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Concatenating and double SHA-256 hashing

"a751efbeabe73bdf9d08df5760104feff915d9d807d4c62178cdeb98d8c25f43"

with itself

"a751efbeabe73bdf9d08df5760104feff915d9d807d4c62178cdeb98d8c25f43"

should output 15eca0aa3e2cc2b9b4fbe0629f1dda87f329500fcdcd6ef546d163211266b3b3

import hashlib header_hex = "a751efbeabe73bdf9d08df5760104feff915d9d807d4c62178cdeb98d8c25f43a751efbeabe73bdf9d08df5760104feff915d9d807d4c62178cdeb98d8c25f43"

header_bin = header_hex.decode('hex')

hash = hashlib.sha256(hashlib.sha256(header_bin).digest()).digest()

hash.encode('hex_codec') '2b4a9fdb97e89e73c4647791e476090eaad04f43c93a3de21a95d2c4fc8e8e0c'

I'm getting the wrong answer :/?

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  • Your code looks correct. Where do you read that the double-SHA256 hash of a751... input concatenated with itself should have 15eca... as output?
    – Pieter Wuille
    Commented Jul 8, 2018 at 1:20
  • bitcointalk.org/index.php?topic=44707.msg534605#msg534605 the bottom of the last page
    – Ben Stolman
    Commented Jul 8, 2018 at 1:29
  • blockchain.com/en/btc/block-index/120872 the Merkle Root in the example on the forum is the same as the one on Blockchain.info
    – Ben Stolman
    Commented Jul 8, 2018 at 1:35
  • Block hashes are often displayed in byte-reversed order. Try reversing the bytes before hashing, and again on the computed hash.
    – Pieter Wuille
    Commented Jul 8, 2018 at 1:37
  • >>> import hashlib >>> header_hex = "435fc2d898ebcd7821c6d407d8d915f9ef4f106057df089ddf3be7abbeef51a7435fc2d898ebcd7821c6d407d8d915f9ef4f106057df089ddf3be7abbeef51a7" >>> header_bin = header_hex.decode('hex') >>> hash = hashlib.sha256(hashlib.sha256(header_bin).digest()).digest() >>> hash.encode('hex_codec') 'b3b366122163d146f56ecdcd0f5029f387da1d9f62e0fbb4b9c22c3eaaa0ec15' >>> hash[::-1].encode('hex_codec') '15eca0aa3e2cc2b9b4fbe0629f1dda87f329500fcdcd6ef546d163211266b3b3' YIPEE THANK YOU PIETER!
    – Ben Stolman
    Commented Jul 8, 2018 at 1:46

1 Answer 1

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2

You have to byte-swap the hex values BEFORE concatenating them. You also have to byte-swap the double hashed hex string!

import hashlib header_hex = "435fc2d898ebcd7821c6d407d8d915f9ef4f106057df089ddf3be7abbeef51a7435fc2d898ebcd7821c6d407d8d915f9ef4f106057df089ddf3be7abbeef51a7"

header_bin = header_hex.decode('hex') hash = hashlib.sha256(hashlib.sha256(header_bin).digest()).digest() hash.encode('hex_codec') 'b3b366122163d146f56ecdcd0f5029f387da1d9f62e0fbb4b9c22c3eaaa0ec15' >>>

hash[::-1].encode('hex_codec') '15eca0aa3e2cc2b9b4fbe0629f1dda87f329500fcdcd6ef546d163211266b3b3'

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