bignum_mod_raw: Added modular negation.
This patch adds the `mpi_mod_raw_neg()` method.
Co-authored-by: Hanno Becker <hanno.becker@arm.com>
Co-authored-by: Minos Galanakis <minos.galanakis@arm.com>
Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>
diff --git a/library/bignum_mod_raw.c b/library/bignum_mod_raw.c
index 22e56b7..138d5a0 100644
--- a/library/bignum_mod_raw.c
+++ b/library/bignum_mod_raw.c
@@ -182,6 +182,18 @@
mbedtls_free( T );
return( 0 );
}
+
+void mbedtls_mpi_mod_raw_neg( mbedtls_mpi_uint *X,
+ const mbedtls_mpi_uint *A,
+ const mbedtls_mpi_mod_modulus *m )
+{
+ mbedtls_mpi_core_sub( X, m->p, A, m->limbs );
+
+ /* If A=0 initially, then X=N now. Detect this by
+ * subtracting N and catching the carry. */
+ mbedtls_mpi_uint borrow = mbedtls_mpi_core_sub( X, X, m->p, m->limbs );
+ (void) mbedtls_mpi_core_add_if( X, m->p, m->limbs, (unsigned) borrow );
+}
/* END MERGE SLOT 7 */
/* BEGIN MERGE SLOT 8 */
diff --git a/library/bignum_mod_raw.h b/library/bignum_mod_raw.h
index d7b6dd1..b512ae0 100644
--- a/library/bignum_mod_raw.h
+++ b/library/bignum_mod_raw.h
@@ -233,6 +233,24 @@
*/
int mbedtls_mpi_mod_raw_from_mont_rep( mbedtls_mpi_uint *X,
const mbedtls_mpi_mod_modulus *m );
+
+/** \brief Perform fixed width modular negation.
+ *
+ * The size of the operation is determined by \p N. \p A must have
+ * the same number of limbs as \p N.
+ *
+ * \p X may be aliased to \p A.
+ *
+ * \param[out] X The result of the modular negation.
+ * This must be initialized. Must have enough limbs to
+ * store the full value of the result.
+ * \param[in] A Little-endian presentation of the input operand. This
+ * must be smaller or equal to \p N.
+ * \param[in] m The address of the modulus related to \p A.
+ */
+void mbedtls_mpi_mod_raw_neg( mbedtls_mpi_uint *X,
+ const mbedtls_mpi_uint *A,
+ const mbedtls_mpi_mod_modulus *m);
/* END MERGE SLOT 7 */
/* BEGIN MERGE SLOT 8 */