diff --git a/library/constant_time.c b/library/constant_time.c
index 60352ae..2952992 100644
--- a/library/constant_time.c
+++ b/library/constant_time.c
@@ -536,3 +536,52 @@
 }
 
 #endif /* MBEDTLS_SSL_SOME_SUITES_USE_TLS_CBC */
+
+#if defined(MBEDTLS_BIGNUM_C)
+
+#define MPI_VALIDATE_RET( cond )                                       \
+    MBEDTLS_INTERNAL_VALIDATE_RET( cond, MBEDTLS_ERR_MPI_BAD_INPUT_DATA )
+
+/*
+ * Conditionally assign X = Y, without leaking information
+ * about whether the assignment was made or not.
+ * (Leaking information about the respective sizes of X and Y is ok however.)
+ */
+int mbedtls_mpi_safe_cond_assign( mbedtls_mpi *X, const mbedtls_mpi *Y, unsigned char assign )
+{
+    int ret = 0;
+    size_t i;
+    mbedtls_mpi_uint limb_mask;
+    MPI_VALIDATE_RET( X != NULL );
+    MPI_VALIDATE_RET( Y != NULL );
+
+    /* 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 assign is 0 or 1 in a time-constant manner */
+    assign = (assign | (unsigned char)-assign) >> (sizeof( assign ) * 8 - 1);
+    /* all-bits 1 if assign is 1, all-bits 0 if assign is 0 */
+    limb_mask = -assign;
+
+#if defined(_MSC_VER)
+#pragma warning( pop )
+#endif
+
+    MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, Y->n ) );
+
+    X->s = mbedtls_cf_cond_select_sign( X->s, Y->s, assign );
+
+    mbedtls_cf_mpi_uint_cond_assign( Y->n, X->p, Y->p, assign );
+
+    for( i = Y->n; i < X->n; i++ )
+        X->p[i] &= ~limb_mask;
+
+cleanup:
+    return( ret );
+}
+
+#endif /* MBEDTLS_BIGNUM_C */