Indent ASN.1 DER in hex

asn1-indent.py

#!/usr/bin/env python3

import sys

from typing import Tuple


def is_container(value: bytes) -> bool:
    return len(value) > 0 and value[0] in {0x30, 0x31}


def decode_next(b: bytes) -> Tuple[Tuple[bytes, bytes, bytes], bytes]:
    if b[1] & 0x80 == 0:
        length = b[1]
        value_start = 2
        value_end = value_start + length
    else:
        length_bytes = b[1] & 0x7f
        value_start = 2 + length_bytes
        length = int.from_bytes(b[2:2+length_bytes], 'big')
        value_end = value_start + length

    return (b[0:1], b[1:value_start], b[value_start:value_end]), b[value_end:]


def treeify(asn1: bytes):
    items = []
    tail = asn1
    while len(tail) > 0:
        (tag, length, value), tail = decode_next(tail)
        if is_container(tag):
            items.append((tag, length, treeify(value)))
        else:
            items.append((tag, length, value))

    if len(items) == 1:
        return items[0]
    else:
        return items


def indent(tree, depth=0) -> str:
    tag, length, value = tree
    if isinstance(value, list):
        s = f"{'  ' * depth}{tag.hex()}  {length.hex()}"
        for subvalue in value:
            s += f"\n{indent(subvalue, depth=depth+1)}"
        return s

    elif isinstance(value, tuple):
        s = f"{'  ' * depth}{tag.hex()}  {length.hex()}"
        s += f"\n{indent(value, depth=depth+1)}"
        return s

    else:
        return f"{'  ' * depth}{tag.hex()}  {length.hex()}  {value.hex()}"


print(indent(treeify(sys.stdin.buffer.read())))