gistfile1.txt

How the blob_versioned_hashes offset is calculated.

class SignedBlobTransaction(Container):
    message: BlobTransaction
    signature: ECDSASignature

class BlobTransaction(Container):
    chain_id: uint256
    nonce: uint64
    max_priority_fee_per_gas: uint256
    max_fee_per_gas: uint256
    gas: uint64
    to: Union[None, Address] # Address = Bytes20
    value: uint256
    data: ByteList[MAX_CALLDATA_SIZE]
    access_list: List[AccessTuple, MAX_ACCESS_LIST_SIZE]
    max_fee_per_data_gas: uint256  # new in PR 5707, a.k.a. fee market change of EIP-4844
    blob_versioned_hashes: List[VersionedHash, MAX_VERSIONED_HASHES_LIST_SIZE]

class AccessTuple(Container):
    address: Address # Bytes20
    storage_keys: List[Hash, MAX_ACCESS_LIST_STORAGE_KEYS]

class ECDSASignature(Container):
    y_parity: boolean
    r: uint256
    s: uint256


path to get blob_versioned_hashes:
message > blob_versioned_hashes


Format in fixed and dynamic ssz sections:


message: 4 bytes - offset to A
signature: 1 + 32 + 32 = 65 bytes

A:
chain_id: 32 bytes
nonce: 8 bytes
priority_fee_per_gas: 32 bytes
max_basefee_per_gas: 32 bytes
gas: 8 bytes
to: 4 bytes - offset to B (relative to A)
value: 32 bytes
data: 4 bytes - offset to C (relative to A)
access_list: 4 bytes - offset to D (relative to A)
max_fee_per_data_gas: 32 bytes
blob_versioned_hashes: 4 bytes - offset to E (relative to A)

B:
to content: 0 or 20 bytes
C:
data content: (dynamic) bytes
D:
acces_list content: (dynamic) bytes
E:
repeated 32 bytes until end of scope

To get to hashes in E directly:
1. we don't really need to read the offset to A, it's always at 4+65 because it's the first offset
2. read offset to E, which is at position A+(fixed data up to offset) == (4+65)+(32+8+32+32+8+4+32+4+32+4) == 4 + 65 + 188 = 257
    - parse byte x = input[257:257+4] as a uint32 to get the offset integer
3. iterate from x to end of data, in steps of 32, to get all hashes

couple possible errors:
L38: signature should be 65 bytes
L64: 96->65 , and also last 32+4 -> 4+32 to make it match the layout. (Having it swapped confused me for a bit...)

@roberto-bayardo
96 -> 65: nice catch, the ECDSASignature is 1+32+32=65. In the consensus specs we use the offset, so the size didn't affect things there.

32+4 -> 4+32

👍