How an array is "tightly packed" in Solidity

Question

I am trying to create a hash (sha256) of an array but I don't understand how it works.

for example :

pragma solidity 0.4.11;
contract test { bytes32 public hash; bytes32 public hash2; bytes32 public hash3; bytes32 public hash4;
function test () {
    address[2] memory _add;
    _add[0] = 0xca35b7d915458ef540ade6068dfe2f44e8fa733c;
    _add[1] = 0xca35b7d915458ef540ade6068dfe2f44e8fa733c;
    hash = sha256(_add[0],_add[1]);
    hash2 = sha256(_add);

    _add[0] = 0;
    _add[1] = 0;
    hash3 = sha256(_add[0],_add[1]);
    hash4 = sha256(_add);
}

hash is different from hash2 and hash3 is different from hash4... why?

The "tightly packed" data should result in the same hash ... no?

Best,

Answer 1

"Tightly packed" no longer applies when dealing with an array. You must zerofill to 32 bytes.

Unfortunately, I couldn't find the docs to validate that, so all I can do is give an example to show it works.

Side Note: Are you sure you want sha256 instead of keccak256, which is used most other places in Ethereum?

Demo

I'll demonstrate with sha3 (the same as keccak256), but the concept is the same.

Baseline

In Remix, you'll find that:

sha3([0xca35b7d915458ef540ade6068dfe2f44e8fa733c, 0xca35b7d915458ef540ade6068dfe2f44e8fa733c]);

gives the result: 0x77e5189111eb6557e8a637b27ef8fbb15bc61d61c2f00cc48878f3a296e5e0ca

Rebuild, with Web3.py

from web3 import Web3, IPCProvider

web3 = Web3(IPCProvider())

addr = 'ca35b7d915458ef540ade6068dfe2f44e8fa733c'
zerofilled = addr.zfill(64)  # 64 hex chars == 32 bytes
two_packed = zerofilled * 2

sha = web3.sha3('0x' + two_packed, encoding='hex')
assert sha == '0x77e5189111eb6557e8a637b27ef8fbb15bc61d61c2f00cc48878f3a296e5e0ca'

For what it's worth, you can also get the result with:

web3.soliditySha3(
  ['address[2]'],
  [[
    '0xca35b7d915458ef540ade6068dfe2f44e8fa733c',
    '0xca35b7d915458ef540ade6068dfe2f44e8fa733c',
  ]],
)

Answer 2

Actually, the issue was that all the elements of the array are converted to Bytes32 (adding a padding of zeros)...

So basically :

data before hash = ca35b7d915458ef540ade6068dfe2f44e8fa733cca35b7d915458ef540ade6068dfe2f44e8fa733c

data before hash2 = 000000000000000000000000ca35b7d915458ef540ade6068dfe2f44e8fa733c000000000000000000000000ca35b7d915458ef540ade6068dfe2f44e8fa733c

data before hash3 = 00000000000000000000000000000000000000000000000000000000000000000000000000000000

data before hash4 = 00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000