BitcoinCash/specification
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protocol/blockchain/hash.md
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@@ -11,7 +11,7 @@ A variety of hashing algorithms are used throughout the Bitcoin Cash protocol.
- This double hash removes the possibility of a [length extension attack](https://en.wikipedia.org/wiki/Length_extension_attack) which a single SHA-256 is vulnerable to. While this is generally not a problem for Bitcoin Cash since the pre-image (the actual data of the block) is available, it trades a minor amount of inefficiency for confidence that this property of SHA-256 cannot be exploited.
- Double SHA-256 has it's own operation for ease-of-use, [OP_HASH256](/protocol/blockchain/script/opcodes/op-hash256)
- Transaction Hashing
- Transactions are hashed using a single application of SHA-256. This is referred to as the transaction hash and is used to uniquely identify the transaction. (NOTE: Historical transaction hashes are not universally unique, there are two sets of two identical coinbase transactions and thus repeated hashes. Since [BIP-34](/protocol/forks/bip-0034), the block height is now required to be in the coinbase transaction, this should not be possible in the future.)
- Transactions are hashed using a single application of SHA-256. This is referred to as the transaction hash and is used to uniquely identify the transaction. (NOTE: Historical transaction hashes are not universally unique, there are two sets of two identical coinbase transactions and thus repeated hashes. Since [BIP-34](/protocol/forks/bip-0034), the block height is now required to be in the coinbase transaction, so this should not be possible in the future.)
- The two cases where this occurred are the following transactions which each appear in two blocks:
- <code>D5D27987D2A3DFC724E359870C6644B40E497BDC0589A033220FE15429D88599</code>
- <code>E3BF3D07D4B0375638D5F1DB5255FE07BA2C4CB067CD81B84EE974B6585FB468</code>