diff --git a/doc/functional-tests.md b/doc/functional-tests.md
--- a/doc/functional-tests.md
+++ b/doc/functional-tests.md
@@ -365,8 +365,5 @@
 #### [test_framework/key.py](/test/functional/test_framework/key.py)
 Test-only secp256k1 elliptic curve implementation
 
-#### [test_framework/bignum.py](/test/functional/test_framework/bignum.py)
-Helpers for script.py
-
 #### [test_framework/blocktools.py](/test/functional/test_framework/blocktools.py)
 Helper functions for creating blocks and transactions.
diff --git a/test/functional/test_framework/bignum.py b/test/functional/test_framework/bignum.py
deleted file mode 100644
--- a/test/functional/test_framework/bignum.py
+++ /dev/null
@@ -1,64 +0,0 @@
-#!/usr/bin/env python3
-#
-# Distributed under the MIT software license, see the accompanying
-# file COPYING or http://www.opensource.org/licenses/mit-license.php.
-"""Big number routines.
-
-This file is copied from python-bitcoinlib.
-"""
-
-import struct
-
-
-# generic big endian MPI format
-
-def bn_bytes(v, have_ext=False):
-    ext = 0
-    if have_ext:
-        ext = 1
-    return ((v.bit_length() + 7) // 8) + ext
-
-
-def bn2bin(v):
-    s = bytearray()
-    i = bn_bytes(v)
-    while i > 0:
-        s.append((v >> ((i - 1) * 8)) & 0xff)
-        i -= 1
-    return s
-
-
-def bn2mpi(v):
-    have_ext = False
-    if v.bit_length() > 0:
-        have_ext = (v.bit_length() & 0x07) == 0
-
-    neg = False
-    if v < 0:
-        neg = True
-        v = -v
-
-    s = struct.pack(b">I", bn_bytes(v, have_ext))
-    ext = bytearray()
-    if have_ext:
-        ext.append(0)
-    v_bin = bn2bin(v)
-    if neg:
-        if have_ext:
-            ext[0] |= 0x80
-        else:
-            v_bin[0] |= 0x80
-    return s + ext + v_bin
-
-
-# bitcoin-specific little endian format, with implicit size
-
-
-def mpi2vch(s):
-    r = s[4:]           # strip size
-    r = r[::-1]         # reverse string, converting BE->LE
-    return r
-
-
-def bn2vch(v):
-    return bytes(mpi2vch(bn2mpi(v)))
diff --git a/test/functional/test_framework/script.py b/test/functional/test_framework/script.py
--- a/test/functional/test_framework/script.py
+++ b/test/functional/test_framework/script.py
@@ -7,7 +7,6 @@
 This file is modified from python-bitcoinlib.
 """
 
-from .bignum import bn2vch
 import hashlib
 import struct
 
@@ -23,7 +22,6 @@
 
 
 MAX_SCRIPT_ELEMENT_SIZE = 520
-
 OPCODE_NAMES = {}
 
 
@@ -31,6 +29,18 @@
     return hashlib.new('ripemd160', sha256(s)).digest()
 
 
+def bn2vch(v):
+    """Convert number to bitcoin-specific little endian format."""
+    # We need v.bit_length() bits, plus a sign bit for every nonzero number.
+    n_bits = v.bit_length() + (v != 0)
+    # The number of bytes for that is:
+    n_bytes = (n_bits + 7) // 8
+    # Convert number to absolute value + sign in top bit.
+    encoded_v = 0 if v == 0 else abs(v) | ((v < 0) << (n_bytes * 8 - 1))
+    # Serialize to bytes
+    return encoded_v.to_bytes(n_bytes, 'little')
+
+
 _opcode_instances = []
 
 
@@ -48,9 +58,11 @@
             # OP_PUSHDATA1
             return b'\x4c' + bytes([len(d)]) + d
         elif len(d) <= 0xffff:
-            return b'\x4d' + struct.pack(b'<H', len(d)) + d  # OP_PUSHDATA2
+            # OP_PUSHDATA2
+            return b'\x4d' + struct.pack(b'<H', len(d)) + d
         elif len(d) <= 0xffffffff:
-            return b'\x4e' + struct.pack(b'<I', len(d)) + d  # OP_PUSHDATA4
+            # OP_PUSHDATA4
+            return b'\x4e' + struct.pack(b'<I', len(d)) + d
         else:
             raise ValueError("Data too long to encode in a PUSHDATA op")