How to encode empty transaction fields?

Question

I want to RLP-encode a(n unsigned) transaction. Some fields of the transaction can be empty/null/zero. I want to know how they are encoded, though.

Let's assume the following transaction fields:

signer_nonce = 0
gas_price = 0
gas_limit = 21000
destination = nil
amount = 0
payload = nil

Nonce, gas price, limit, and amount are easily encoded using any RLP-encoder and a value of 0x00 is perfectly fine.

But how do I encode empty fields if I don't have any data or any recipient address?

Eth::Util.bin_to_hex(RLP.encode(nil))
/home/user/.gem/ruby/3.0.0/gems/rlp-0.7.3/lib/rlp/sedes.rb:31:in `infer': Did not find sedes handling type NilClass (TypeError)
    from /home/user/.gem/ruby/3.0.0/gems/rlp-0.7.3/lib/rlp/encode.rb:51:in `encode'
    from (irb):14:in `<main>'
    from /usr/lib/ruby/gems/3.0.0/gems/irb-1.3.6/exe/irb:11:in `<top (required)>'
    from /bin/irb:23:in `load'
    from /bin/irb:23:in `<main>'

I cannot set the address to 0x00 as this would be a valid address afaik. Should I use the empty string byte 0x80? How would I distinguish and empty string from the zero byte 0x80?

Eth::Util.bin_to_hex(RLP.encode(""))
=> "80"
Eth::Util.bin_to_hex(RLP.encode(0))
=> "80"

How to encode empty transaction fields such as empty data or no recipient address?

Answer 1

All of these field have to be provided and should not be nil. There are generally 2 different types when encoding a transaction with RLP: 256-bit Number and a Bytes.

So if we have an empty data we would encode it as an empty bytes (e.g. 0x). In this case an empty string and an empty bytes are the same, as a string would be converted to bytes before RLP encoding.

For the address it is a little bit more tricky. It is defined as a Bytes that has a length of 20. Unless you have a creation transaction then it is an empty bytes.

You can find an example how this is implemented in the Kethereum repositors:

• RLP Encoding
• Type Mapping

You can find the more formal definition in the Ethereum Yellow Paper at the beginning of page 5.

A list of all fields that should be encoded in a Transaction can for example be found in EIP-1559 for the latest format.