I am very new to Bitcoin. Recently checked Bitcoin explorer. All blocks and transactions and blocks stored in the blockchain are identified with a SHA-256 hash. This hash is calculated from the contents of the block or transaction. The website indexes these hashes in its database and provides a reverse lookup to the original string against which a hash is computed.
These SHA-256 hashes are not unique to bitcoin only. Although the block hashes have a specific pattern which may be meaningful for bitcoin only, the transactions do not. Is it possible that this reverse lookup can be used as a rainbow table in future ? Till now how much of the hash space has been exhausted ? At what rate is it growing ? Even if it is 1% it means for every 1000 hashes there are ~10 hashes that are compromised. Of course the lookup table may not yield the original text but often any original text may work. I understand that it is not a problem for most of the passwords (for well designed websites), because the passwords are hashed with a salt. But there can be other scenarios where a reverse lookup table can be considered as a threat to the security of the system.
Is this a threat or bitcoin have something to protect against such attack ? Should other systems use any precaution against this ?
There are an estimated 650 million transactions on the chain (as of June 2021).
sha256 has 2^256 possible values.
The chain therefore has 0.00000000000000000000000000000000000000000000000000000000000000000056% of the possible values of sha256.
6.5e8 / 2^256 * 100 = 5.6e-67
While possible, it's highly unlikely that a hash you are attempting to find a collision for exists on the chain.