asn1

library3.3.0pypypi✓ verified 24d ago

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.

serialization

pip install asn1

github ↗package ↗

INSTALL

IMPORT

SIG · ASN1

asn1

serializationpythonv3.3.0

Install

1.6s avg

Import

18ms

Disk

16MB

Pass rate

10/ 10

Env Coverage10 / 10

glibc

3.9–3.13

musl

3.9–3.13

Install & Compatibility

Where this runs

tested against v3.3.0 · pip install

no network on importno background threads

Install × environment matrix

Each cell = how many times install + import succeeded across repeated harness runs. Partial = flaky.
glibc = Debian/Ubuntu slim · musl = Alpine Linux

musl

py 3.10–3.95 runs

installs and imports cleanly · install 0.0s · import 0.018s · 17.9MB

glibc

py 3.10–3.95 runs

installs and imports cleanly · install 1.6s · import 0.018s · 18MB

16MB installed

● package 16MB

Code

Verified usage

Verified import paths — ran on the pinned version, not inferred.

Encoder

✓ from asn1 import Encoder

Decoder

✓ from asn1 import Decoder

Numbers

✓ from asn1 import Numbers

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)

Debug

Known issues

breakingIn 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.

affects: 3.0.0

gotchaDue 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.

affects: All versions

gotchaVersion 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.

affects: Prior to 3.2.0

Errors

Common errors & fixes

ModuleNotFoundError: No module named 'asn1'

The 'asn1' module is not installed in the Python environment.

fix

Install the module using pip: 'pip install asn1'.

ImportError: cannot import name 'asn1' from 'cryptography.hazmat.bindings._rust' (unknown location)

The 'cryptography' package is not properly installed or is missing dependencies.

fix

Reinstall the 'cryptography' package ensuring all dependencies are met: 'pip install --force-reinstall cryptography'.

ImportError: cannot import name asn1

Attempting to import 'asn1' from 'pysnmp' which does not contain an 'asn1' module.

fix

Import 'asn1' directly: 'import asn1'.

asn1.Error: Premature end of input.

The decoder encountered incomplete or truncated ASN.1 data.

fix

Ensure the entire ASN.1 encoded data is read before decoding.

No field named 'r' defined for ASNBitcoinSignature

Incorrect import of 'Sequence' from 'asn1crypto.core' instead of 'pyasn1.type.univ'.

fix

Import 'Sequence' from 'pyasn1.type.univ': 'from pyasn1.type.univ import Sequence'.

Upgrade

Version history

3.3.0latest on PyPI · released May 15, 2026

Audit

Dependencies

futureoptionalRequired for Python 2.7 and 3.x compatibility in older environments. Not strictly needed for modern Python 3.5+ installations.

typingoptionalRequired for type hints in Python 2.7. Not needed for Python 3.5+ as 'typing' is part of the standard library.

Agent activity

33 hits · last 30 days

node

OpenAI (training)

Resources

githubhttps://github.com/andrivet/python-asn1 ↗

packagehttps://pypi.org/project/asn1/ ↗