eth2.0-specs/specs/deneb/polynomial-commitments.md

# Deneb -- Polynomial Commitments

## Table of contents

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- [Introduction](#introduction)
- [Custom types](#custom-types)
- [Constants](#constants)
- [Preset](#preset)
  - [Blob](#blob)
  - [Crypto](#crypto)
  - [Trusted setup](#trusted-setup)
- [Helper functions](#helper-functions)
  - [Bit-reversal permutation](#bit-reversal-permutation)
    - [`is_power_of_two`](#is_power_of_two)
    - [`reverse_bits`](#reverse_bits)
    - [`bit_reversal_permutation`](#bit_reversal_permutation)
  - [BLS12-381 helpers](#bls12-381-helpers)
    - [`hash_to_bls_field`](#hash_to_bls_field)
    - [`bytes_to_bls_field`](#bytes_to_bls_field)
    - [`validate_kzg_g1`](#validate_kzg_g1)
    - [`bytes_to_kzg_commitment`](#bytes_to_kzg_commitment)
    - [`bytes_to_kzg_proof`](#bytes_to_kzg_proof)
    - [`blob_to_polynomial`](#blob_to_polynomial)
    - [`compute_challenge`](#compute_challenge)
    - [`bls_modular_inverse`](#bls_modular_inverse)
    - [`div`](#div)
    - [`g1_lincomb`](#g1_lincomb)
    - [`compute_powers`](#compute_powers)
  - [Polynomials](#polynomials)
    - [`evaluate_polynomial_in_evaluation_form`](#evaluate_polynomial_in_evaluation_form)
  - [KZG](#kzg)
    - [`blob_to_kzg_commitment`](#blob_to_kzg_commitment)
    - [`verify_kzg_proof`](#verify_kzg_proof)
    - [`verify_kzg_proof_impl`](#verify_kzg_proof_impl)
    - [`verify_kzg_proof_multi`](#verify_kzg_proof_multi)
    - [`compute_kzg_proof`](#compute_kzg_proof)
    - [`compute_kzg_proof_impl`](#compute_kzg_proof_impl)
    - [`compute_blob_kzg_proof`](#compute_blob_kzg_proof)
    - [`verify_blob_kzg_proof`](#verify_blob_kzg_proof)
    - [`verify_blob_kzg_proof_multi`](#verify_blob_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 Deneb specification. The implementations are not optimized for performance, but readability. All practical implementations should optimize the polynomial operations.

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.

Public functions MUST accept raw bytes as input and perform the required cryptographic normalization before invoking any internal functions.

## Custom types

| Name | SSZ equivalent | Description |
| - | - | - |
| `G1Point` | `Bytes48` | |
| `G2Point` | `Bytes96` | |
| `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 |
| `KZGProof` | `Bytes48` | Same as for `KZGCommitment` |
| `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 |

## Constants

| Name | Value | Notes |
| - | - | - |
| `BLS_MODULUS` | `52435875175126190479447740508185965837690552500527637822603658699938581184513` | Scalar field modulus of BLS12-381 |
| `BYTES_PER_FIELD_ELEMENT` | `uint64(32)` | Bytes used to encode a BLS scalar field element |
| `G1_POINT_AT_INFINITY` | `Bytes48(b'\xc0' + b'\x00' * 47)` | Serialized form of the point at infinity on the G1 group |


## Preset

### Blob

| Name | Value |
| - | - |
| `FIELD_ELEMENTS_PER_BLOB` | `uint64(4096)` |
| `FIAT_SHAMIR_PROTOCOL_DOMAIN` | `b'FSBLOBVERIFY_V1_'` |
| `RANDOM_CHALLENGE_KZG_MULTI_DOMAIN` | `b'RCKZGMULTI___V1_'` |

### 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 |

### 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 |
| - | - |
| `KZG_SETUP_G2_LENGTH` | `65` |
| `KZG_SETUP_G1` | `Vector[G1Point, FIELD_ELEMENTS_PER_BLOB]`, contents TBD |
| `KZG_SETUP_G2` | `Vector[G2Point, KZG_SETUP_G2_LENGTH]`, contents TBD |
| `KZG_SETUP_LAGRANGE` | `Vector[KZGCommitment, FIELD_ELEMENTS_PER_BLOB]`, contents TBD |

## Helper functions

### 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:
    """
    Reverse the bit order of an integer ``n``.
    """
    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
def bit_reversal_permutation(sequence: Sequence[T]) -> Sequence[T]:
    """
    Return a copy with bit-reversed permutation. The permutation is an involution (inverts itself).

    The input and output are a sequence of generic type ``T`` objects.
    """
    return [sequence[reverse_bits(i, len(sequence))] for i in range(len(sequence))]
```

### BLS12-381 helpers

#### `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)
```

#### `bytes_to_bls_field`

```python
def bytes_to_bls_field(b: Bytes32) -> BLSFieldElement:
    """
    Convert untrusted bytes to a trusted and validated BLS scalar field element.
    This function does not accept inputs greater than the BLS modulus.
    """
    field_element = int.from_bytes(b, ENDIANNESS)
    assert field_element < BLS_MODULUS
    return BLSFieldElement(field_element)
```


#### `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)
```

#### `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):
        value = bytes_to_bls_field(blob[i * BYTES_PER_FIELD_ELEMENT: (i + 1) * BYTES_PER_FIELD_ELEMENT])
        polynomial[i] = value
    return polynomial
```

#### `compute_challenge`

```python
def compute_challenge(blob: Blob,
                      commitment: KZGCommitment) -> BLSFieldElement:
    """
    Return the Fiat-Shamir challenge required by the rest of the protocol.
    The Fiat-Shamir logic works as per the following pseudocode:
    """

    # Append the number of polynomials and the degree of each polynomial as a domain separator
    num_polynomials = int.to_bytes(1, 8, ENDIANNESS)
    degree_poly = int.to_bytes(FIELD_ELEMENTS_PER_BLOB, 8, ENDIANNESS)
    data = FIAT_SHAMIR_PROTOCOL_DOMAIN + degree_poly + num_polynomials

    # Append each polynomial which is composed by field elements
    data += blob

    # Append serialized G1 points
    data += commitment

    # Transcript has been prepared: time to create the challenges
    return hash_to_bls_field(data)
```

#### `bls_modular_inverse`

```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
    """
    return BLSFieldElement(pow(x, -1, BLS_MODULUS)) if x != 0 else BLSFieldElement(0)
```

#### `div`

```python
def div(x: BLSFieldElement, y: BLSFieldElement) -> BLSFieldElement:
    """
    Divide two field elements: ``x`` by `y``.
    """
    return BLSFieldElement((int(x) * int(bls_modular_inverse(y))) % BLS_MODULUS)
```

#### `g1_lincomb`

```python
def g1_lincomb(points: Sequence[KZGCommitment], scalars: Sequence[BLSFieldElement]) -> KZGCommitment:
    """
    BLS multiscalar multiplication. This function can be optimized using Pippenger's algorithm and variants.
    """
    assert len(points) == len(scalars)
    result = bls.Z1
    for x, a in zip(points, scalars):
        result = bls.add(result, bls.multiply(bls.bytes48_to_G1(x), a))
    return KZGCommitment(bls.G1_to_bytes48(result))
```

#### `compute_powers`

```python
def compute_powers(x: BLSFieldElement, n: uint64) -> Sequence[BLSFieldElement]:
    """
    Return ``x`` to power of [0, n-1], if n > 0. When n==0, an empty array is returned.
    """
    current_power = 1
    powers = []
    for _ in range(n):
        powers.append(BLSFieldElement(current_power))
        current_power = current_power * int(x) % BLS_MODULUS
    return powers
```


### Polynomials

#### `evaluate_polynomial_in_evaluation_form`

```python
def evaluate_polynomial_in_evaluation_form(polynomial: Polynomial,
                                           z: BLSFieldElement) -> BLSFieldElement:
    """
    Evaluate a polynomial (in evaluation form) at an arbitrary point ``z`` that is not in the domain.
    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
    inverse_width = bls_modular_inverse(BLSFieldElement(width))

    roots_of_unity_brp = bit_reversal_permutation(ROOTS_OF_UNITY)

    # 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])

    result = 0
    for i in range(width):
        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)
```

### KZG

KZG core functions. These are also defined in Deneb execution specs.

#### `blob_to_kzg_commitment`

```python
def blob_to_kzg_commitment(blob: Blob) -> KZGCommitment:
    """
    Public method.
    """
    return g1_lincomb(bit_reversal_permutation(KZG_SETUP_LAGRANGE), blob_to_polynomial(blob))
```

#### `verify_kzg_proof`

```python
def verify_kzg_proof(commitment_bytes: Bytes48,
                     z: Bytes32,
                     y: Bytes32,
                     proof_bytes: Bytes48) -> bool:
    """
    Verify KZG proof that ``p(z) == y`` where ``p(z)`` is the polynomial represented by ``polynomial_kzg``.
    Receives inputs as bytes.
    Public method.
    """
    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))
```


#### `verify_kzg_proof_impl`

```python
def verify_kzg_proof_impl(commitment: KZGCommitment,
                          z: BLSFieldElement,
                          y: BLSFieldElement,
                          proof: KZGProof) -> bool:
    """
    Verify KZG proof that ``p(z) == y`` where ``p(z)`` is the polynomial represented by ``polynomial_kzg``.
    """
    # 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))
    P_minus_y = bls.add(bls.bytes48_to_G1(commitment), bls.multiply(bls.G1, BLS_MODULUS - y))
    return bls.pairing_check([
        [P_minus_y, bls.neg(bls.G2)],
        [bls.bytes48_to_G1(proof), X_minus_z]
    ])
```

#### `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 all inputs to the transcript before we hash
    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

    r = hash_to_bls_field(data)
    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.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(bls.G1_to_bytes48(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]
    ])
```

#### `compute_kzg_proof`

```python
def compute_kzg_proof(blob: Blob, z: Bytes32) -> KZGProof:
    """
    Compute KZG proof at point `z` for the polynomial represented by `blob`.
    Do this by computing the quotient polynomial in evaluation form: q(x) = (p(x) - p(z)) / (x - z).
    Public method.
    """
    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().
    """
    y = evaluate_polynomial_in_evaluation_form(polynomial, z)
    polynomial_shifted = [BLSFieldElement((int(p) - int(y)) % BLS_MODULUS) for p in polynomial]

    # Make sure we won't divide by zero during division
    assert z not in ROOTS_OF_UNITY
    denominator_poly = [BLSFieldElement((int(x) - int(z)) % BLS_MODULUS)
                        for x in bit_reversal_permutation(ROOTS_OF_UNITY)]

    # Calculate quotient polynomial by doing point-by-point division
    quotient_polynomial = [div(a, b) for a, b in zip(polynomial_shifted, denominator_poly)]
    return KZGProof(g1_lincomb(bit_reversal_permutation(KZG_SETUP_LAGRANGE), quotient_polynomial))
```

#### `compute_blob_kzg_proof`

```python
def compute_blob_kzg_proof(blob: Blob) -> KZGProof:
    """
    Given a blob, return the KZG proof that is used to verify it against the commitment.
    Public method.
    """
    commitment = blob_to_kzg_commitment(blob)
    polynomial = blob_to_polynomial(blob)
    evaluation_challenge = compute_challenge(blob, commitment)
    return compute_kzg_proof_impl(polynomial, evaluation_challenge)
```

#### `verify_blob_kzg_proof`

```python
def verify_blob_kzg_proof(blob: Blob,
                          commitment_bytes: Bytes48,
                          proof_bytes: Bytes48) -> bool:
    """
    Given a blob and a KZG proof, verify that the blob data corresponds to the provided commitment.

    Public method.
    """
    commitment = bytes_to_kzg_commitment(commitment_bytes)

    polynomial = blob_to_polynomial(blob)
    evaluation_challenge = compute_challenge(blob, commitment)

    # Evaluate polynomial at `evaluation_challenge`
    y = evaluate_polynomial_in_evaluation_form(polynomial, evaluation_challenge)

    # Verify proof
    proof = bytes_to_kzg_proof(proof_bytes)
    return verify_kzg_proof_impl(commitment, evaluation_challenge, y, proof)
```

#### `verify_blob_kzg_proof_multi`

```python
def verify_blob_kzg_proof_multi(blobs: Sequence[Blob],
                                commitments_bytes: Sequence[Bytes48],
                                proofs_bytes: Sequence[Bytes48]) -> bool:
    """
    Given a list of blobs and blob KZG proofs, verify that they correspond to the provided commitments.

    Public method.
    """

    assert len(blobs) == len(commitments_bytes) == len(proofs_bytes)
    
    commitments, evaluation_challenges, ys, proofs = [], [], [], []
    for blob, commitment_bytes, proof_bytes in zip(blobs, commitments_bytes, proofs_bytes):
        commitment = bytes_to_kzg_commitment(commitment_bytes)
        commitments.append(commitment)
        polynomial = blob_to_polynomial(blob)
        evaluation_challenge = compute_challenge(blob, commitment)
        evaluation_challenges.append(evaluation_challenge)
        ys.append(evaluate_polynomial_in_evaluation_form(polynomial, evaluation_challenge))
        proofs.append(bytes_to_kzg_proof(proof_bytes))

    return verify_kzg_proof_multi(commitments, evaluation_challenges, ys, proofs)
```