bitcoin.py: make int_to_hex throw on overflow

lib/bitcoin.py

@@ -50,12 +50,18 @@ def rev_hex(s):
     return bh2u(bfh(s)[::-1])
 
 
-def int_to_hex(i, length=1):
+def int_to_hex(i: int, length: int=1) -> str:
+    """Converts int to little-endian hex string.
+    `length` is the number of bytes available
+    """
     if not isinstance(i, int):
         raise TypeError('{} instead of int'.format(i))
+    range_size = pow(256, length)
+    if i < -range_size/2 or i >= range_size:
+        raise OverflowError('cannot convert int {} to hex ({} bytes)'.format(i, length))
     if i < 0:
         # two's complement
-        i = pow(256, length) + i
+        i = range_size + i
     s = hex(i)[2:].rstrip('L')
     s = "0"*(2*length - len(s)) + s
     return rev_hex(s)

lib/tests/test_bitcoin.py

@@ -12,7 +12,7 @@ from lib.bitcoin import (
     deserialize_privkey, serialize_privkey, is_segwit_address,
     is_b58_address, address_to_scripthash, is_minikey, is_compressed, is_xpub,
     xpub_type, is_xprv, is_bip32_derivation, seed_type, EncodeBase58Check,
-    script_num_to_hex, push_script, add_number_to_script)
+    script_num_to_hex, push_script, add_number_to_script, int_to_hex)
 from lib import ecc, crypto, ecc_fast
 from lib.ecc import number_to_string, string_to_number
 from lib.transaction import opcodes
@@ -227,6 +227,26 @@ class Test_bitcoin(SequentialTestCase):
         result = Hash(payload)
         self.assertEqual(expected, result)
 
+    def test_int_to_hex(self):
+        self.assertEqual('00', int_to_hex(0, 1))
+        self.assertEqual('ff', int_to_hex(-1, 1))
+        self.assertEqual('00000000', int_to_hex(0, 4))
+        self.assertEqual('01000000', int_to_hex(1, 4))
+        self.assertEqual('7f', int_to_hex(127, 1))
+        self.assertEqual('7f00', int_to_hex(127, 2))
+        self.assertEqual('80', int_to_hex(128, 1))
+        self.assertEqual('80', int_to_hex(-128, 1))
+        self.assertEqual('8000', int_to_hex(128, 2))
+        self.assertEqual('ff', int_to_hex(255, 1))
+        self.assertEqual('ff7f', int_to_hex(32767, 2))
+        self.assertEqual('0080', int_to_hex(-32768, 2))
+        self.assertEqual('ffff', int_to_hex(65535, 2))
+        with self.assertRaises(OverflowError): int_to_hex(256, 1)
+        with self.assertRaises(OverflowError): int_to_hex(-129, 1)
+        with self.assertRaises(OverflowError): int_to_hex(-257, 1)
+        with self.assertRaises(OverflowError): int_to_hex(65536, 2)
+        with self.assertRaises(OverflowError): int_to_hex(-32769, 2)
+
     def test_var_int(self):
         for i in range(0xfd):
             self.assertEqual(var_int(i), "{:02x}".format(i) )