +99,8 @@
                                                   MBEDTLS_LN_2_DIV_LN_10_SCALE100 - 1) / \
                                                  MBEDTLS_LN_2_DIV_LN_10_SCALE100) + 10 + 6)
 
+#if !defined(MBEDTLS_BIGNUM_ALT)
+
 /*
  * Define the base integer type, architecture-wise.
  *
@@ -1065,6 +1067,9 @@ typedef enum {
 int mbedtls_mpi_gen_prime(mbedtls_mpi *X, size_t nbits, int flags,
                           int (*f_rng)(void *, unsigned char *, size_t),
                           void *p_rng);
+#else /* MBEDTLS_BIGNUM_ALT */
+#include "bignum_alt.h"
+#endif /* MBEDTLS_BIGNUM_ALT */
 
 #if defined(MBEDTLS_SELF_TEST)
 
diff --git a/library/bignum.c b/library/bignum.c
index 424490951d..38df1addb6 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -38,7 +38,7 @@
 
 #include "mbedtls/platform.h"
 
-
+#if !defined(MBEDTLS_BIGNUM_ALT)
 
 /*
  * Conditionally select an MPI sign in constant time.
@@ -1187,6 +1187,7 @@ int mbedtls_mpi_sub_int(mbedtls_mpi *X, const mbedtls_mpi *A, mbedtls_mpi_sint b
     return mbedtls_mpi_sub_mpi(X, A, &B);
 }
 
+#if !defined(MBEDTLS_MPI_MUL_MPI_ALT)
 /*
  * Baseline multiplication: X = A * B  (HAC 14.12)
  */
@@ -1282,6 +1283,7 @@ int mbedtls_mpi_mul_int(mbedtls_mpi *X, const mbedtls_mpi *A, mbedtls_mpi_uint b
 cleanup:
     return ret;
 }
+#endif
 
 /*
  * Unsigned integer divide - double mbedtls_mpi_uint dividend, u1/u0, and
@@ -1728,13 +1730,19 @@ cleanup:
 
     return ret;
 }
-
+ 
+#if !defined(MBEDTLS_MPI_EXP_MOD_ALT)
+ 
+/*
+* Sliding-window exponentiation: X = A^E mod N  (HAC 14.85)
+*/
 int mbedtls_mpi_exp_mod(mbedtls_mpi *X, const mbedtls_mpi *A,
                         const mbedtls_mpi *E, const mbedtls_mpi *N,
                         mbedtls_mpi *prec_RR)
 {
     return mbedtls_mpi_exp_mod_optionally_safe(X, A, E, MBEDTLS_MPI_IS_SECRET, N, prec_RR);
 }
+#endif
 
 int mbedtls_mpi_exp_mod_unsafe(mbedtls_mpi *X, const mbedtls_mpi *A,
                                const mbedtls_mpi *E, const mbedtls_mpi *N,
@@ -1742,6 +1750,7 @@ int mbedtls_mpi_exp_mod_unsafe(mbedtls_mpi *X, const mbedtls_mpi *A,
 {
     return mbedtls_mpi_exp_mod_optionally_safe(X, A, E, MBEDTLS_MPI_IS_PUBLIC, N, prec_RR);
 }
+ 
 
 /*
  * Greatest common divisor: G = gcd(A, B)  (HAC 14.54)
@@ -2298,6 +2307,7 @@ cleanup:
 }
 
 #endif /* MBEDTLS_GENPRIME */
+#endif /* MBEDTLS_BIGNUM_ALT */
 
 #if defined(MBEDTLS_SELF_TEST)