fix: DER encoding for tag and length in TLV (#282)

Author: fsamier

src/erc7730/common/binary.py

@@ -13,43 +13,53 @@ def from_hex(value: str) -> bytes:
     return bytes.fromhex(value.removeprefix("0x"))
 
 
-def tlv(tag: int | IntEnum, *value: bytes | str | None) -> bytes:
+def tlv(tag: int | IntEnum, value: bytes | str | None = None) -> bytes:
     """
     Encode a value in TLV format (Tag-Length-Value)
 
-    If value is not encoded, it will be encoded as ASCII.
-    If input string is not ASCII, and UnicodeEncodeError is raised.
+    Tag and length are DER encoded. If tag value or length exceed 255, an OverflowError is raised.
 
-    If encoded value is longer than 255 bytes, an OverflowError is raised.
+    If value is not encoded, it will be encoded as ASCII.
+    If input string is not ASCII, a UnicodeEncodeError is raised.
 
     @param tag: the tag (can be an enum)
-    @param value: the value (can be already encoded, or a string)
+    @param value: the value (can be already encoded, a string or None)
     @return: encoded TLV
     """
-    values_encoded = bytearray()
-    for v in value:
-        if v is not None:
-            values_encoded.extend(v.encode("ascii", errors="strict") if isinstance(v, str) else v)
-    return (
-        (tag.value if isinstance(tag, IntEnum) else tag).to_bytes(1, "big")
-        + len(values_encoded).to_bytes(1, "big")
-        + values_encoded
-    )
 
+    return der_encode_int(tag.value if isinstance(tag, IntEnum) else tag) + length_value(value)
 
-def length_value(value: bytes | str | None) -> bytes:
+
+def length_value(
+    value: bytes | str | None,
+) -> bytes:
     """
-    Prepend the length of the value encoded on 1 byte to the value itself.
+    Prepend the length (DER encoded) of the value encoded to the value itself.
+    If length exceeds 255 bytes, an OverflowError is raised.
 
     If value is not encoded, it will be encoded as ASCII.
-    If input string is not ASCII, and UnicodeEncodeError is raised.
-
-    If encoded value is longer than 255 bytes, an OverflowError is raised.
+    If input string is not ASCII, a UnicodeEncodeError is raised.
 
     @param value: the value (can be already encoded, or a string)
     @return: encoded TLV
     """
     if value is None:
         return (0).to_bytes(1, "big")
-    value_encoded = value.encode("ascii", errors="strict") if isinstance(value, str) else value
-    return len(value_encoded).to_bytes(1, "big") + value_encoded
+    match value:
+        case bytes():
+            value_encoded = value
+        case str():
+            value_encoded = value.encode("ascii", errors="strict")
+    return der_encode_int(len(value_encoded)) + value_encoded
+
+
+def der_encode_int(value: int) -> bytes:
+    """
+    Encode an integer in DER format.
+    If value exceeds 255, an OverflowError is raised.
+
+    @param value: the integer to encode
+    @return: DER encoded byte array
+    """
+    value_bytes = value.to_bytes(1, "big")  # raises OverflowError if value >= 256
+    return (0x81).to_bytes(1, "big") + value_bytes if value >= 0x80 else value_bytes

tests/common/test_binary.py

@@ -0,0 +1,37 @@
+from enum import IntEnum
+
+import pytest
+
+from erc7730.common.binary import tlv
+
+
+class _Tag(IntEnum):
+    FIELD = 5
+
+
+@pytest.mark.parametrize(
+    "tag, value, expected",
+    [
+        (1, None, b"\x01\x00"),
+        (1, b"\xab", b"\x01\x01\xab"),
+        (1, "hi", b"\x01\x02hi"),
+        (_Tag.FIELD, b"\xff", b"\x05\x01\xff"),
+        (0x80, None, b"\x81\x80\x00"),
+        (1, b"\x00" * 128, b"\x01\x81\x80" + b"\x00" * 128),
+    ],
+)
+def test_tlv(tag: int | IntEnum, value: bytes | str | None, expected: bytes) -> None:
+    assert tlv(tag, value) == expected
+
+
+@pytest.mark.parametrize(
+    "tag, value, exc",
+    [
+        (256, None, OverflowError),
+        (1, b"\x00" * 256, OverflowError),
+        (1, "là-haut", UnicodeEncodeError),
+    ],
+)
+def test_tlv_errors(tag: int, value: bytes | str | None, exc: type[Exception]) -> None:
+    with pytest.raises(exc):
+        tlv(tag, value)