fix(emulated): pad a constant custom modulus to the full number of limbs - #1854
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The variable-modulus methods document that the modulus may be a constant, but NewElement stores a constant on the minimal number of limbs while the hints (mulHint, computeSubPaddingHint) read NbLimbs modulus limbs from their inputs. With a small constant modulus they read the wrong inputs: ModMul fails its check and ModAdd/ModExp/ModAssertIsEqual fail with "input length mismatch". Pad the modulus with zero limbs in checkModulus.
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ModMul,ModAdd,ModExpandModAssertIsEqualsay the modulus can be a witness or a constant, but a small constant modulus doesn't work:Before:
After: all four work with a constant modulus (test engine, r1cs, scs).
NewElementstores a constant on the minimal number of limbs (1 limb for 7), whilecallMulHint/mulHintandcomputeSubPaddingHintalways readNbLimbs()modulus limbs from their inputs, so they either pick up the wrong inputs or get the wrong input count.checkModulusnow returns the modulus padded with zero limbs up toNbLimbs(), and the four methods use that. A witness modulus already has all its limbs and is returned unchanged.Added
TestConstantModulus(Secp256k1Fp and Mod1e512, ModMulCanonical / ModAdd / ModAssertIsEqual / ModExp with a 20-bit constant modulus). It fails on master.std/math/emulated, the MODEXP precompile tests andinternal/stats(no change) pass.