{% comment %}
This file is licensed under the MIT License (MIT) available on
http://opensource.org/licenses/MIT.
{% endcomment %}
{% assign filename="_includes/ref/bitcoin-core/api-intro.md" %}

## Bitcoin Core APIs
{% include helpers/subhead-links.md %}

### Hash Byte Order
{% include helpers/subhead-links.md %}

{% autocrossref %}

Bitcoin Core RPCs accept and return hashes in the reverse of their
normal byte order. For example, the Unix `sha256sum` command would display the
SHA256(SHA256()) hash of mainnet block 300,000's header as the
following string:

    5472ac8b1187bfcf91d6d218bbda1eb2405d7c55f1f8cc820000000000000000

The string 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 Bitcoin RPCs use the reverse byte order for hashes, so if you
want to get information about block 300,000 using the `getblock` RPC,
you need to reverse the byte order:

    > bitcoin-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 rational for the reversal is unknown, but it likely stems from
Bitcoin's use of hash digests (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.

Off-site documentation such as the Bitcoin Wiki tends to use the terms
big endian and little endian as shown in the table below, but they
aren't always consistent. Worse, these two different ways of
representing a hash digest can confuse anyone who looks at the Bitcoin
Core source code and finds a so-called "big endian" value being stored
in a little-endian data type.

As header hashes and TXIDs are widely used as global identifiers in
other Bitcoin 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 ("Big Endian") | RPC Byte Order ("Little Endian") |
|---------------|---------------------|-----------------|
| 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.
{% endautocrossref %}

{% highlight python %}
#!/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')
{% endhighlight %}
