Python-ASN1

3.2.0 · active · verified Sat Apr 11

Python-ASN1 is a straightforward library for encoding and decoding ASN.1 data, compatible with Python 2.7+ and 3.5+. It supports both BER (Basic Encoding Rules) for parsing and DER (Distinguished Encoding Rules) for parsing and generation, including indefinite lengths. The library is written entirely in Python and supports most common ASN.1 types, including the REAL type. The latest version is 3.2.0, with a regular release cadence addressing new Python versions and API enhancements.

Warnings

breaking In version 3.0.0, the `Encoder` incorrectly defaulted to CER encoding, diverging from previous versions which used DER. This was a significant behavioral change for users who did not explicitly specify the encoding method.
Fix: Upgrade to version 3.0.1 or later. If staying on 3.0.0, explicitly specify `asn1.Encoder.DER` when initializing the encoder or `start()` method if DER is desired, or be aware that CER will be used by default without a stream, and DER with a stream.
gotcha Due to ASN.1 having more distinct data types than Python, a single Python type might map to multiple ASN.1 types. The library uses the most frequently expected ASN.1 type by default. If a different ASN.1 type (e.g., 'IA5String' vs. 'UTF8String' for a Python string) is intended, it must be explicitly specified during encoding using `encoder.write(value, asn1.Numbers.DesiredType)`.
Fix: Always explicitly specify the ASN.1 `Numbers` type (e.g., `asn1.Numbers.IA5String`) when encoding a Python value if the default mapping is not the desired ASN.1 type.
gotcha Version 3.2.0 introduced a new, more Pythonic context manager API for `Encoder` and `Decoder` (e.g., `with asn1.Encoder() as encoder:`). While the older `encoder.start()` / `encoder.output()` methods remain backward-compatible, the context manager approach is recommended for cleaner code and proper resource handling, especially for constructed types and streams.
Fix: Migrate encoding and decoding logic to use the `with asn1.Encoder(...) as encoder:` and `with asn1.Decoder(...) as decoder:` syntax for improved readability and maintainability. Review the documentation for examples of the new API.

Install

pip install asn1 Install latest version

Imports

Encoder
```
from asn1 import Encoder
```
Decoder
```
from asn1 import Decoder
```
Numbers
```
from asn1 import Numbers
```

Quickstart

This quickstart demonstrates encoding and decoding a simple ObjectIdentifier and a Sequence containing an Integer and a UTF8String. It utilizes the modern context manager API introduced in version 3.2.0 for both encoding and decoding, which simplifies resource management and constructed type handling.

import asn1

# 1. Encoding an ObjectIdentifier and a Sequence
with asn1.Encoder() as encoder:
    # Encode an ObjectIdentifier (e.g., "1.2.3")
    encoder.write('1.2.3', asn1.Numbers.ObjectIdentifier)

    # Encode a Sequence containing an Integer and a UTF8String
    with encoder.enter(asn1.Numbers.Sequence) as sequence_encoder:
        sequence_encoder.write(123, asn1.Numbers.Integer)
        sequence_encoder.write('hello', asn1.Numbers.UTF8String)

    encoded_bytes = encoder.output()
print(f"Encoded bytes: {encoded_bytes.hex()}")

# 2. Decoding the ASN.1 data
decoded_elements = []
with asn1.Decoder(stream=encoded_bytes) as decoder:
    # Decode the first element (ObjectIdentifier)
    tag, value = decoder.read()
    decoded_elements.append(f"Tag: {tag.nr} (ObjectIdentifier), Class: {tag.cls}, Value: {value}")

    # Decode the next element, which is the Sequence
    tag, value = decoder.read()
    if tag.nr == asn1.Numbers.Sequence:
        decoded_elements.append(f"Tag: {tag.nr} (Sequence), Class: {tag.cls}")
        # Enter the sequence to decode its elements
        with decoder.enter() as sequence_decoder:
            while not sequence_decoder.eof():
                tag, value = sequence_decoder.read()
                decoded_elements.append(f"  -> Tag: {tag.nr}, Class: {tag.cls}, Value: {value}")

print("\nDecoded data:")
for item in decoded_elements:
    print(item)

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