HASH BYTE ORDER

PAI 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

The result above is also how the hash appears in the previous-header-hash part of block 300,001’s header:

<pre>02000000<b>5472ac8b1187bfcf91d6d218bbda1eb2405d7c55f1f8cc82000\
0000000000000</b>ab0aaa377ca3f49b1545e2ae6b0667a08f42e72d8c24ae\
237140e28f14f3bb7c6bcc6d536c890019edd83ccf</pre>

However, PAI Core’s RPCs use the byte-wise reverse for hashes, so if you want to get information about block 300,000 using the getblock RPC, you need to reverse the requested hash:

> PAI-cli getblock \
  000000000000000082ccf8f1557c5d40b21edabb18d2d691cfbf87118bac7254

(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 PAI 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 TXIDs and merkle roots.

As header hashes and TXIDs are widely used as global identifiers in other PAI 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.

Data Internal Byte Order RPC Byte Order
Example: SHA256(SHA256(0x00)) Hash: 1406…539a Hash: 9a53…0614
Header Hashes: SHA256(SHA256(block header)) Used when constructing block headers Used by RPCs such as getblock; widely used in block explorers
Merkle Roots: SHA256(SHA256(TXIDs and merkle rows)) Used when constructing block headers Returned by RPCs such as getblock
TXIDs: SHA256(SHA256(transaction)) Used in transaction inputs Used 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 scripts N/A: RPCs use addresses which use internal byte order
P2SH Hashes: RIPEMD160(SHA256(redeem script)) Used in both addresses and pubkey scripts N/A: RPCs use addresses which use internal byte order

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 order).

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

#!/usr/bin/env python

from sys import byteorder
from hashlib import sha256

## You can put in $data an 80-byte block header to get its header hash,
## or a raw transaction to get its txid
data = "00".decode("hex")
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: ", hash.encode('hex_codec')
print "RPC-Byte-Order Hash:      ", hash[::-1].encode('hex_codec')