ecp_curves: Re-introduced `mbedtls_ecp_fix_negative()`
This patch re-introduces `mbedtls_ecp_fix_negative` and
appropriately adjusts its' define guards.
Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>
diff --git a/library/ecp_curves.c b/library/ecp_curves.c
index c23ff2c..6ee3d6c 100644
--- a/library/ecp_curves.c
+++ b/library/ecp_curves.c
@@ -5299,6 +5299,39 @@
MBEDTLS_ECP_DP_SECP256R1_ENABLED ||
MBEDTLS_ECP_DP_SECP384R1_ENABLED */
+#if defined(MBEDTLS_TEST_HOOKS) && defined(MBEDTLS_ECP_C)
+MBEDTLS_STATIC_TESTABLE
+void mbedtls_ecp_fix_negative(mbedtls_mpi *N, signed char c, size_t bits)
+{
+ size_t i;
+
+ /* Set N := 2^bits - 1 - N. We know that 0 <= N < 2^bits, so
+ * set the absolute value to 0xfff...fff - N. There is no carry
+ * since we're subtracting from all-bits-one. */
+ for (i = 0; i <= bits / 8 / sizeof(mbedtls_mpi_uint); i++) {
+ N->p[i] = ~(mbedtls_mpi_uint) 0 - N->p[i];
+ }
+ /* Add 1, taking care of the carry. */
+ i = 0;
+ do {
+ ++N->p[i];
+ } while (N->p[i++] == 0 && i <= bits / 8 / sizeof(mbedtls_mpi_uint));
+ /* Invert the sign.
+ * Now N = N0 - 2^bits where N0 is the initial value of N. */
+ N->s = -1;
+
+ /* Add |c| * 2^bits to the absolute value. Since c and N are
+ * negative, this adds c * 2^bits. */
+ mbedtls_mpi_uint msw = (mbedtls_mpi_uint) -c;
+#if defined(MBEDTLS_HAVE_INT64)
+ if (bits == 224) {
+ msw <<= 32;
+ }
+#endif
+ N->p[bits / 8 / sizeof(mbedtls_mpi_uint)] += msw;
+}
+#endif /* MBEDTLS_TEST_HOOKS & MBEDTLS_ECP_C */
+
#if defined(MBEDTLS_ECP_DP_SECP521R1_ENABLED)
/* Size of p521 in terms of mbedtls_mpi_uint */
#define P521_WIDTH (521 / 8 / sizeof(mbedtls_mpi_uint) + 1)