How do I use Nethereum to extract Chain Id from a raw transaction?

Question

Here is the F# code I have so far -

let parseChainId rawTransaction =
    let tx = TransactionFactory.CreateTransaction rawTransaction
    let chainIdWithAddend = (IntTypeDecoder().DecodeBigInteger tx.Signature.V - BigInteger 35) / BigInteger 2
    let addend = ???
    let chainId = chainIdWithAddend - addend
    chainId

However, I'm sure this code is incomplete in some cases because I can't figure out how to determine the {0, 1} addend as specified in https://github.com/ethereum/EIPs/blob/master/EIPS/eip-155.md

If I could figure out how to determine the addend, then I believe the above algorithm for chain id extraction would be complete.

Do you have it working in C#? – Jim Foye Jun 17 '21 at 17:35 — Jim Foye, Jun 17 '21 at 17:35

Brian Berns · Answer 1 · 2021-06-18T13:23:36.887

I don't know anything about Ethereum (or cryptology in general), but from reading the linked document, I don't think your logic is quite right. What you're calling the "addend" is defined as a parity value, which is always either 0 or 1. This is why the spec says that v = CHAIN_ID * 2 + 35 or v = CHAIN_ID * 2 + 36, which means that CHAIN_ID = (v - {35,36}) / 2.

This is a complete guess, but I think you can determine the parity simply by knowing whether v is even or odd. If it's odd, you want to subtract 35, and if it's even, you want to subtract 36, so that in either case you can divide the result exactly by 2.

If this is true, we can rewrite your code like this:

let v = IntTypeDecoder().DecodeBigInteger tx.Signature.V
let parity = (if v % 2 = 0 then 1 else 0)
let chainId = (v - BigInteger (35 + parity)) / BigInteger 2   // no remainder possible

I think you could also make the parity implicit by relying on F#'s integer division to discard the remainder:

let v = IntTypeDecoder().DecodeBigInteger tx.Signature.V
let chainId = (v - BigInteger 35) / BigInteger 2   // ignore remainder

Be careful about off-by-one errors, though. I haven't tested (or even tried to compile) any of this code.

Another caveat: All of the above only seems to apply in certain conditions, such as when nine elements have been hashed, instead of six. I assume you've already accounted for that.

Thank you for your time, I have up-voted this! While the code is working on my end and the explanation seems accurate, I can't yet mark it as the answer until someone with a bit more expertise can confirm this for sure. — Bryan Edds, Jun 19 '21 at 00:50

How do I use Nethereum to extract Chain Id from a raw transaction?

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