Convert strings with an unknown number of hex strings embedded in them to strings using regex-CodePudding

So I have a list of strings (content from Snort rules), and I am trying to convert the hex portions of them to UTF-8/ASCII, so I can send the content over netcat.

The method I have now works fine for strings with single hex characters (i.e. 3A), but breaks when there's a series of hex characters (i.e. 3A 4B 00 FF)

My current solution is:

import re
import codecs

def convert_hex(match):
  string = match.group(1)
  string = string.replace(" ", "")
  decode_hex = codecs.getdecoder("hex_codec")
  try:
    result = decode_hex(string)[0]
  except:
    result = bytes.fromhex((lambda s: ("%s%s00" * (len(s)//2)) % tuple(s))(string)).decode('utf-16-le')
  return result.decode("utf-8")


strings = ['|0A|Referer|3A| res|3A|/C|3A|', 'RemoteNC Control Password|3A|', '/bbs/search.asp', 'User-Agent|3A| Mozilla/4.0 |28|compatible|3B| MSIE 5.0|3B| Windows NT 5.0|29|']

converted_strings = []

for string in strings:
    for i in range(len(string)):
        string = re.sub(r"\|(.{2})\|", convert_hex, string)
    converted_strings.append(string)

For the strings in strings, this works, but for a string like:

|08 00 00 00 27 C7 CC 6B C2 FD 13 0E|

it breaks.

I tried changing the regex to:

re.sub(r"\|.*([A-Fa-f0-9]{2}).*\|")

but that only converts the last hex.

I need this solution to work for strings like Hello|3A|World, |3A 00 FF|, and Hello|3A 00|World

I know it's an issue with the regexp, but I'm not sure what exactly.

Any help would be much appreciated.

CodePudding user response：

It looks like a substring is either always hex i.e. (?:[A-Fa-f0-9]{2}\s) [A-Fa-f0-9]{2} or not hex at all between | symbols?

This works:

for string in strings:
    for i in range(len(string)):
        string = re.sub(r"(?<=\|)((?:[A-Fa-f0-9]{2}\s)*[A-Fa-f0-9]{2})(?=\|)", convert_hex, string)
    converted_strings.append(string)

(extra parentheses for a capturing group 1 - you could leave out one pair of parentheses and change your function to act on group(0) instead)

But it breaks on your example |08 00 00 00 27 C7 CC 6B C2 FD 13 0E|, as that doesn't appear to be a valid UTF-8 encoding. The resulting error:

UnicodeDecodeError: 'utf-8' codec can't decode byte 0xc7 in position 5: invalid continuation byte

However, a valid UTF-8 encoded multi-byte string like '|74 65 73 74 20 f0 9f 98 80|' works just fine:

import re
import codecs

def convert_hex(match):
  string = match.group(1)
  string = string.replace(" ", "")
  decode_hex = codecs.getdecoder("hex_codec")
  try:
    result = decode_hex(string)[0]
  except:
    result = bytes.fromhex((lambda s: ("%s%s00" * (len(s)//2)) % tuple(s))(string)).decode('utf-16-le')
  return result.decode("utf-8")


strings = ['|74 65 73 74 20 f0 9f 98 80|']

converted_strings = []

for string in strings:
    for i in range(len(string)):
        string = re.sub(r"(?<=\|)((?:[A-Fa-f0-9]{2}\s)*[A-Fa-f0-9]{2})(?=\|)", convert_hex, string)
    converted_strings.append(string)

print(converted_strings)

Result:

['|test