diff --git a/library/bignum_new.c b/library/bignum_new.c
index 3292861..977b5d7 100644
--- a/library/bignum_new.c
+++ b/library/bignum_new.c
@@ -21,10 +21,19 @@
 
 #if defined(MBEDTLS_BIGNUM_C)
 
+#include <string.h>
+
+#include "mbedtls/platform_util.h"
 #include "mbedtls/error.h"
 #include "mbedtls/bignum.h"
+#include "bignum_core.h"
 #include "bignum_mod.h"
 
+#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 */
@@ -73,6 +82,10 @@
     return( ( i * biL ) + j );
 }
 
+/* Get a specific byte, without range checks. */
+#define GET_BYTE( X, i )                                \
+    ( ( ( X )[( i ) / ciL] >> ( ( ( i ) % ciL ) * 8 ) ) & 0xff )
+
 void mbedtls_mpi_mod_residue_release( mbedtls_mpi_mod_residue *r )
 {
     if ( r == NULL )
@@ -135,4 +148,263 @@
     return( 0 );
 }
 
+/* Check X to have at least n limbs and set it to 0. */
+static int mpi_core_clear( mbedtls_mpi_uint *X,
+                           size_t nx,
+                           size_t limbs )
+{
+    if( X == NULL )
+        return( MBEDTLS_ERR_MPI_BAD_INPUT_DATA );
+
+    else if( nx < limbs )
+        return( MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL );
+
+    else
+    {
+        memset( X, 0, nx * ciL );
+        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 x )
+{
+    uint8_t i;
+    unsigned char *x_ptr;
+    mbedtls_mpi_uint tmp = 0;
+
+    for( i = 0, x_ptr = (unsigned char*) &x; i < ciL; i++, x_ptr++ )
+    {
+        tmp <<= CHAR_BIT;
+        tmp |= (mbedtls_mpi_uint) *x_ptr;
+    }
+
+    return( tmp );
+}
+
+static mbedtls_mpi_uint mpi_bigendian_to_host( mbedtls_mpi_uint x )
+{
+#if defined(__BYTE_ORDER__)
+
+/* Nothing to do on bigendian systems. */
+#if ( __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__ )
+    return( x );
+#endif /* __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__ */
+
+#if ( __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ )
+
+/* For GCC and Clang, have builtins for byte swapping. */
+#if defined(__GNUC__) && defined(__GNUC_PREREQ)
+#if __GNUC_PREREQ(4,3)
+#define have_bswap
+#endif
+#endif
+
+#if defined(__clang__) && defined(__has_builtin)
+#if __has_builtin(__builtin_bswap32)  &&                 \
+    __has_builtin(__builtin_bswap64)
+#define have_bswap
+#endif
+#endif
+
+#if defined(have_bswap)
+    /* The compiler is hopefully able to statically evaluate this! */
+    switch( sizeof(mbedtls_mpi_uint) )
+    {
+        case 4:
+            return( __builtin_bswap32(x) );
+        case 8:
+            return( __builtin_bswap64(x) );
+    }
+#endif
+#endif /* __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ */
+#endif /* __BYTE_ORDER__ */
+
+    /* 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( x ) );
+}
+
+static void mpi_core_bigendian_to_host( mbedtls_mpi_uint * const X,
+                                        size_t limbs )
+{
+    mbedtls_mpi_uint *cur_limb_left;
+    mbedtls_mpi_uint *cur_limb_right;
+    if( limbs == 0 )
+        return;
+
+    /*
+     * Traverse limbs and
+     * - adapt byte-order in each limb
+     * - swap the limbs themselves.
+     * For that, simultaneously traverse the limbs from left to right
+     * and from right to left, as long as the left index is not bigger
+     * than the right index (it's not a problem if limbs is odd and the
+     * indices coincide in the last iteration).
+     */
+    for( cur_limb_left = X, cur_limb_right = X + ( limbs - 1 );
+         cur_limb_left <= cur_limb_right;
+         cur_limb_left++, cur_limb_right-- )
+    {
+        mbedtls_mpi_uint tmp;
+        /* Note that if cur_limb_left == cur_limb_right,
+         * this code effectively swaps the bytes only once. */
+        tmp             = mpi_bigendian_to_host( *cur_limb_left  );
+        *cur_limb_left  = mpi_bigendian_to_host( *cur_limb_right );
+        *cur_limb_right = tmp;
+    }
+}
+
+/*
+ * Import X from unsigned binary data, little endian
+ *
+ * This function is guaranteed to return an MPI with at least the necessary
+ * number of limbs (in particular, it does not skip 0s in the input).
+ */
+int mbedtls_mpi_core_read_le( mbedtls_mpi_uint *X,
+                              size_t nx,
+                              const unsigned char *buf,
+                              size_t buflen )
+{
+    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
+    size_t i;
+    size_t const limbs = CHARS_TO_LIMBS( buflen );
+
+    /* Ensure that target MPI has at least the necessary number of limbs */
+    MBEDTLS_MPI_CHK( mpi_core_clear( X, nx, limbs ) );
+
+    for( i = 0; i < buflen; i++ )
+        X[i / ciL] |= ((mbedtls_mpi_uint) buf[i]) << ((i % ciL) << 3);
+
+cleanup:
+    return( ret );
+}
+
+/*
+ * Import X from unsigned binary data, big endian
+ *
+ * This function is guaranteed to return an MPI with exactly the necessary
+ * number of limbs (in particular, it does not skip 0s in the input).
+ */
+int mbedtls_mpi_core_read_be( mbedtls_mpi_uint *X,
+                              size_t nx,
+                              const unsigned char *buf,
+                              size_t buflen )
+{
+    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
+    size_t const limbs    = CHARS_TO_LIMBS( buflen );
+    size_t const overhead = ( limbs * ciL ) - buflen;
+    unsigned char *Xp;
+
+    MPI_VALIDATE_RET( X != NULL );
+    MPI_VALIDATE_RET( buflen == 0 || buf != NULL );
+
+    /* Ensure that target MPI has at least the necessary number of limbs */
+    MBEDTLS_MPI_CHK( mpi_core_clear( X, nx, limbs ) );
+
+    /* Avoid calling `memcpy` with NULL source or destination argument,
+     * even if buflen is 0. */
+    if( buflen != 0 )
+    {
+        Xp = (unsigned char*) X;
+        memcpy( Xp + overhead, buf, buflen );
+
+        mpi_core_bigendian_to_host( X, nx );
+    }
+
+cleanup:
+    return( ret );
+}
+
+/*
+ * Export X into unsigned binary data, little endian
+ */
+int mbedtls_mpi_core_write_le( const mbedtls_mpi_uint *X,
+                               size_t nx,
+                               unsigned char *buf,
+                               size_t buflen )
+{
+    size_t stored_bytes = nx * ciL;
+    size_t bytes_to_copy;
+    size_t i;
+
+    if( stored_bytes < buflen )
+    {
+        bytes_to_copy = stored_bytes;
+    }
+    else
+    {
+        bytes_to_copy = buflen;
+
+        /* The output buffer is smaller than the allocated size of X.
+         * However X may fit if its leading bytes are zero. */
+        for( i = bytes_to_copy; i < stored_bytes; i++ )
+        {
+            if( GET_BYTE( X, i ) != 0 )
+                return( MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL );
+        }
+    }
+
+    for( i = 0; i < bytes_to_copy; i++ )
+        buf[i] = GET_BYTE( X, i );
+
+    if( stored_bytes < buflen )
+    {
+        /* Write trailing 0 bytes */
+        memset( buf + stored_bytes, 0, buflen - stored_bytes );
+    }
+
+    return( 0 );
+}
+
+/*
+ * Export X into unsigned binary data, big endian
+ */
+int mbedtls_mpi_core_write_be( const mbedtls_mpi_uint *X,
+                               size_t nx,
+                               unsigned char *buf,
+                               size_t buflen )
+{
+    size_t stored_bytes;
+    size_t bytes_to_copy;
+    unsigned char *p;
+    size_t i;
+
+    MPI_VALIDATE_RET( X != NULL );
+    MPI_VALIDATE_RET( buflen == 0 || buf != NULL );
+
+    stored_bytes = nx * ciL;
+
+    if( stored_bytes < buflen )
+    {
+        /* There is enough space in the output buffer. Write initial
+         * null bytes and record the position at which to start
+         * writing the significant bytes. In this case, the execution
+         * trace of this function does not depend on the value of the
+         * number. */
+        bytes_to_copy = stored_bytes;
+        p = buf + buflen - stored_bytes;
+        memset( buf, 0, buflen - stored_bytes );
+    }
+    else
+    {
+        /* The output buffer is smaller than the allocated size of X.
+         * However X may fit if its leading bytes are zero. */
+        bytes_to_copy = buflen;
+        p = buf;
+        for( i = bytes_to_copy; i < stored_bytes; i++ )
+        {
+            if( GET_BYTE( X, i ) != 0 )
+                return( MBEDTLS_ERR_MPI_BUFFER_TOO_SMALL );
+        }
+    }
+
+    for( i = 0; i < bytes_to_copy; i++ )
+        p[bytes_to_copy - i - 1] = GET_BYTE( X, i );
+
+    return( 0 );
+}
+
 #endif /* MBEDTLS_BIGNUM_C */