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Reject zero inputs in bls_modular_inverse() #3299

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merged 2 commits into from
Mar 20, 2023
Merged

Reject zero inputs in bls_modular_inverse() #3299

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0e0403d
Reject zero inputs in bls_modular_inverse()
asn-d6 Mar 16, 2023
1219bea
Add some basic bls_modular_inverse() tests
asn-d6 Mar 17, 2023
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7 changes: 4 additions & 3 deletions specs/deneb/polynomial-commitments.md
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Original file line number Diff line number Diff line change
Expand Up @@ -252,10 +252,11 @@ def compute_challenge(blob: Blob,
```python
def bls_modular_inverse(x: BLSFieldElement) -> BLSFieldElement:
"""
Compute the modular inverse of x
i.e. return y such that x * y % BLS_MODULUS == 1 and return 0 for x == 0
Compute the modular inverse of x (for x != 0)
i.e. return y such that x * y % BLS_MODULUS == 1
"""
return BLSFieldElement(pow(x, -1, BLS_MODULUS)) if x != 0 else BLSFieldElement(0)
assert (int(x) % BLS_MODULUS) != 0
return BLSFieldElement(pow(x, -1, BLS_MODULUS))
```

#### `div`
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23 changes: 23 additions & 0 deletions ...yspec/eth2spec/test/deneb/unittests/polynomial_commitments/test_polynomial_commitments.py
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Expand Up @@ -215,6 +215,29 @@ def test_verify_blob_kzg_proof_incorrect_proof(spec):
assert not spec.verify_blob_kzg_proof(blob, commitment, proof)


@with_deneb_and_later
@spec_test
@single_phase
def test_bls_modular_inverse(spec):
"""
Verify computation of multiplicative inverse
"""
rng = random.Random(5566)

# Should fail for x == 0
expect_assertion_error(lambda: spec.bls_modular_inverse(0))
expect_assertion_error(lambda: spec.bls_modular_inverse(spec.BLS_MODULUS))
expect_assertion_error(lambda: spec.bls_modular_inverse(2 * spec.BLS_MODULUS))

# Test a trivial inversion
assert 1 == int(spec.bls_modular_inverse(1))

# Test a random inversion
r = rng.randint(0, spec.BLS_MODULUS - 1)
r_inv = int(spec.bls_modular_inverse(r))
assert r * r_inv % BLS_MODULUS == 1


@with_deneb_and_later
@spec_test
@single_phase
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