[ageprocpp]

Description

Fixes Fenrir #17457

For heap math, mp_exptmod can select single-digit 2k reduction for an even modulus such as 2^DIGIT_BIT. Its setup value is 2^DIGIT_BIT, which does not fit in one digit, so the operation returns MP_VAL. Reject multi-digit moduli with a zero low digit in mp_reduce_is_2k, allowing the existing Barrett path to handle them when available.

Testing

• Built with --enable-heapmath --disable-shared --enable-static and CFLAGS='-O0 -DWOLFSSL_PUBLIC_MP'.
• Ran ./tests/unit.test --api -test_mp_exptmod_even_2k. The test checks 2^DIGIT_BIT, 3 * 2^DIGIT_BIT, and the adjacent valid 2k modulus, using (p - 1)^2 mod p = 1.
• Confirmed that the regression test fails against the unmodified mp_reduce_is_2k.

Checklist

• added tests
• updated/added doxygen
• updated appropriate READMEs
• Updated manual and documentation

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has insufficient remaining quota. Try requesting a review at a lower effort level.

[github-actions]

retest this please

[github-actions]

MemBrowse Memory Report · Explore what changed →

zephyr-frdm_rw612

• FLASH: datas +16 B, log_const_area -8 B, rodata -8 B, text +3,872 B (+1.4%, 286,336 B / 67,108,864 B, total: 0% used)
• RAM: bss +49 B, datas +16 B (+0.1%, 58,423 B / 983,040 B, total: 6% used)

No memory changes detected for:

• gcc-arm-cortex-m0plus
• gcc-arm-cortex-m3
• gcc-arm-cortex-m4
• gcc-arm-cortex-m4-baremetal
• gcc-arm-cortex-m4-crypto-only
• gcc-arm-cortex-m4-dtls13
• gcc-arm-cortex-m4-min-ecc
• gcc-arm-cortex-m4-openssl-compat
• gcc-arm-cortex-m4-pkcs7
• gcc-arm-cortex-m4-pq
• gcc-arm-cortex-m4-rsa-only
• gcc-arm-cortex-m4-sp-math
• gcc-arm-cortex-m4-tls12
• gcc-arm-cortex-m4-tls13
• gcc-arm-cortex-m7
• gcc-arm-cortex-m7-pq
• gcc-arm-cortex-m7-tls13
• stm32-sim-stm32h753

---

_{Catch memory regressions in CI with MemBrowse. Free for open source.}

[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #12081

Scan targets checked: wolfcrypt-src, wolfcrypt-bugs, wolfssl-bugs
Not reviewed (no scan target covers them): tests/api/test_wolfmath.h
Coverage: 2 of 2 in-scope changed file(s) opened by the reviewer

Fenrir result: Approved ✅

No new issues found in the changed files.

Advisory only — this automated result does not count as a GitHub approval.

Review tier: Max

[ageprocpp]

retest this please