如何从pyasn1对象恢复底层数据并提取原编码T61Strings？

Absolutely, you can solve this problem cleanly with pyasn1—let’s break down exactly how to grab those raw, unmodified T61String (and similar type) substrate bytes without relying on re-encoding.

The Key: Retain Substrate Data During Decoding #

By default, pyasn1’s BER decoder discards the original raw bytes once it parses the ASN.1 structure. But you can override this by enabling the withSubstrate=True flag when decoding. This tells the decoder to attach the original BER-encoded substrate (including tag, length, and content bytes) to each parsed ASN.1 object, which you can then retrieve with the get_substrate() method.

Step-by-Step Implementation #

1. Decode with Substrate Preservation
   First, decode your raw ASN.1 byte stream while keeping the original substrate data:

   from pyasn1.codec.ber import decoder
   from pyasn1.type import univ, char
   
   # Replace with your actual raw ASN.1 BER bytes
   raw_asn1_bytes = b"...your nested ASN.1 bytes here..."
   
   # Decode with substrate retention enabled
   decoded_object, remaining_bytes = decoder.decode(raw_asn1_bytes, withSubstrate=True)
   

2. Recursively Traverse and Extract T61String Substrates
   Write a recursive function to walk the nested ASN.1 structure, identify T61String (and related) objects, and pull their raw substrate bytes:

   def extract_raw_t61_strings(asn1_object):
       raw_t61_data = []
       
       # Check if current object is a T61String (add similar types here if needed)
       if isinstance(asn1_object, char.T61String):
           raw_t61_data.append(asn1_object.get_substrate())
       
       # Recurse into sequence/set types (common nested structures)
       elif isinstance(asn1_object, (univ.Sequence, univ.Set, univ.SequenceOf, univ.SetOf)):
           for component in asn1_object:
               raw_t61_data.extend(extract_raw_t61_strings(component))
       
       # Handle choice types (another common nested pattern)
       elif hasattr(asn1_object, "getComponent"):
           try:
               active_component = asn1_object.getComponent()
               raw_t61_data.extend(extract_raw_t61_strings(active_component))
           except Exception:
               pass
       
       return raw_t61_data
   
   # Get all raw T61String substrate bytes from your decoded structure
   all_raw_t61 = extract_raw_t61_strings(decoded_object)
   

Why This Works #

• The get_substrate() method returns the exact bytes that were parsed from the original ASN.1 stream for that T61String—no re-encoding, no modifications. This is perfect for your encryption logic that requires strict byte matching.
• You can easily extend the isinstance check to include other similar character types (like char.UniversalString or char.PrintableString) if you need to capture those too.

Quick Note #

Make sure you’re using a recent version of pyasn1—older releases may have limited support for withSubstrate. Update it with:

pip install --upgrade pyasn1

内容的提问来源于stack exchange，提问作者PythonAteMyHamster