P-256 scalar mul and modular inverse from s2n-bignum #1794
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When the usual architectural constraints are met, the preprocessor macro EC_P256_USE_S2N_BIGNUM is set. That in turn triggers the replacement of the existing Fermat inverse in p256-nistz.c with the markedly faster and formally verified divstep-based code from s2n-bignum.
This replaces the general (fresh, not precomputed, point) scalar multiplication with the corresponding function p256_montjscalarmul or p256_montjscalarmul_alt from s2n-bignum.
This should make FIPS builds work.
Changing #(I) to #(1*I) in the new macro blocks.
Codecov ReportAll modified and coverable lines are covered by tests ✅
Additional details and impacted files@@ Coverage Diff @@
## main #1794 +/- ##
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- Coverage 78.34% 78.31% -0.03%
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Files 580 581 +1
Lines 97255 97228 -27
Branches 13945 13939 -6
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- Hits 76190 76147 -43
- Misses 20444 20457 +13
- Partials 621 624 +3 ☔ View full report in Codecov by Sentry. |
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Issues:
Resolves #ISSUE-NUMBER1
Addresses #ISSUE-NUMBER2
Description of changes:
This makes use of s2n-bignum's P-256 functions for both the fresh-point scalar multiplication and the modular (actually Montgomery) inverse that arises in conversion from Jacobian to affine.
Call-outs:
Basic #if/#else/#endif versioning is a bit crude and maybe there is a better way to do this stylistically.
Only tested on a few platforms and may need more work to make it completely portable, robust for FIPS builds etc.
Testing:
Ran "ninja run_tests" and a few other aws-lc tests.
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.