Add optionally unsafe variant of exp_mod for perf
Attempt to partially solve the performance regression in 3.6.0 without
adding too much code size.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/bignum_core.h b/library/bignum_core.h
index 92c8d47..c63cdee 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -605,6 +605,44 @@
size_t mbedtls_mpi_core_exp_mod_working_limbs(size_t AN_limbs, size_t E_limbs);
/**
+ * \brief Perform a modular exponentiation with public or secret exponent:
+ * X = A^E mod N, where \p A is already in Montgomery form.
+ *
+ * \p X may be aliased to \p A, but not to \p RR or \p E, even if \p E_limbs ==
+ * \p AN_limbs.
+ *
+ * \param[out] X The destination MPI, as a little endian array of length
+ * \p AN_limbs.
+ * \param[in] A The base MPI, as a little endian array of length \p AN_limbs.
+ * Must be in Montgomery form.
+ * \param[in] N The modulus, as a little endian array of length \p AN_limbs.
+ * \param AN_limbs The number of limbs in \p X, \p A, \p N, \p RR.
+ * \param[in] E The exponent, as a little endian array of length \p E_limbs.
+ * \param E_limbs The number of limbs in \p E.
+ * \param[in] RR The precomputed residue of 2^{2*biL} modulo N, as a little
+ * endian array of length \p AN_limbs.
+ * \param[in,out] T Temporary storage of at least the number of limbs returned
+ * by `mbedtls_mpi_core_exp_mod_working_limbs()`.
+ * Its initial content is unused and its final content is
+ * indeterminate.
+ * It must not alias or otherwise overlap any of the other
+ * parameters.
+ * It is up to the caller to zeroize \p T when it is no
+ * longer needed, and before freeing it if it was dynamically
+ * allocated.
+ * \param[in] E_public Set to MBEDTLS_MPI_IS_PUBLIC to gain some performance
+ * when the value of E is public.
+ * Set to MBEDTLS_MPI_IS_SECRET when the value of E is secret.
+ */
+void mbedtls_mpi_core_exp_mod_optionally_safe(mbedtls_mpi_uint *X,
+ const mbedtls_mpi_uint *A,
+ const mbedtls_mpi_uint *N, size_t AN_limbs,
+ const mbedtls_mpi_uint *E, size_t E_limbs,
+ const mbedtls_mpi_uint *RR,
+ mbedtls_mpi_uint *T,
+ int E_public);
+
+/**
* \brief Perform a modular exponentiation with secret exponent:
* X = A^E mod N, where \p A is already in Montgomery form.
*