I am confused about miners being able to modify timestamp data within the header. I thought the only modifiable piece of data within the header was the nonce. I understand changing the Merkle root, through different transactions being included in a trial block, but how are timestamps modifiable?
It is impossible to enforce that the timestamp be exactly a specific time due to network latency and the imprecision of clocks in computers. So the timestamp in a block just has to be within a range of times that goes from the median time of the last 11 blocks to a few hours ahead of current time. A miner can modify the timestamp as long as it fits within this range of time. I thought the timestamps where reliant upon the sender, and when they pushed their transaction, is this incorrect?
No. Transactions do not have timestamps and their times have no effect on the block's timestamp.Now I'm wondering what happened if/when SHA-256 becomes compromised to some degree.
There are two forms of "compromised" for hash functions: a preimage attack and a collision attack. A preimage attack allows the attacker to determine the data that a given hash came from. Basically a preimage attack allows you to reverse a hash. This kind of attack is difficult, and AFAIK, no known preimage attack has been found for any hash function, even ones considered broken. If a preimage attack were found on SHA256, it may allow miners to simply mine blocks faster. Depending on how the attack works. this could mean that blocks are found at a constant time regardless of the difficulty. But such attacks are unlikely, and even if one existed, it may not even result in finding a valid block header since, technically, there are infinitely many pieces of data that hash to a given hash.Collision attacks are a different story. Collision attacks are much easier to find and possibly more problematic than a preimage attack. All broken hash functions are broken because of collision attacks. A collision attack is where two pieces of data can be found that hash to the same thing. Although technically there exists infinitely many pieces of data that hash to the same thing, finding those pieces of data is incredibly hard and there are so many possible hashes that finding a collision through brute force would take a very very long time. However, a collision attack allows someone to find two colliding pieces of data in a practical time frame (e.g. several years). If someone were able to find another valid block which has a hash that matches one of another existing block (so a collision), then there would be a lot of problems since the hashes would match but the transaction history could be different.
