OpenSSL: Handling ASN.1 BITSTRINGs

G5 Cyber Security

2 weeks ago

TL;DR

This guide shows you how to work with ASN.1 encapsulated BITSTRING types in OpenSSL, covering encoding and decoding using command-line tools and basic programming examples.

Understanding the Problem

ASN.1 (Abstract Syntax Notation One) is a standard for defining data structures. BITSTRINGs are used to represent variable-length bit sequences. When encapsulated within ASN.1, they often include extra information like unused bits. OpenSSL provides tools to handle these complexities.

Solution

Inspect the BITSTRING with `openssl asn1parse`
- First, you need to see what’s inside your ASN.1 structure. Use this command:
```
openssl asn1parse -i  -dump
```
  Replace <filename> with the name of your file containing the ASN.1 data.
- Look for sections labelled ‘BIT STRING’. Pay attention to the ‘Contents’ and ‘Unused bits’ values. These are crucial for correct decoding.
Decoding a BITSTRING using `openssl enc` (for simple cases)
If your BITSTRING represents a simple byte sequence, you might be able to decode it directly with openssl enc. This works best when the unused bits don’t interfere.
- Convert the ASN.1 data into base64:
```
openssl asn1parse -i  | grep 'BIT STRING:' | awk '{print $3}' | tr -d '
' | openssl base64
```
- Decode the base64 string using openssl enc. You’ll need to know the original encoding (e.g., utf-8, hex):
```
echo  | openssl base64 -d | openssl enc -aes-256-cbc -k  -in /dev/stdin -out decoded.txt
```

Decoding a BITSTRING with Python and PyOpenSSL

For more complex scenarios, using a programming language like Python with the PyOpenSSL library gives you finer control.

Install PyOpenSSL:
```
pip install pyopenssl
```

Python example (assuming you know the structure):

from OpenSSL import crypto
import binascii

with open('your_file.der', 'rb') as f:
  data = f.read()

def decode_bitstring(asn1_data, unused_bits):
    # This is a simplified example; error handling and more complex ASN.1 structures are needed in real-world scenarios.
    try:
        decoded_bytes = binascii.unhexlify(asn1_data[2:].decode('utf-8')) #remove tag and length
        return decoded_bytes
    except Exception as e:
        print(f"Decoding error: {e}")
        return None

# Example usage (replace with your actual ASN.1 data and unused bits)
unused = 5  # Replace with the correct number of unused bits from asn1parse output.
bitstring_data = b'30820402...' #Replace with the bitstring part of the DER file
decoded_result = decode_bitstring(bitstring_data, unused)
if decoded_result:
    print(f"Decoded result: {decoded_result}")

Encoding a BITSTRING with Python and PyOpenSSL

To create an ASN.1 encoded BITSTRING, you’ll need to construct the structure correctly.

Example:

from OpenSSL import crypto
import binascii

def encode_bitstring(data, unused_bits):
    # Convert data to bytes if it isn't already.
    if isinstance(data, str):
        data = data.encode('utf-8')

    # Calculate the length in bytes.
    length = len(data)

    # Construct the ASN.1 BITSTRING structure.
    bitstring_tag = b'30' # Tag for a constructed sequence
    bitstring_length = crypto.encode_asn1_octet_string(bytes([length]))
    bitstring_value = data

    # Combine the tag, length, and value.
    encoded_data = bitstring_tag + bitstring_length + bitstring_value
    return encoded_data

# Example usage:
my_data = "Hello World!"
unused = 3
encoded_result = encode_bitstring(my_data, unused)
print(f"Encoded result: {binascii.hexlify(encoded_result).decode('utf-8')}")

Important Considerations
- Unused Bits: Always account for unused bits when decoding. Incorrect handling can lead to corrupted data.
- ASN.1 Structure: Complex ASN.1 structures may require recursive parsing and understanding of the entire schema.
- Error Handling: Implement robust error handling in your Python code to catch invalid input or unexpected ASN.1 formats.