pycddl

PyCDDL: Deserialize CBOR and/or do CDDL schema validation

CDDL is a schema language for the CBOR serialization format. pycddl allows you to:

• Validate CBOR documents match a particular CDDL schema, based on the Rust cddl library.
• Optionally, decode CBOR documents.

Usage

Validation

Here we use the cbor2 library to serialize a dictionary to CBOR, and then validate it:

from pycddl import Schema
import cbor2

uint_schema = Schema("""
    object = {
        xint: uint
    }
"""
)
uint_schema.validate_cbor(cbor2.dumps({"xint", -2}))

If validation fails, a pycddl.ValidationError is raised.

Validation + deserialization

You can deserialize CBOR to Python objects using cbor.loads(). However:

• cbor2 uses C code by default, and the C programming language is prone to memory safety issues. If you are reading untrusted CBOR, better to use a Rust library to decode the data.
• You will need to parse the CBOR twice, once for validation and once for decoding, adding performance overhead.

By deserializing with pycddl, you solve the first problem, and a future version of pycddl will solve the second problem (see https://gitlab.com/tahoe-lafs/pycddl/-/issues/37).

from pycddl import Schema
import cbor2

uint_schema = Schema("""
    object = {
        xint: uint
    }
"""
)
deserialized = uint_schema.validate_cbor(cbor2.dumps({"xint", -2}), True)
assert deserialized == {"xint": -2}

Deserializing without schema validation

If you don't care about schemas, you can just deserialize the CBOR like so:

from pycddl import Schema

ACCEPT_ANYTHING = Schema("main = any")

def loads(encoded_cbor_bytes):
    return ACCEPT_ANYTHING.validate_cbor(encoded_cbor_bytes, True)

In a future release this will become a standalone, more efficient API, see https://gitlab.com/tahoe-lafs/pycddl/-/issues/36

Reducing memory usage and safety constraints

In order to reduce memory usage, you can pass in any Python object that implements the buffer API and stores bytes, e.g. a memoryview() or a mmap object.

The passed-in object must be read-only, and the data must not change during validation! If you mutate the data while validation is happening the result can be memory corruption or other undefined behavior.

Supported CBOR types for deserialization

If you are deserializing a CBOR document into Python objects, you can deserialize:

• Null/None.
• Booleans.
• Floats.
• Integers up to 64-bit size. Larger integers aren't supported yet.
• Bytes.
• Strings.
• Lists.
• Maps/dictionaries.
• Sets.

Other types will be added in the future if there is user demand.

Schema validation is not restricted to this list, but rather is limited by the functionality of the cddl Rust crate.

Release notes

0.6.4

Features:

• Update Rust CDDL dependency to update transitive dependency lexical-core which had been revised (i.a.) because of soundness/safety issues.

0.6.3

Features:

• Support final 3.13.

0.6.2

Features:

• Added support for Python 3.13.

0.6.1

Bug fixes:

• Allow PyPy to deserialize CBOR too.

0.6.0

Features:

• validate_cbor(serialized_cbor, True) will deserialize the CBOR document into Python objects, so you don't need to use e.g. cbor2.loads(serialized_cbor) separately.

Bug fixes:

• Release the GIL in a much more useful way.

Misc:

• Upgrade to newer cddl crate (0.9.4).

0.5.3

• Upgrade to newer cddl crate (0.9.3).
• Add Python 3.12 support.

0.5.2

• Upgrade to newer cddl crate (0.9.2), improving validation functionality.

0.5.1

• Upgrade to newer cddl crate, fixing some validation bugs.

0.5.0

• Support for ARM macOS.
• Dropped Python 3.7 support.

0.4.1

• Test fixes, with no user-relevant changes.

0.4.0

• validate_cbor() now accepts read-only buffers, not just bytes. This is useful if you want to e.g. validate a large file, since you can mmap() it.
• The GIL is released when parsing documents larger than 10KiB.

0.3.0

• Fixed major bug where if the document was valid UTF-8, the library would attempt to parse it as JSON!
• Added support for ARM macOS.

0.2.2

• Updated to cddl 0.9.1.

0.2.1

• Added PyPy wheels.

0.2.0

• Schemas are now only parsed once (when the Schema() object is created), instead of every time validation happens, which should improve validation performance.
• Updated to a newer version of underlying CDDL library, which should make CDDL parsing more compliant.
• Added a repr() implementation to Schema for easier debugging.

0.1.11

• Initial release.