sp_int: fixes and added testing

[SparkiDev]

Description

sp_set_bit(): check i is in range before use.
_sp_div_2(): Use a constant-time clamp as called by a constant-time function.
_sp_sqr(): static buffer needs to be one larger for when ECC with P-521 is the largest size.

Add tests:

• Testing negative numbers with mp_read_raidx/mp_to_radix 10/16
• Testing negative numbers with mp_add_d/mp_sub_d
• Testing of mp_gcd without mp_lcm
• More testing of mp_mod_d and when negative numbers are used
• Check maximum values work for square. Check of _sp_sqr() bug
• Add testing of mp_add/sub_mod_ct
• Add testing of mp_cmp_mag
• Add testing of mp_mulmod/mp_sqrmod
• Add testing of mp_exch
• Add testing of mp_to_unsigned_bin_len_ct
• Add testing of mp_exptmod that uses base-2 windowing method.
• Add testing of mp_invmod_mont_ct

Testing

./configure --enable-all

[SparkiDev]

_sp_sqr() code generated with PR:
https://github.com/wolfSSL/scripts/pull/578

[Copilot]

Pull request overview

Fixes three bugs in wolfcrypt/src/sp_int.c and substantially extends the multi-precision integer tests in wolfcrypt/test/test.c to exercise the affected paths and other previously-untested behaviour.

Changes:

• Bug fixes in sp_set_bit (range check on i), _sp_div_2 (constant-time clamp to keep the helper constant-time for its CT callers), and _sp_sqr (static buffer enlarged by one digit so ECC P-521 squaring fits). Also small sign-cast/sp_size_t cast cleanups.
• Many new test cases in mp_test: negative-value radix conversions, negative mp_add_d/mp_sub_d, mp_mod_d fast paths and negative input, multi-word mp_gcd (now also tested without mp_lcm), max-value mp_sqr, mp_addmod_ct/mp_submod_ct/mp_div_2_mod_ct, mp_cmp_mag, mp_mulmod/mp_sqrmod result-aliasing, mp_exch, mp_to_unsigned_bin_len_ct, base-2/windowed mp_exptmod, and mp_invmod_mont_ct.
• Test plumbing: mp_test_add_sub_d gains a b operand, mp_test_exptmod gains a t scratch operand, and the mp_test_lcm_gcd guards are split via a new MP_TEST_HAVE_LCM macro so mp_gcd is exercised independently.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
wolfcrypt/src/sp_int.c	Range check in sp_set_bit, constant-time clamp in _sp_div_2, +1 digit on _sp_sqr static buffer, minor signed/sp_size_t cast cleanups.
wolfcrypt/test/test.c	Extensive new mp_test coverage and updated dispatcher/signatures to feed the new tests.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #10529

Scan targets checked: wolfcrypt-bugs, wolfcrypt-src

No new issues found in the changed files. ✅