#!/usr/bin/env python3
"""
Usage: $0 HEXDATA
       ... | $0
Parse HEXDATA as the hexadecimal representation of ASN.1 structured data and
print the data out with indentation reflecting the ASN.1 nested structure.
Whitespace in HEXDATA is ignored.
"""

import binascii
import re
import sys

def process(data, offset, slice_end, level):
    while offset < slice_end:
        tag = data[offset]
        is_constructed = tag & 0x20
        content_offset = offset + 2
        length = data[offset + 1]
        if length == 0x80:
            raise Exception("Indefinite length is not supported at {}".format(offset))
        elif length & 0x80:
            content_offset += length & 0x7f
            length_bytes = data[offset+2:content_offset]
            length = int(binascii.hexlify(length_bytes).decode('ascii'), 16)
        content_end = content_offset+length
        if is_constructed:
            maybe_content = ''
        else:
            maybe_content = binascii.hexlify(data[content_offset:content_end]).decode('ascii')
            if tag & 0x3f == 0x03: # BIT STRING
                maybe_content = maybe_content[:2] + ' ' + maybe_content[2:]
        print(' ' * level +
              binascii.hexlify(data[offset:content_offset]).decode('ascii'),
              maybe_content)
        if is_constructed:
            process(data, content_offset, content_offset + length, level + 1)
        offset = content_offset + length

BLANK_RE = re.compile(r'\s+')

def prepare(raw):
    return binascii.unhexlify(BLANK_RE.sub('', raw))

def main(args):
    if len(args) == 0:
        raw = sys.stdin.read()
    elif args[0] == '--help':
        sys.stdout.write(usage)
        exit(0)
    elif len(args) == 1:
        raw = args[0]
    else:
        sys.stderr.write('Too many arguments!\n')
        sys.stderr.write(__doc__)
        exit(2)
    data = prepare(raw)
    process(data, 0, len(data), 0)

if __name__ == '__main__':
    main(sys.argv[1:])