diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index 4d04b336e7..3deedfedd9 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -131,6 +131,8 @@
 #define MBEDTLS_LN_2_DIV_LN_10_SCALE100                 332
 #define MBEDTLS_MPI_RW_BUFFER_SIZE             ( ((MBEDTLS_MPI_MAX_BITS_SCALE100 + 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.
  *
@@ -990,6 +992,9 @@ 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 540f7523b5..b861d5c1d2 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -84,6 +84,8 @@
 #define mbedtls_free       free
 #endif
 
+#if !defined(MBEDTLS_BIGNUM_ALT)
+
 #define MPI_VALIDATE_RET( cond )                                       \
     MBEDTLS_INTERNAL_VALIDATE_RET( cond, MBEDTLS_ERR_MPI_BAD_INPUT_DATA )
 #define MPI_VALIDATE( cond )                                           \
@@ -1553,6 +1555,8 @@ int mbedtls_mpi_sub_int( mbedtls_mpi *X, const mbedtls_mpi *A, mbedtls_mpi_sint
     return( mbedtls_mpi_sub_mpi( X, A, &_B ) );
 }
 
+#if !defined(MBEDTLS_MPI_MUL_MPI_ALT) || !defined(MBEDTLS_MPI_EXP_MOD_ALT)
+
 /*
  * Helper for mbedtls_mpi multiplication
  */
@@ -1564,6 +1568,7 @@ static
  */
 __attribute__ ((noinline))
 #endif
+
 void mpi_mul_hlp( size_t i, mbedtls_mpi_uint *s, mbedtls_mpi_uint *d, mbedtls_mpi_uint b )
 {
     mbedtls_mpi_uint c = 0, t = 0;
@@ -1625,6 +1630,9 @@ void mpi_mul_hlp( size_t i, mbedtls_mpi_uint *s, mbedtls_mpi_uint *d, mbedtls_mp
     while( c != 0 );
 }
 
+#endif
+
+#if !defined(MBEDTLS_MPI_MUL_MPI_ALT)
 /*
  * Baseline multiplication: X = A * B  (HAC 14.12)
  */
@@ -1676,6 +1684,7 @@ cleanup:
 
     return( ret );
 }
+#endif
 
 /*
  * Baseline multiplication: X = A * b
@@ -2013,6 +2022,8 @@ int mbedtls_mpi_mod_int( mbedtls_mpi_uint *r, const mbedtls_mpi *A, mbedtls_mpi_
     return( 0 );
 }
 
+#if !defined(MBEDTLS_MPI_EXP_MOD_ALT)
+
 /*
  * Fast Montgomery initialization (thanks to Tom St Denis)
  */
@@ -2414,6 +2425,7 @@ cleanup:
 
     return( ret );
 }
+#endif
 
 /*
  * Greatest common divisor: G = gcd(A, B)  (HAC 14.54)
@@ -3002,6 +3014,7 @@ cleanup:
 }
 
 #endif /* MBEDTLS_GENPRIME */
+#endif /* MBEDTLS_BIGNUM_ALT */
 
 #if defined(MBEDTLS_SELF_TEST)