test: add compact block filter (BIP158) helper routines

By now, we add one helper for calculating ranged hashes and another one
for finding relevant scriptPubKeys given a block.

test/functional/test_framework/blockfilter.py

@@ -0,0 +1,49 @@
+#!/usr/bin/env python3
+# Copyright (c) 2022 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Helper routines relevant for compact block filters (BIP158).
+"""
+from .siphash import siphash
+
+
+def bip158_basic_element_hash(script_pub_key, N, block_hash):
+    """ Calculates the ranged hash of a filter element as defined in BIP158:
+
+    'The first step in the filter construction is hashing the variable-sized
+    raw items in the set to the range [0, F), where F = N * M.'
+
+    'The items are first passed through the pseudorandom function SipHash, which takes a
+    128-bit key k and a variable-sized byte vector and produces a uniformly random 64-bit
+    output. Implementations of this BIP MUST use the SipHash parameters c = 2 and d = 4.'
+
+    'The parameter k MUST be set to the first 16 bytes of the hash (in standard
+    little-endian representation) of the block for which the filter is constructed. This
+    ensures the key is deterministic while still varying from block to block.'
+    """
+    M = 784931
+    block_hash_bytes = bytes.fromhex(block_hash)[::-1]
+    k0 = int.from_bytes(block_hash_bytes[0:8], 'little')
+    k1 = int.from_bytes(block_hash_bytes[8:16], 'little')
+    return (siphash(k0, k1, script_pub_key) * (N * M)) >> 64
+
+
+def bip158_relevant_scriptpubkeys(node, block_hash):
+    """ Determines the basic filter relvant scriptPubKeys as defined in BIP158:
+
+    'A basic filter MUST contain exactly the following items for each transaction in a block:
+       - The previous output script (the script being spent) for each input, except for
+         the coinbase transaction.
+       - The scriptPubKey of each output, aside from all OP_RETURN output scripts.'
+    """
+    spks = set()
+    for tx in node.getblock(blockhash=block_hash, verbosity=3)['tx']:
+        # gather prevout scripts
+        for i in tx['vin']:
+            if 'prevout' in i:
+                spks.add(bytes.fromhex(i['prevout']['scriptPubKey']['hex']))
+        # gather output scripts
+        for o in tx['vout']:
+            if o['scriptPubKey']['type'] != 'nulldata':
+                spks.add(bytes.fromhex(o['scriptPubKey']['hex']))
+    return spks