diff --git a/library/constant_time.c b/library/constant_time.c
index 2952992..b2203ba 100644
--- a/library/constant_time.c
+++ b/library/constant_time.c
@@ -584,4 +584,57 @@
     return( ret );
 }
 
+/*
+ * Conditionally swap X and Y, without leaking information
+ * about whether the swap was made or not.
+ * Here it is not ok to simply swap the pointers, which whould lead to
+ * different memory access patterns when X and Y are used afterwards.
+ */
+int mbedtls_mpi_safe_cond_swap( mbedtls_mpi *X, mbedtls_mpi *Y, unsigned char swap )
+{
+    int ret, s;
+    size_t i;
+    mbedtls_mpi_uint limb_mask;
+    mbedtls_mpi_uint tmp;
+    MPI_VALIDATE_RET( X != NULL );
+    MPI_VALIDATE_RET( Y != NULL );
+
+    if( X == Y )
+        return( 0 );
+
+    /* MSVC has a warning about unary minus on unsigned integer types,
+     * but this is well-defined and precisely what we want to do here. */
+#if defined(_MSC_VER)
+#pragma warning( push )
+#pragma warning( disable : 4146 )
+#endif
+
+    /* make sure swap is 0 or 1 in a time-constant manner */
+    swap = (swap | (unsigned char)-swap) >> (sizeof( swap ) * 8 - 1);
+    /* all-bits 1 if swap is 1, all-bits 0 if swap is 0 */
+    limb_mask = -swap;
+
+#if defined(_MSC_VER)
+#pragma warning( pop )
+#endif
+
+    MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, Y->n ) );
+    MBEDTLS_MPI_CHK( mbedtls_mpi_grow( Y, X->n ) );
+
+    s = X->s;
+    X->s = mbedtls_cf_cond_select_sign( X->s, Y->s, swap );
+    Y->s = mbedtls_cf_cond_select_sign( Y->s, s, swap );
+
+
+    for( i = 0; i < X->n; i++ )
+    {
+        tmp = X->p[i];
+        X->p[i] = ( X->p[i] & ~limb_mask ) | ( Y->p[i] & limb_mask );
+        Y->p[i] = ( Y->p[i] & ~limb_mask ) | (     tmp & limb_mask );
+    }
+
+cleanup:
+    return( ret );
+}
+
 #endif /* MBEDTLS_BIGNUM_C */