Bitcoin safety by no means trusted resistance to length-extension as a result of preimages are public anyway, and customary perception is that the double was used only for defense-in-depth.
I do not assume Bitcoin ever makes use of hashes in a means that might undergo from size extensions, however I suppose Satoshi went with the secure alternative of stopping it all over the place.
To keep away from this property, Ferguson and Schneier advised utilizing SHA256d = SHA256(SHA256(x)) which avoids length-extension assaults. This building has some minor weaknesses (not related to bitcoin), so I would not suggest it for brand spanking new protocols, and would use HMAC with fixed key, or truncated SHA512 as a substitute.
https://bitcoin.stackexchange.com/a/8461/137501
The paper’s discovery is attention-grabbing in that it could transfer SHA256d additional away from a random oracle which has implications for secondary on-chain makes use of (e.g. in sensible contracts or as 32-byte P2SH wrapper).
Apparently, Bitcoin builders did not assume that securing in opposition to length-extension issues in order that they went with plain SHA256 for SegWit P2WSH tackle hashes.
Later, Bitcoin Money builders selected SHA256d for P2SH32, thus sustaining consistency with the remainder of the protocol, and unlinkability between never-spent-from addresses.
Readers is perhaps fascinated with some older associated work, that has already proven a weak spot in opposition to an unique use-case (Dodis et al., 2013):
We exhibit a cryptographic setting, known as mutual proofs of labor, through which the highlighted construction of H2 may be exploited. In mutual proofs of labor, two events show to one another that they’ve computed some asserted quantity of computational effort. This process is impressed by, and much like, consumer puzzles [20, 21, 27, 28, 40] and puzzle auctions [42]. We give a protocol for mutual proofs of labor whose computational process is computing hash chains. This protocol is safe when utilizing a random oracle, however when utilizing as a substitute H2 an attacker can cheat by abusing the structural properties mentioned above.
