diff --git a/src/atmst/blockstore/car_file.py b/src/atmst/blockstore/car_file.py
index 6f33bc9..7b36d35 100644
--- a/src/atmst/blockstore/car_file.py
+++ b/src/atmst/blockstore/car_file.py
@@ -6,20 +6,24 @@
 from . import BlockStore
 
 # should be equivalent to multiformats.varint.decode(), but not extremely slow for no reason.
-def parse_varint(stream: BinaryIO):
+def decode_varint(stream: BinaryIO):
 	n = 0
-	shift = 0
-	while True:
+	for shift in range(0, 63, 7):
 		val = stream.read(1)
 		if not val:
-			raise ValueError("eof") # match varint.decode()
+			raise ValueError("unexpected end of varint input")
 		val = val[0]
 		n |= (val & 0x7f) << shift
 		if not val & 0x80:
+			if shift and not val:
+				raise ValueError("varint not minimally encoded")
 			return n
 		shift += 7
+	raise ValueError("varint too long")
 
 def encode_varint(n: int) -> bytes:
+	if not 0 <= n < 2**63:
+		raise ValueError("integer out of encodable varint range")
 	res = []
 	while n > 0x7f:
 		res.append(0x80 | (n & 0x7f))
@@ -47,7 +51,7 @@ def __init__(self, file: BinaryIO, validate_hashes: bool=True) -> None:
 		file.seek(0)
 
 		# parse out CAR header
-		header_len = parse_varint(file)
+		header_len = decode_varint(file)
 		header = file.read(header_len)
 		if len(header) != header_len:
 			raise EOFError("not enough CAR header bytes")
@@ -62,7 +66,7 @@ def __init__(self, file: BinaryIO, validate_hashes: bool=True) -> None:
 		self.block_offsets = {}
 		while True:
 			try:
-				length = parse_varint(file)
+				length = decode_varint(file)
 			except ValueError:
 				break # EOF
 			start = file.tell()
diff --git a/tests/test_varint.py b/tests/test_varint.py
new file mode 100644
index 0000000..effe9d2
--- /dev/null
+++ b/tests/test_varint.py
@@ -0,0 +1,28 @@
+import unittest
+import io
+
+from atmst.blockstore.car_file import decode_varint, encode_varint
+
+class MSTDiffTestCase(unittest.TestCase):
+	def test_varint_encode(self):
+		self.assertEqual(encode_varint(0), b"\x00")
+		self.assertEqual(encode_varint(1), b"\x01")
+		self.assertEqual(encode_varint(127), b"\x7f")
+		self.assertEqual(encode_varint(128), b"\x80\x01")
+		self.assertEqual(encode_varint(2**63-1), b'\xff\xff\xff\xff\xff\xff\xff\xff\x7f')
+		self.assertRaises(ValueError, encode_varint, 2**63)
+		self.assertRaises(ValueError, encode_varint, -1)
+
+	def test_varint_decode(self):
+		self.assertEqual(decode_varint(io.BytesIO(b"\x00")), 0)
+		self.assertEqual(decode_varint(io.BytesIO(b"\x01")), 1)
+		self.assertEqual(decode_varint(io.BytesIO(b"\x7f")), 127)
+		self.assertEqual(decode_varint(io.BytesIO(b"\x80\x01")), 128)
+		self.assertEqual(decode_varint(io.BytesIO(b'\xff\xff\xff\xff\xff\xff\xff\xff\x7f')), 2**63-1)
+		self.assertRaises(ValueError, decode_varint, io.BytesIO(b'\xff\xff\xff\xff\xff\xff\xff\xff\xff\x7f')) # too big
+		self.assertRaises(ValueError, decode_varint, io.BytesIO(b"")) # too short
+		self.assertRaises(ValueError, decode_varint, io.BytesIO(b'\xff')) # truncated
+		self.assertRaises(ValueError, decode_varint, io.BytesIO(b"\x80\x00")) # too minimally encoded
+
+if __name__ == '__main__':
+	unittest.main(module="tests.test_varint")