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# EIP-4844 -- Polynomial Commitments
## Table of contents
<!-- TOC -->
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- [Introduction ](#introduction )
- [Custom types ](#custom-types )
- [Constants ](#constants )
- [Preset ](#preset )
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- [Blob ](#blob )
- [Crypto ](#crypto )
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- [Trusted setup ](#trusted-setup )
- [Helper functions ](#helper-functions )
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- [Bit-reversal permutation ](#bit-reversal-permutation )
- [`is_power_of_two` ](#is_power_of_two )
- [`reverse_bits` ](#reverse_bits )
- [`bit_reversal_permutation` ](#bit_reversal_permutation )
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- [BLS12-381 helpers ](#bls12-381-helpers )
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- [`hash_to_bls_field` ](#hash_to_bls_field )
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- [`bytes_to_bls_field` ](#bytes_to_bls_field )
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- [`validate_kzg_g1` ](#validate_kzg_g1 )
- [`bytes_to_kzg_commitment` ](#bytes_to_kzg_commitment )
- [`bytes_to_kzg_proof` ](#bytes_to_kzg_proof )
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- [`blob_to_polynomial` ](#blob_to_polynomial )
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- [`compute_challenges` ](#compute_challenges )
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- [`bls_modular_inverse` ](#bls_modular_inverse )
- [`div` ](#div )
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- [`g1_lincomb` ](#g1_lincomb )
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- [`poly_lincomb` ](#poly_lincomb )
- [`compute_powers` ](#compute_powers )
- [Polynomials ](#polynomials )
- [`evaluate_polynomial_in_evaluation_form` ](#evaluate_polynomial_in_evaluation_form )
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- [KZG ](#kzg )
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- [`blob_to_kzg_commitment` ](#blob_to_kzg_commitment )
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- [`verify_kzg_proof` ](#verify_kzg_proof )
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- [`verify_kzg_proof_impl` ](#verify_kzg_proof_impl )
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- [`verify_kzg_proof_multi` ](#verify_kzg_proof_multi )
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- [`compute_kzg_proof` ](#compute_kzg_proof )
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- [`compute_kzg_proof_impl` ](#compute_kzg_proof_impl )
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- [`compute_aggregated_poly_and_commitment` ](#compute_aggregated_poly_and_commitment )
- [`compute_aggregate_kzg_proof` ](#compute_aggregate_kzg_proof )
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- [`verify_aggregate_kzg_proof_aggregation` ](#verify_aggregate_kzg_proof_aggregation )
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- [`verify_aggregate_kzg_proof` ](#verify_aggregate_kzg_proof )
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- [`verify_aggregate_kzg_proof_multi` ](#verify_aggregate_kzg_proof_multi )
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## Introduction
This document specifies basic polynomial operations and KZG polynomial commitment operations as they are needed for the EIP-4844 specification. The implementations are not optimized for performance, but readability. All practical implementations should optimize the polynomial operations.
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Functions flagged as "Public method" MUST be provided by the underlying KZG library as public functions. All other functions are private functions used internally by the KZG library.
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Public functions MUST accept raw bytes as input and perform the required cryptographic normalization before invoking any internal functions.
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## Custom types
| Name | SSZ equivalent | Description |
| - | - | - |
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| `G1Point` | `Bytes48` | |
| `G2Point` | `Bytes96` | |
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| `BLSFieldElement` | `uint256` | Validation: `x < BLS_MODULUS` |
| `KZGCommitment` | `Bytes48` | Validation: Perform [BLS standard's ](https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-bls-signature-04#section-2.5 ) "KeyValidate" check but do allow the identity point |
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| `KZGProof` | `Bytes48` | Same as for `KZGCommitment` |
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| `Polynomial` | `Vector[BLSFieldElement, FIELD_ELEMENTS_PER_BLOB]` | A polynomial in evaluation form |
| `Blob` | `ByteVector[BYTES_PER_FIELD_ELEMENT * FIELD_ELEMENTS_PER_BLOB]` | A basic blob data |
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## Constants
| Name | Value | Notes |
| - | - | - |
| `BLS_MODULUS` | `52435875175126190479447740508185965837690552500527637822603658699938581184513` | Scalar field modulus of BLS12-381 |
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| `BYTES_PER_FIELD_ELEMENT` | `uint64(32)` | Bytes used to encode a BLS scalar field element |
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| `G1_POINT_AT_INFINITY` | `Bytes48(b'\xc0' + b'\x00' * 47)` | Serialized form of the point at infinity on the G1 group |
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## Preset
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### Blob
| Name | Value |
| - | - |
| `FIELD_ELEMENTS_PER_BLOB` | `uint64(4096)` |
| `FIAT_SHAMIR_PROTOCOL_DOMAIN` | `b'FSBLOBVERIFY_V1_'` |
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| `RANDOM_CHALLENGE_KZG_MULTI_DOMAIN` | `b'RCKZGMULTI___V1_'` |
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### Crypto
| Name | Value | Notes |
| - | - | - |
| `ROOTS_OF_UNITY` | `Vector[BLSFieldElement, FIELD_ELEMENTS_PER_BLOB]` | Roots of unity of order FIELD_ELEMENTS_PER_BLOB over the BLS12-381 field |
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### Trusted setup
The trusted setup is part of the preset: during testing a `minimal` insecure variant may be used,
but reusing the `mainnet` settings in public networks is a critical security requirement.
| Name | Value |
| - | - |
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| `KZG_SETUP_G2_LENGTH` | `65` |
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| `KZG_SETUP_G1` | `Vector[G1Point, FIELD_ELEMENTS_PER_BLOB]` , contents TBD |
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| `KZG_SETUP_G2` | `Vector[G2Point, KZG_SETUP_G2_LENGTH]` , contents TBD |
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| `KZG_SETUP_LAGRANGE` | `Vector[KZGCommitment, FIELD_ELEMENTS_PER_BLOB]` , contents TBD |
## Helper functions
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### Bit-reversal permutation
All polynomials (which are always given in Lagrange form) should be interpreted as being in
bit-reversal permutation. In practice, clients can implement this by storing the lists
`KZG_SETUP_LAGRANGE` and `ROOTS_OF_UNITY` in bit-reversal permutation, so these functions only
have to be called once at startup.
#### `is_power_of_two`
```python
def is_power_of_two(value: int) -> bool:
"""
Check if ``value`` is a power of two integer.
"""
return (value > 0) and (value & (value - 1) == 0)
```
#### `reverse_bits`
```python
def reverse_bits(n: int, order: int) -> int:
"""
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Reverse the bit order of an integer ``n``.
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"""
assert is_power_of_two(order)
# Convert n to binary with the same number of bits as "order" - 1, then reverse its bit order
return int(('{:0' + str(order.bit_length() - 1) + 'b}').format(n)[::-1], 2)
```
#### `bit_reversal_permutation`
```python
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def bit_reversal_permutation(sequence: Sequence[T]) -> Sequence[T]:
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"""
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Return a copy with bit-reversed permutation. The permutation is an involution (inverts itself).
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The input and output are a sequence of generic type ``T`` objects.
"""
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return [sequence[reverse_bits(i, len(sequence))] for i in range(len(sequence))]
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```
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### BLS12-381 helpers
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#### `hash_to_bls_field`
```python
def hash_to_bls_field(data: bytes) -> BLSFieldElement:
"""
Hash ``data`` and convert the output to a BLS scalar field element.
The output is not uniform over the BLS field.
"""
hashed_data = hash(data)
return BLSFieldElement(int.from_bytes(hashed_data, ENDIANNESS) % BLS_MODULUS)
```
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#### `bytes_to_bls_field`
```python
def bytes_to_bls_field(b: Bytes32) -> BLSFieldElement:
"""
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Convert untrusted bytes to a trusted and validated BLS scalar field element.
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This function does not accept inputs greater than the BLS modulus.
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"""
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field_element = int.from_bytes(b, ENDIANNESS)
assert field_element < BLS_MODULUS
return BLSFieldElement(field_element)
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```
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#### `validate_kzg_g1`
```python
def validate_kzg_g1(b: Bytes48) -> None:
"""
Perform BLS validation required by the types `KZGProof` and `KZGCommitment` .
"""
if b == G1_POINT_AT_INFINITY:
return
assert bls.KeyValidate(b)
```
#### `bytes_to_kzg_commitment`
```python
def bytes_to_kzg_commitment(b: Bytes48) -> KZGCommitment:
"""
Convert untrusted bytes into a trusted and validated KZGCommitment.
"""
validate_kzg_g1(b)
return KZGCommitment(b)
```
#### `bytes_to_kzg_proof`
```python
def bytes_to_kzg_proof(b: Bytes48) -> KZGProof:
"""
Convert untrusted bytes into a trusted and validated KZGProof.
"""
validate_kzg_g1(b)
return KZGProof(b)
```
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#### `blob_to_polynomial`
```python
def blob_to_polynomial(blob: Blob) -> Polynomial:
"""
Convert a blob to list of BLS field scalars.
"""
polynomial = Polynomial()
for i in range(FIELD_ELEMENTS_PER_BLOB):
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value = bytes_to_bls_field(blob[i * BYTES_PER_FIELD_ELEMENT: (i + 1) * BYTES_PER_FIELD_ELEMENT])
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polynomial[i] = value
return polynomial
```
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#### `compute_challenges`
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```python
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def compute_challenges(polynomials: Sequence[Polynomial],
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commitments: Sequence[KZGCommitment]) -> Tuple[Sequence[BLSFieldElement], BLSFieldElement]:
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"""
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Return the Fiat-Shamir challenges required by the rest of the protocol.
The Fiat-Shamir logic works as per the following pseudocode:
hashed_data = hash(DOMAIN_SEPARATOR, polynomials, commitments)
r = hash(hashed_data, 0)
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r_powers = [1, r, r**2, r**3, ...]
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eval_challenge = hash(hashed_data, 1)
Then return `r_powers` and `eval_challenge` after converting them to BLS field elements.
The resulting field elements are not uniform over the BLS field.
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"""
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# Append the number of polynomials and the degree of each polynomial as a domain separator
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num_polynomials = int.to_bytes(len(polynomials), 8, ENDIANNESS)
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degree_poly = int.to_bytes(FIELD_ELEMENTS_PER_BLOB, 8, ENDIANNESS)
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data = FIAT_SHAMIR_PROTOCOL_DOMAIN + degree_poly + num_polynomials
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# Append each polynomial which is composed by field elements
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for poly in polynomials:
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for field_element in poly:
data += int.to_bytes(field_element, BYTES_PER_FIELD_ELEMENT, ENDIANNESS)
# Append serialized G1 points
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for commitment in commitments:
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data += commitment
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# Transcript has been prepared: time to create the challenges
hashed_data = hash(data)
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r = hash_to_bls_field(hashed_data + b'\x00')
r_powers = compute_powers(r, len(commitments))
eval_challenge = hash_to_bls_field(hashed_data + b'\x01')
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return r_powers, eval_challenge
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```
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#### `bls_modular_inverse`
```python
def bls_modular_inverse(x: BLSFieldElement) -> BLSFieldElement:
"""
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Compute the modular inverse of x
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i.e. return y such that x * y % BLS_MODULUS == 1 and return 0 for x == 0
"""
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return BLSFieldElement(pow(x, -1, BLS_MODULUS)) if x != 0 else BLSFieldElement(0)
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```
#### `div`
```python
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def div(x: BLSFieldElement, y: BLSFieldElement) -> BLSFieldElement:
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"""
Divide two field elements: ``x`` by `y` `.
"""
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return BLSFieldElement((int(x) * int(bls_modular_inverse(y))) % BLS_MODULUS)
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```
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#### `g1_lincomb`
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```python
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def g1_lincomb(points: Sequence[KZGCommitment], scalars: Sequence[BLSFieldElement]) -> KZGCommitment:
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"""
BLS multiscalar multiplication. This function can be optimized using Pippenger's algorithm and variants.
"""
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assert len(points) == len(scalars)
result = bls.Z1
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for x, a in zip(points, scalars):
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result = bls.add(result, bls.multiply(bls.bytes48_to_G1(x), a))
return KZGCommitment(bls.G1_to_bytes48(result))
```
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#### `poly_lincomb`
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```python
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def poly_lincomb(polys: Sequence[Polynomial],
scalars: Sequence[BLSFieldElement]) -> Polynomial:
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"""
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Given a list of ``polynomials``, interpret it as a 2D matrix and compute the linear combination
of each column with `scalars` : return the resulting polynomials.
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"""
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assert len(polys) == len(scalars)
result = [0] * FIELD_ELEMENTS_PER_BLOB
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for v, s in zip(polys, scalars):
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for i, x in enumerate(v):
result[i] = (result[i] + int(s) * int(x)) % BLS_MODULUS
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return Polynomial([BLSFieldElement(x) for x in result])
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```
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#### `compute_powers`
```python
def compute_powers(x: BLSFieldElement, n: uint64) -> Sequence[BLSFieldElement]:
"""
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Return ``x`` to power of [0, n-1], if n > 0. When n==0, an empty array is returned.
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"""
current_power = 1
powers = []
for _ in range(n):
powers.append(BLSFieldElement(current_power))
current_power = current_power * int(x) % BLS_MODULUS
return powers
```
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### Polynomials
#### `evaluate_polynomial_in_evaluation_form`
```python
def evaluate_polynomial_in_evaluation_form(polynomial: Polynomial,
z: BLSFieldElement) -> BLSFieldElement:
"""
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Evaluate a polynomial (in evaluation form) at an arbitrary point ``z`` that is not in the domain.
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Uses the barycentric formula:
f(z) = (z**WIDTH - 1) / WIDTH * sum_(i=0)^WIDTH (f(DOMAIN[i]) * DOMAIN[i]) / (z - DOMAIN[i])
"""
width = len(polynomial)
assert width == FIELD_ELEMENTS_PER_BLOB
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inverse_width = bls_modular_inverse(BLSFieldElement(width))
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roots_of_unity_brp = bit_reversal_permutation(ROOTS_OF_UNITY)
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# If we are asked to evaluate within the domain, we already know the answer
if z in roots_of_unity_brp:
eval_index = roots_of_unity_brp.index(z)
return BLSFieldElement(polynomial[eval_index])
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result = 0
for i in range(width):
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a = BLSFieldElement(int(polynomial[i]) * int(roots_of_unity_brp[i]) % BLS_MODULUS)
b = BLSFieldElement((int(BLS_MODULUS) + int(z) - int(roots_of_unity_brp[i])) % BLS_MODULUS)
result += int(div(a, b) % BLS_MODULUS)
result = result * int(pow(z, width, BLS_MODULUS) - 1) * int(inverse_width)
return BLSFieldElement(result % BLS_MODULUS)
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```
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### KZG
KZG core functions. These are also defined in EIP-4844 execution specs.
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#### `blob_to_kzg_commitment`
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```python
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def blob_to_kzg_commitment(blob: Blob) -> KZGCommitment:
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"""
Public method.
"""
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return g1_lincomb(bit_reversal_permutation(KZG_SETUP_LAGRANGE), blob_to_polynomial(blob))
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```
#### `verify_kzg_proof`
```python
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def verify_kzg_proof(commitment_bytes: Bytes48,
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z: Bytes32,
y: Bytes32,
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proof_bytes: Bytes48) -> bool:
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"""
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Verify KZG proof that ``p(z) == y`` where ``p(z)`` is the polynomial represented by ``polynomial_kzg``.
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Receives inputs as bytes.
Public method.
"""
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return verify_kzg_proof_impl(bytes_to_kzg_commitment(commitment_bytes),
bytes_to_bls_field(z),
bytes_to_bls_field(y),
bytes_to_kzg_proof(proof_bytes))
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```
#### `verify_kzg_proof_impl`
```python
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def verify_kzg_proof_impl(commitment: KZGCommitment,
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z: BLSFieldElement,
y: BLSFieldElement,
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proof: KZGProof) -> bool:
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"""
Verify KZG proof that ``p(z) == y`` where ``p(z)`` is the polynomial represented by ``polynomial_kzg``.
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"""
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# Verify: P - y = Q * (X - z)
X_minus_z = bls.add(bls.bytes96_to_G2(KZG_SETUP_G2[1]), bls.multiply(bls.G2, BLS_MODULUS - z))
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P_minus_y = bls.add(bls.bytes48_to_G1(commitment), bls.multiply(bls.G1, BLS_MODULUS - y))
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return bls.pairing_check([
[P_minus_y, bls.neg(bls.G2)],
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[bls.bytes48_to_G1(proof), X_minus_z]
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])
```
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#### `verify_kzg_proof_multi`
```python
def verify_kzg_proof_multi(commitments: Sequence[KZGCommitment],
zs: Sequence[BLSFieldElement],
ys: Sequence[BLSFieldElement],
proofs: Sequence[KZGProof]) -> bool:
"""
Verify multiple KZG proofs efficiently.
"""
assert len(commitments) == len(zs) == len(ys) == len(proofs)
# Compute a random challenge. Note that it does not have to be computed from a hash,
# r just has to be random.
degree_poly = int.to_bytes(FIELD_ELEMENTS_PER_BLOB, 8, ENDIANNESS)
num_commitments = int.to_bytes(len(commitments), 8, ENDIANNESS)
data = RANDOM_CHALLENGE_KZG_MULTI_DOMAIN + degree_poly + num_commitments
# Append each polynomial which is composed by field elements
for commitment, z, y, proof in zip(commitments, zs, ys, proofs):
data += commitment \
+ int.to_bytes(z, BYTES_PER_FIELD_ELEMENT, ENDIANNESS) \
+ int.to_bytes(y, BYTES_PER_FIELD_ELEMENT, ENDIANNESS) \
+ proof
hashed_data = hash(data)
r = hash_to_bls_field(hashed_data + b'\x00')
r_powers = compute_powers(r, len(commitments))
# Verify: e(sum r^i proof_i, [s]) ==
# e(sum r^i (commitment_i - [y_i]) + sum r^i z_i proof_i, [1])
proof_lincomb = g1_lincomb(proofs, r_powers)
proof_z_lincomb = g1_lincomb(proofs, [z * r_power for z, r_power in zip(zs, r_powers)])
C_minus_ys = [bls.G1_to_bytes48(bls.add(bls.bytes48_to_G1(commitment), bls.multiply(bls.G1, BLS_MODULUS - y)))
for commitment, y in zip(commitments, ys)]
C_minus_y_as_KZGCommitments = [KZGCommitment(x) for x in C_minus_ys]
C_minus_y_lincomb = g1_lincomb(C_minus_y_as_KZGCommitments, r_powers)
return bls.pairing_check([
[proof_lincomb, bls.neg(KZG_SETUP_G2[1])],
[bls.add(C_minus_y_lincomb, proof_z_lincomb), bls.G2]
])
```
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#### `compute_kzg_proof`
```python
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def compute_kzg_proof(blob: Blob, z: Bytes32) -> KZGProof:
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"""
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Compute KZG proof at point `z` for the polynomial represented by `blob` .
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Do this by computing the quotient polynomial in evaluation form: q(x) = (p(x) - p(z)) / (x - z).
Public method.
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"""
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polynomial = blob_to_polynomial(blob)
return compute_kzg_proof_impl(polynomial, bytes_to_bls_field(z))
```
#### `compute_kzg_proof_impl`
```python
def compute_kzg_proof_impl(polynomial: Polynomial, z: BLSFieldElement) -> KZGProof:
"""
Helper function for compute_kzg_proof() and compute_aggregate_kzg_proof().
"""
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y = evaluate_polynomial_in_evaluation_form(polynomial, z)
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polynomial_shifted = [BLSFieldElement((int(p) - int(y)) % BLS_MODULUS) for p in polynomial]
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# Make sure we won't divide by zero during division
assert z not in ROOTS_OF_UNITY
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denominator_poly = [BLSFieldElement((int(x) - int(z)) % BLS_MODULUS)
for x in bit_reversal_permutation(ROOTS_OF_UNITY)]
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# Calculate quotient polynomial by doing point-by-point division
quotient_polynomial = [div(a, b) for a, b in zip(polynomial_shifted, denominator_poly)]
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return KZGProof(g1_lincomb(bit_reversal_permutation(KZG_SETUP_LAGRANGE), quotient_polynomial))
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```
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#### `compute_aggregated_poly_and_commitment`
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```python
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def compute_aggregated_poly_and_commitment(
blobs: Sequence[Blob],
kzg_commitments: Sequence[KZGCommitment]) -> Tuple[Polynomial, KZGCommitment, BLSFieldElement]:
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"""
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Return (1) the aggregated polynomial, (2) the aggregated KZG commitment,
and (3) the polynomial evaluation random challenge.
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This function should also work with blobs == [] and kzg_commitments == []
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"""
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assert len(blobs) == len(kzg_commitments)
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# Convert blobs to polynomials
polynomials = [blob_to_polynomial(blob) for blob in blobs]
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# Generate random linear combination and evaluation challenges
r_powers, evaluation_challenge = compute_challenges(polynomials, kzg_commitments)
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# Create aggregated polynomial in evaluation form
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aggregated_poly = poly_lincomb(polynomials, r_powers)
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# Compute commitment to aggregated polynomial
aggregated_poly_commitment = KZGCommitment(g1_lincomb(kzg_commitments, r_powers))
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return aggregated_poly, aggregated_poly_commitment, evaluation_challenge
```
#### `compute_aggregate_kzg_proof`
```python
def compute_aggregate_kzg_proof(blobs: Sequence[Blob]) -> KZGProof:
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"""
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Given a list of blobs, return the aggregated KZG proof that is used to verify them against their commitments.
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Public method.
"""
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commitments = [blob_to_kzg_commitment(blob) for blob in blobs]
aggregated_poly, aggregated_poly_commitment, evaluation_challenge = compute_aggregated_poly_and_commitment(
blobs,
commitments
)
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return compute_kzg_proof_impl(aggregated_poly, evaluation_challenge)
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```
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#### `verify_aggregate_kzg_proof_aggregation`
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```python
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def verify_aggregate_kzg_proof_aggregation(blobs: Sequence[Blob],
commitments_bytes: Sequence[Bytes48]) \
-> Tuple[KZGCommitment, BLSFieldElement, BLSFieldElement]:
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"""
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Given a list of blobs and an aggregated KZG proof, verify that they correspond to the provided commitments.
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Public method.
"""
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commitments = [bytes_to_kzg_commitment(c) for c in commitments_bytes]
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aggregated_poly, aggregated_poly_commitment, evaluation_challenge = compute_aggregated_poly_and_commitment(
blobs,
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commitments
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)
# Evaluate aggregated polynomial at `evaluation_challenge` (evaluation function checks for div-by-zero)
y = evaluate_polynomial_in_evaluation_form(aggregated_poly, evaluation_challenge)
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return (aggregated_poly_commitment, evaluation_challenge, y)
```
#### `verify_aggregate_kzg_proof`
```python
def verify_aggregate_kzg_proof(blobs: Sequence[Blob],
commitments_bytes: Sequence[Bytes48],
aggregated_proof_bytes: Bytes48) -> bool:
"""
Given a list of blobs and an aggregated KZG proof, verify that they correspond to the provided commitments.
Public method.
"""
aggregated_poly_commitment, evaluation_challenge, y = \
verify_aggregate_kzg_proof_aggregation(blobs, commitments_bytes)
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aggregated_proof = bytes_to_kzg_proof(aggregated_proof_bytes)
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return verify_kzg_proof_impl(aggregated_poly_commitment, evaluation_challenge, y, aggregated_proof)
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```
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#### `verify_aggregate_kzg_proof_multi`
```python
def verify_aggregate_kzg_proof_multi(list_blobs: Sequence[Sequence[Blob]],
list_commitments_bytes: Sequence[Sequence[Bytes48]],
list_aggregated_proof_bytes: Sequence[Bytes48]) -> bool:
"""
Given a list of blobs and an aggregated KZG proof, verify that they correspond to the provided commitments.
Public method.
"""
aggregated_poly_commitments, evaluation_challenges, ys = [], [], []
for blobs, commitments_bytes in zip(list_blobs, list_commitments_bytes):
aggregated_poly_commitment, evaluation_challenge, y = \
verify_aggregate_kzg_proof_aggregation(blobs, commitments_bytes)
aggregated_poly_commitments.append(aggregated_poly_commitment)
evaluation_challenges.append(evaluation_challenge)
ys.append(y)
list_aggregated_proof = [bytes_to_kzg_proof(proof) for proof in list_aggregated_proof_bytes]
return verify_kzg_proof_multi(aggregated_poly_commitments, evaluation_challenges, ys, list_aggregated_proof)
```