crypto: Use Almost Montgomery Multiplication in modexp #1427
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Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## master #1427 +/- ##
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+ Coverage 81.71% 81.97% +0.26%
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Files 152 152
Lines 13606 13806 +200
Branches 3222 3223 +1
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+ Hits 11118 11318 +200
Misses 343 343
Partials 2145 2145
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Pull request overview
Updates the expmod precompile’s modular exponentiation to use Almost Montgomery Multiplication (AMM) for improved performance, and extends unit coverage with additional vectors targeting AMM edge cases.
Changes:
- Replace classic Montgomery multiplication with AMM in the odd-modulus exponentiation path.
- Add a final conditional reduction step after converting out of Montgomery form to ensure canonical output.
- Add extensive unit test vectors intended to exercise AMM-related corner cases.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.
| File | Description |
|---|---|
| lib/evmone_precompiles/modexp.cpp | Switches modexp’s multiplication primitive to AMM and adjusts the final reduction step. |
| test/unittests/precompiles_expmod_test.cpp | Adds new expmod test vectors covering AMM-specific scenarios and boundary conditions. |
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Pull request overview
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lib/evmone_precompiles/modexp.cpp
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| /// The AMM is relaxed version of the Montgomery multiplication which produces | ||
| /// results in the range result < 2*mod, i.e., it may be larger than the modulus. | ||
| /// This allows to skip the final conditional subtraction in most cases, improving performance. | ||
| /// The inputs are expected to be in the Montgomery form and also in the range < 2*mod. |
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The mul_amm doc says inputs are expected to be in Montgomery form, but modexp_odd() calls it with y = 1 to convert from Montgomery form. Please clarify the contract/docstring to mention this non-Montgomery operand use (or adjust wording so it matches actual usage).
Suggested change
| /// The inputs are expected to be in the Montgomery form and also in the range < 2*mod. | |
| /// In the typical case, both inputs x and y are expected to be in Montgomery form and also in | |
| /// the range < 2*mod. A notable supported special case is y == 1, which is used to convert | |
| /// a value from Montgomery form back to the standard representation. |
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Use the relaxed version of the Montgomery multiplication called Almost Montgomery Multiplication from "Efficient Software Implementations of Modular Exponentiation" (https://eprint.iacr.org/2011/239.pdf). ``` │ old │ new │ │ gas/s │ gas/s vs base │ modexp<expmod_execute>/mod_len:8/exp_bits:33-14 902.5M ± 0% 941.3M ± 0% +4.29% (p=0.000 n=11) modexp<expmod_execute>/mod_len:16/exp_bits:33-14 896.6M ± 1% 931.5M ± 0% +3.88% (p=0.000 n=11) modexp<expmod_execute>/mod_len:24/exp_bits:33-14 233.0M ± 1% 241.2M ± 0% +3.53% (p=0.000 n=11) modexp<expmod_execute>/mod_len:32/exp_bits:33-14 233.9M ± 1% 242.0M ± 0% +3.43% (p=0.000 n=11) modexp<expmod_execute>/mod_len:32/exp_bits:256-14 245.3M ± 1% 254.7M ± 0% +3.83% (p=0.000 n=11) modexp<expmod_execute>/mod_len:32/exp_bits:8192-14 488.2M ± 0% 505.2M ± 0% +3.48% (p=0.000 n=11) modexp<expmod_execute>/mod_len:40/exp_bits:11-14 204.1M ± 0% 212.3M ± 1% +3.99% (p=0.000 n=11) modexp<expmod_execute>/mod_len:48/exp_bits:8-14 277.2M ± 0% 288.6M ± 0% +4.14% (p=0.000 n=11) modexp<expmod_execute>/mod_len:48/exp_bits:256-14 338.4M ± 0% 351.9M ± 0% +4.01% (p=0.000 n=11) modexp<expmod_execute>/mod_len:56/exp_bits:6-14 359.1M ± 0% 371.5M ± 0% +3.46% (p=0.000 n=11) modexp<expmod_execute>/mod_len:64/exp_bits:5-14 434.1M ± 0% 446.5M ± 0% +2.87% (p=0.000 n=11) modexp<expmod_execute>/mod_len:72/exp_bits:4-14 160.5M ± 1% 161.3M ± 0% +0.48% (p=0.001 n=11) modexp<expmod_execute>/mod_len:112/exp_bits:4-14 368.1M ± 0% 369.0M ± 0% +0.24% (p=0.002 n=11) modexp<expmod_execute>/mod_len:136/exp_bits:3-14 132.1M ± 0% 133.2M ± 1% +0.79% (p=0.001 n=11) modexp<expmod_execute>/mod_len:192/exp_bits:2-14 186.7M ± 1% 186.7M ± 2% ~ (p=0.199 n=11) modexp<expmod_execute>/mod_len:504/exp_bits:2-14 82.04M ± 0% 81.86M ± 1% -0.21% (p=0.010 n=11) modexp<expmod_execute>/mod_len:512/exp_bits:2-14 84.51M ± 1% 84.42M ± 0% ~ (p=0.270 n=11) modexp<expmod_execute>/mod_len:512/exp_bits:8192-14 348.0M ± 0% 348.4M ± 0% +0.12% (p=0.034 n=11) modexp<expmod_execute>/mod_len:520/exp_bits:2-14 87.13M ± 0% 87.00M ± 0% ~ (p=0.300 n=11) modexp<expmod_execute>/mod_len:1016/exp_bits:2-14 310.5M ± 1% 309.7M ± 0% ~ (p=0.171 n=11) modexp<expmod_execute>/mod_len:1024/exp_bits:2-14 315.2M ± 0% 314.7M ± 0% ~ (p=0.151 n=11) modexp<expmod_execute>/mod_len:1024/exp_bits:256-14 704.2M ± 0% 705.1M ± 0% ~ (p=0.217 n=11) modexp<expmod_execute>/mod_len:1024/exp_bits:2048-14 1.327G ± 0% 1.325G ± 0% ~ (p=0.519 n=11) geomean 290.3M 295.5M +1.79% ```
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Use the relaxed version of the Montgomery multiplication called Almost Montgomery Multiplication from
"Efficient Software Implementations of Modular Exponentiation" (https://eprint.iacr.org/2011/239.pdf).