diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index c61db82c6..95b9485ac 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -112,6 +112,8 @@
 #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.
  *
@@ -1085,6 +1087,9 @@ 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 37193f55a..a5d6bb6a2 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -48,6 +48,8 @@
 
 #include "mbedtls/platform.h"
 
+#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 )                                           \
@@ -1359,6 +1361,8 @@ 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.
  *
  * Add \p b * \p s to \p d.
@@ -1383,6 +1387,7 @@ static
  */
 __attribute__ ((noinline))
 #endif
+
 void mpi_mul_hlp( size_t i,
                   const mbedtls_mpi_uint *s,
                   mbedtls_mpi_uint *d,
@@ -1447,6 +1452,9 @@ void mpi_mul_hlp( size_t i,
     }
 }
 
+#endif
+
+#if !defined(MBEDTLS_MPI_MUL_MPI_ALT)
 /*
  * Baseline multiplication: X = A * B  (HAC 14.12)
  */
@@ -1536,6 +1544,7 @@ int mbedtls_mpi_mul_int( mbedtls_mpi *X, const mbedtls_mpi *A, mbedtls_mpi_uint
 cleanup:
     return( ret );
 }
+#endif
 
 /*
  * Unsigned integer divide - double mbedtls_mpi_uint dividend, u1/u0, and
@@ -1865,6 +1874,8 @@ 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)
  */
@@ -2299,6 +2310,7 @@ cleanup:
 
     return( ret );
 }
+#endif
 
 /*
  * Greatest common divisor: G = gcd(A, B)  (HAC 14.54)
@@ -2977,6 +2989,7 @@ cleanup:
 }
 
 #endif /* MBEDTLS_GENPRIME */
+#endif /* MBEDTLS_BIGNUM_ALT */
 
 #if defined(MBEDTLS_SELF_TEST)