bytes32 same length hex strings cost different gas why?

Question

I can not understand the difference in gas for the following problem. I generated a sha256 hash value and the output i chose to be lowercase hexadecimal. So the output value is hexadecimal string of 64 characters.Then i saved this value to a bytes32 variable inside a smart contract. The smart contract's code is:

pragma solidity ^0.4.23;

contract Project {
    bytes32 public value;

    function addValue(bytes32 val) public {
        value = val;
    }
}

Using remix, I noticed that the operation to add a value cost 43629 for some values and other 43693. So the difference is 64 gas. Can anyone help me understand why this is happening? I tested many values and the difference was 64 gas, was it random? I also tested to store it as string. Then the cost was constant for all inputs, maybe because the values have the same length as i said the inputs values have 64 characters.

Answer 1

From the yellow paper:

Gtxdatazero 4 Paid for every zero byte of data or code for a transaction.

Gtxdatanonzero 68 Paid for every non-zero byte of data or code for a transaction

So if you replace a non-zero with a zero in your transaction data, you'll reduce the cost by 64 (68-4).

Note that consecutive 0s don't necessarily matter, just if they represent a full byte. It's easier to see if you separate bytes: d5 bf 2c b4 91 dd 63 f8 00 37 3f 16 ab 0f c4 86 ff e0 40 92 35 87 c0 0a dc ea d3 dc 4a 45 44 b6. So the second value you mentioned in your comment has exactly one zero byte.