Recover Ethereum legacy transaction public key in Elixir

ethereum_transaction_revocer.livemd

Recover Ethereum transaction public key

Mix.install([:ethers, :ex_secp256k1])

Procedure for legacy transactions

raw_transaction =
  "0xf86b35850861c4680082520894dec50599773e005112663e4e35528460319c5507875f53e91d066f0b8025a07bbf52ea4ee9b12ca3f1e5d63e93eee550d111225457e0b59d7698324b04993aa06ab577a1f38941cc08d7345d8aee9fc62024bca37da2115d8420baa7e3fcc300"

"0xf86b35850861c4680082520894dec50599773e005112663e4e35528460319c5507875f53e91d066f0b8025a07bbf52ea4ee9b12ca3f1e5d63e93eee550d111225457e0b59d7698324b04993aa06ab577a1f38941cc08d7345d8aee9fc62024bca37da2115d8420baa7e3fcc300"

decoded_raw_transaction = Ethers.Utils.hex_decode!(raw_transaction)

<<248, 107, 53, 133, 8, 97, 196, 104, 0, 130, 82, 8, 148, 222, 197, 5, 153, 119, 62, 0, 81, 18, 102,
  62, 78, 53, 82, 132, 96, 49, 156, 85, 7, 135, 95, 83, 233, 29, 6, 111, 11, 128, 37, 160, 123, 191,
  82, 234, 78, 233, ...>>

This is the structure of legacy ethereum transactions.

[nonce, gas_price, gas, to, value, data, v, r, s] = ExRLP.decode(decoded_raw_transaction)

[
  "5",
  <<8, 97, 196, 104, 0>>,
  "R\b",
  <<222, 197, 5, 153, 119, 62, 0, 81, 18, 102, 62, 78, 53, 82, 132, 96, 49, 156, 85, 7>>,
  <<95, 83, 233, 29, 6, 111, 11>>,
  "",
  "%",
  <<123, 191, 82, 234, 78, 233, 177, 44, 163, 241, 229, 214, 62, 147, 238, 229, 80, 209, 17, 34, 84,
    87, 224, 181, 157, 118, 152, 50, 75, 4, 153, 58>>,
  <<106, 181, 119, 161, 243, 137, 65, 204, 8, 215, 52, 93, 138, 238, 159, 198, 32, 36, 188, 163,
    125, 162, 17, 93, 132, 32, 186, 167, 227, 252, 195, 0>>
]

Re-construct transaction body and calculate recovery_id based on EIP-155

v_int = :binary.decode_unsigned(v)

{tx_body, parity} =
  if v_int > 27 do
    m = v_int - 35
    parity = rem(m, 2)
    chain_id = div(m - parity, 2)

    {[nonce, gas_price, gas, to, value, data, chain_id, 0, 0], parity}
  else
    {[nonce, gas_price, gas, to, value, data], v_int - 27}
  end

{[
   "5",
   <<8, 97, 196, 104, 0>>,
   "R\b",
   <<222, 197, 5, 153, 119, 62, 0, 81, 18, 102, 62, 78, 53, 82, 132, 96, 49, 156, 85, 7>>,
   <<95, 83, 233, 29, 6, 111, 11>>,
   "",
   1,
   0,
   0
 ], 0}

RLP encode and Hash the body

tx_body_hash = ExRLP.encode(tx_body) |> ExKeccak.hash_256()

<<152, 121, 142, 245, 217, 136, 37, 192, 238, 92, 189, 151, 145, 126, 37, 43, 195, 38, 80, 60, 92,
  47, 60, 44, 197, 2, 131, 195, 179, 173, 212, 165>>

Recover public key

{:ok, pub_key} = ExSecp256k1.recover(tx_body_hash, r, s, parity)

{:ok,
 <<4, 151, 206, 146, 242, 45, 215, 166, 141, 181, 30, 205, 84, 73, 151, 234, 173, 231, 63, 182, 120,
   35, 117, 51, 226, 227, 195, 92, 141, 221, 98, 121, 152, 231, 183, 11, 129, 58, 154, 14, 48, 102,
   215, 221, 133, 5, 87, 44, ...>>}

Calculate address of a public key

Ethers.Utils.public_key_to_address(pub_key)

"0xb412E9D7c64638bDD554EcaDf2520633147Eaa44"

i think the if should be > 28 as i just hit that case in testing with a type:0 transaction signed in ethers.js . >27 produces a -7 in that case..

could likely be more like >=35 as there's no case we want a signed int there

uuugh. these fkn magic number holders from btc.. silly.. anyway since we also subtract 27 should likely put this in a cond where if v >= 35 do w/chain_id etc.. if >=27 do w/o and :else throw maybe? or just return with the v_int being whatever it is...

i'll likely assign some const-type things like @155_magic_number 35 and @legacy_magic_number 27

@alisinabh

looks to be working however ++++

some interesting points to add along with the above for the upcoming PR:
ethersjs may send 0x01<sig> or 0x02<sig> in addition to the "type 0" 0x<sig> . in the case of the first two we'll need to remove those prefixes before calling hex decode on them. not terribly hard but something to note.

more interesting is that in the "type 1" case there are more fields in the initial RLP decode so knowing what to expect when assigning [nonce, gas...] or if its [nonce, maxFee...] will be critical. I'm fairly certain we can use the detected type to dictate the expected array but it may be worth introducing them in separate PRS.

i.e PR for "legacy" (type 0). then 1 (1559) and 2 etc... there's some unknowns in there for me (access lists etc)