Padding bytes were not empty : while decoding txn input data

Question

I'm decoding the raw input data of an eth transaction. I have the function signature to decode the input with.

I'm using the eth_abi library for python.

res = eth_abi.decode(['address', 'address'], eth_utils.decode_hex(input))

eth_utils.decode_hex() converts hex to bytes.

this throws:

Padding bytes were not empty: b'\x1c\xffy\xcd\x00\x00\x00\x00\x00\x00\x00\x00'

Some examples:

paramList= ['address', 'bool']
input= "0xa9059cbb000000000000000000000000b25cd920f3cb29e9de2a1f55577a3fe539d187a70000000000000000000000000000000000000000000000000000000159b4fa00"

paramList= ['uint256']
input= "0x2e1a7d4d0000000000000000000000000000000000000000000000000214e8348c4f0000"

Can anyone help me out??

Answer 1

There are two problems:

1. You must skip the 4-byte function signature in your input data before calling eth_abi.decode
2. You used the wrong paramList in one of your example.

To decode it, you can use:

eth_abi.decode(paramList, eth_utils.decode_hex(input)[4:])

The first example you gave should be decoded as transfer(address,uint256) because the second argument is not "bool" encoded, See the function signature here

paramList = ['address', 'uint256']
input= "0xa9059cbb000000000000000000000000b25cd920f3cb29e9de2a1f55577a3fe539d187a70000000000000000000000000000000000000000000000000000000159b4fa00"
eth_abi.decode(paramList, eth_utils.decode_hex(input)[4:])

Output: ('0xb25cd920f3cb29e9de2a1f55577a3fe539d187a7', 5800000000)

The second example is ok to be decoded using the above:

eth_abi.decode(['uint256'], eth_utils.decode_hex("0x2e1a7d4d0000000000000000000000000000000000000000000000000214e8348c4f0000")[4:])

Output : 150000000000000000

Read more on ABI encoding and why paramList is wrong and you should skip the first 4-byte: Solidity Documentation on ABI encoding

Answer 2

'padding bytes were not empty' means the hex value you are passing has more than one value in it.

log out the input you are feeding eth_utils.decode_hex(), and slice the signature (or other value) from the front of the hex literal. Append 0x to the front of the resulting hex, if necessary, and try again.