Overview

DERlite is a encoder and parser for DER-encoded (and some BER-encoded) ASN.1 data.

DERlite requires Python 3.5 (particularly for some features of the bytes and int classes). It has no hard dependencies beyond the standard library, but requires T.61 and ISO-2022 codecs to be available in order to decode all valid string types.

Features

• Straightforward encoding of most primitive types (except character strings), SETs, SEQUENCEs, and implicit and explicit tags.
• Correct decoding of character string types.
• Complex application-specific types can be supported in an object-oriented way.
• Fairly easy to hand-write a parser and an encoder for your chosen ASN.1 structures.
• Easy to mix ad-hoc and more structured styles.
• 100% python, compatible with version 3.5 and higher
• the module is a single file and is MIT-licensed; it can be used via PyPI or simply included in a larger project

Missing Features

Datetimes always use GeneralizedTime rather than UTCTime. This is usually preferable, except that some specifications such as PKIX require the use of UTCTime for dates during the late 20th century.

There's no support for many BER features, in particular, indefinite-length encodings and constructed primitives.

The entire encoded object must fit in a bytes object; there is no support for streaming either during encoding or decoding.

Usage Example

Decoding a PKIX RSAPublicKey structure:

from derlite import Decoder, Tag

decoder = Decoder(some_bytes)
decoder.enter(Tag.Sequence)
modulus = decoder.read_integer()
exponent = decoder.read_integer()
decoder.leave()

Creating an LDAP AddRequest message to add two attributes to a given DN, assuming that dn, attr1, and attr2 are objects implementing the .encode_der() or .as_der() informal protocol methods:

from derlite import Encoder, Tag

addRequestTag = Tag(16, cls=Tag.Application)

encoder = Encoder()
encoder.enter(abandonRequestTag)
encoder.enter(Tag.Sequence)
encoder.write(dn)
encoder.write( [ attr1, attr2 ] )
encoder.leave()
encoder.leave()

some_bytes = encoder.getvalue()

In addition to the Encoder and Decoder classes, DERlite provides a Tag class for storing tag numbers along with their tag-class and constructed flag; Oid for manipulating OIDs; and OptionBitSet for handling sets of flags stored in BIT STRINGs (as is common in some ASN.1 specifications).

Other Resources

Other modules providing DER/BER functionality:

• pyasn1 is a full-featured ASN.1 implementation, including the ability to parse ASN.1 specifications and generate codec classes.
• asn1 is similar to (and the inspiration for) DERlite. It is less featureful, but a more mature project.
• python-asn1crypto is focused specifically on PKIX/X.500 structures.

To get a quick understanding of BER and DER, see A Layman's Guide to a Subset of ASN.1, BER, and DER. For a more comprehensive description of the standard, try the books on this page, in particular Olivier Dubuisson's, which can be read for free.

For a look into some of the horrors of real-world ASN.1 implementations, see Peter Gutmann's X.509 Style Guide.

And finally, some (but not all) of the underlying standards can be downloaded from ITU or ECMA.

History

DERlite began as a set of conveniences and extensions to the asn1 module by Geert Jansen and Sebastien Andrivet. The current interface is incompatible in many ways (hence the new name), and little or none of the original code remains, but usage should be similar.

The name DERlite was chosen because this module is lighter-weight than a full ASN.1 implementation such as pyasn1, and is hopefully pleasant to use (though it would be an exaggeration to say that working with ASN.1 is a "delight").

Author

DERlite was written by Wim Lewis (wiml@hhhh.org). It may be used under the terms of the MIT License.

Bug reports, suggestions, and patches are gratefully accepted via Github or via email.