pyasn1

Common errors

Warnings

Install

Install compatibility verified last tested: 2026-05-12

Imports

• Integer, OctetString, ObjectIdentifier

  from pyasn1.type.univ import Integer, OctetString, ObjectIdentifier

  All primitive ASN.1 scalar types live in pyasn1.type.univ

• Sequence, SequenceOf, Set, SetOf, Choice

  from pyasn1.type.univ import Sequence, SequenceOf, Set, SetOf, Choice

  Structured/constructed types also in pyasn1.type.univ

• NamedType, OptionalNamedType, DefaultedNamedType, NamedTypes

  from pyasn1.type.namedtype import NamedType, OptionalNamedType, DefaultedNamedType, NamedTypes

  Required when declaring SEQUENCE field names; wrong module is a common ImportError

• Tag, tagClassContext, tagFormatSimple

  from pyasn1.type.tag import Tag, tagClassContext, tagFormatSimple

  Used with .subtype(implicitTag=...) or .subtype(explicitTag=...) for context tagging

• encode (DER)
  wrong

  from pyasn1.codec.ber.encoder import encode

  correct

  from pyasn1.codec.der.encoder import encode

  Use the DER encoder for certificates and strict protocols; BER allows non-canonical forms that some validators reject

• decode (DER)

  from pyasn1.codec.der.decoder import decode

  Always unpack as (value, remainder) — decode() returns a 2-tuple, not just the value

• encode/decode (BER)

  from pyasn1.codec.ber.encoder import encode
  from pyasn1.codec.ber.decoder import decode

  BER is the permissive superset; use for parsing untrusted or legacy inputs where indefinite length is possible

• encode/decode (native Python dicts)

  from pyasn1.codec.native.encoder import encode
  from pyasn1.codec.native.decoder import decode

  Converts pyasn1 objects to/from plain Python dicts/lists/ints; useful for JSON interop

• PyAsn1Error

  from pyasn1.error import PyAsn1Error

  Base exception for all pyasn1 errors; catch this for encoder/decoder failures

Quickstart verified last tested: 2026-04-23

Define an ASN.1 SEQUENCE schema, populate it, DER-encode it, then decode it back and verify round-trip equality.

from pyasn1.type.univ import Sequence, Integer
from pyasn1.type.namedtype import NamedType, OptionalNamedType, DefaultedNamedType, NamedTypes
from pyasn1.type.tag import Tag, tagClassContext, tagFormatSimple
from pyasn1.codec.der.encoder import encode
from pyasn1.codec.der.decoder import decode
from pyasn1.error import PyAsn1Error

# 1. Define ASN.1 schema as a Python class
class Record(Sequence):
    componentType = NamedTypes(
        NamedType('id', Integer()),
        OptionalNamedType(
            'room',
            Integer().subtype(implicitTag=Tag(tagClassContext, tagFormatSimple, 0))
        ),
        DefaultedNamedType(
            'house',
            Integer(0).subtype(implicitTag=Tag(tagClassContext, tagFormatSimple, 1))
        ),
    )

# 2. Populate the schema object
record = Record()
record['id'] = 123
record['room'] = 321

# 3. DER-encode to bytes
substrate = encode(record)
print('DER bytes:', substrate.hex())  # e.g. 30070201 7b800201 41

# 4. Decode back — ALWAYS unpack the 2-tuple (value, remainder)
try:
    received, remainder = decode(substrate, asn1Spec=Record())
except PyAsn1Error as exc:
    raise SystemExit(f'Decode failed: {exc}')

assert remainder == b'', 'Unexpected trailing bytes'
assert received['id'] == record['id']
print('Round-trip OK:', received.prettyPrint())