OpenSSL: Handling ASN.1 BITSTRINGs

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

1. 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.
2. 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

3. 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}")
     

4. 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')}")
     

5. 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.