Add mbedtls_mpi_core_ct_uint_table_lookup
This will be needed for extracting modular exponentiation from the
prototype. The function signature is kept aligned to the prototype, but
the implementation is new. (The implementation of this function in the
prototype has further optimisations which are out of scope for now.)
The function is not reused in the bignum counterpart as it will become
redundant soon.
This function is meant to be static, but doesn't have the qualifier as
it is not used yet and would cause compiler warnings. The
MBEDTLS_STATIC_TESTABLE macro will be added in a later commit.
Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/library/bignum_core.h b/library/bignum_core.h
index 9a5b89f..3618e42 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -452,4 +452,23 @@
int mbedtls_mpi_core_get_mont_r2_unsafe( mbedtls_mpi *X,
const mbedtls_mpi *N );
+/**
+ * Select an MPI from a table without leaking the index.
+ *
+ * \param dest The destination buffer. This must point to a writable
+ * buffer of at least \p limbs limbs.
+ * \param table The address of the table. This must point to a readable
+ * array of \p count elements of
+ * \p limbs limbs each each.
+ * \param limbs The length of a table entry in limbs.
+ * \param count The number of elements in \p table.
+ * \param index The secret table index to look up. This must be in the
+ * range `0,..,count-1`.
+ */
+void mbedtls_mpi_core_ct_uint_table_lookup( mbedtls_mpi_uint *dest,
+ const mbedtls_mpi_uint *table,
+ size_t limbs,
+ size_t count,
+ size_t index );
+
#endif /* MBEDTLS_BIGNUM_CORE_H */