Hash Byte Order

Dash Core RPCs accept and return the byte-wise reverse of computed SHA-256 hash values. For example, the Unix sha256sum command displays the SHA256(SHA256()) hash of mainnet block 300,000's header as:

> /bin/echo -n '020000007ef055e1674d2e6551dba41cd214debbee34aeb544c7ec670000000000000000d3998963f80c5bab43fe8c26228e98d030edf4dcbe48a666f5c39e2d7a885c9102c86d536c890019593a470d' \
    | xxd -r -p \
    | sha256sum -b \
    | xxd -r -p \
    | sha256sum -b

5472ac8b1187bfcf91d6d218bbda1eb2405d7c55f1f8cc820000000000000000 (Resulting hash)

The result above is also how the hash appears in the previous-header-hash part of blockblock - One or more transactions prefaced by a block header and protected by proof of work. Blocks are the data stored on the block chain. 300,001's header:

020000005472ac8b1187bfcf91d6d218bbda1eb2405d7c55f1f8cc82000\
0000000000000ab0aaa377ca3f49b1545e2ae6b0667a08f42e72d8c24ae\
237140e28f14f3bb7c6bcc6d536c890019edd83ccf

However, Dash Core's RPCs use the byte-wise reverse for hashes, so if you want to get information about block 675,776 using the getblock RPC, you need to reverse the requested hash:

> dash-cli getblock \
    000000000000327a66cd1011b2d1defd1417b7d9e39b439e8e67ba996ee92602

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Note: hex representation uses two characters to display each byte of data, which is why the reversed string looks somewhat mangled.

The rationale for the reversal is unknown, but it likely stems from Dash Core's use of hashes (which are byte arrays in C++) as integers for the purpose of determining whether the hash is below the network target. Whatever the reason for reversing header hashes, the reversal also extends to other hashes used in RPCs, such as TXIDsTXIDs - An identifier used to uniquely identify a particular transaction; specifically, the sha256d hash of the transaction. and merkle roots.

As header hashes and TXIDs are widely used as global identifiers in other Dash software, this reversal of hashes has become the standard way to refer to certain objects. The table below should make clear where each byte order is used.

DataInternal Byte OrderRPC Byte Order
Example: SHA256(SHA256(0x00))Hash: 1406...539aHash: 9a53...0614
-----------------------------------------------------
Header Hashes: SHA256(SHA256(block header))Used when constructing block headersUsed by RPCs such as getblock; widely used in block explorers
-----------------------------------------------------
Merkle Roots: SHA256(SHA256(TXIDs and merkle rows))Used when constructing block headersReturned by RPCs such as getblock
-----------------------------------------------------
TXIDs: SHA256(SHA256(transaction))Used in transaction inputsUsed by RPCs such as gettransaction and transaction data parts of getblock; widely used in wallet programs
-----------------------------------------------------
P2PKH Hashes: RIPEMD160(SHA256(pubkey))Used in both addresses and pubkey scriptsN/A: RPCs use addresses which use internal byte order
-----------------------------------------------------
P2SH Hashes: RIPEMD160(SHA256(redeem script))Used in both addresses and pubkey scriptsN/A: RPCs use addresses which use internal byte order
-----------------------------------------------------

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Note: RPCs which return raw results, such as getrawtransaction or the raw mode of getblock, always display hashes as they appear in blocks (internal byte orderinternal byte order - The standard order in which hash digests are displayed as strings---the same format used in serialized blocks and transactions.).

The code below may help you check byte order by generating hashes from raw hex.

from sys import byteorder
from hashlib import sha256
import codecs

decode_hex = codecs.getdecoder('hex_codec')
encode_hex = codecs.getencoder('hex_codec')

# You can put in $data an 80-byte block header to get its header hash,
# or a raw transaction to get its txid
data = decode_hex('00')[0]
data_hash = sha256(sha256(data).digest()).digest()

print("Warning: this code only tested on a little-endian x86_64 arch")
print()
print("System byte order:        ", byteorder)
print("Internal-Byte-Order Hash: ", encode_hex(data_hash)[0])
print("RPC-Byte-Order Hash:      ", encode_hex(data_hash[::-1])[0])

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