Implement CBOR parser following ASN.1 paradigm with full test coverage

[Copilot]

Overview

Implements RFC 8949 CBOR (Concise Binary Object Representation) parser and encoder in Scapy, following the same architectural pattern as ASN.1 (separation of type system from codec).

Implementation

Core Components (scapy/cbor/)

• cbor.py: Type system with CBOR_Object base class, major type objects (0-7), and CBOR_MajorTypes registry using metaclass pattern
• cborcodec.py: Codec with CBORcodec_Object encoders/decoders, including IEEE 754 half-precision float conversion
• RandCBORObject: Fuzzing support with gaussian distribution for integers, exponential sizing for collections, depth limiting for nested structures

Type Coverage

• Integers (unsigned/negative), byte/text strings (UTF-8), arrays, maps, semantic tags
• Simple values (true, false, null, undefined)
• Floats (16/32/64-bit IEEE 754)

Testing

Test Suite (test/scapy/layers/cbor.uts)

• 161 total tests: 119 core tests + 42 cbor2 interoperability tests
• Roundtrip encoding/decoding validation
• RFC 8949 compliance verification (specific hex encodings, boundary values)
• Stress tests with random generation (100+ objects)
• Performance optimized: auto-switch to simple types in recursive structures to prevent exponential nesting

Interoperability

• Bidirectional compatibility with cbor2 library (test-only dependency)
• Both Scapy→cbor2 and cbor2→Scapy tested

Platform Fixes

Windows UTF-8 handling (scapy/tools/UTscapy.py)

# Fixed file opens to use UTF-8 explicitly
with open(test_file, encoding='utf-8') as f:  # Was: open(test_file)

Windows defaults to cp1252, causing 'café' to misread as 'café'.

UTScapy test format

• Added blank lines between compound statements and assertions for Python's interactive parser

Documentation

Added CBOR section to doc/scapy/advanced_usage.rst mirroring ASN.1 structure with usage examples, type descriptions, and interoperability notes.

Dependencies

Added cbor2 to tox.ini test dependencies for interoperability validation (used only in tests, not in implementation).

Example Usage

from scapy.cbor import *

# Create and encode
obj = CBOR_MAP({
    'key': CBOR_TEXT_STRING('value'),
    'number': CBOR_UNSIGNED_INTEGER(42)
})
encoded = bytes(obj)

# Decode
decoded, remainder = CBOR_Codecs.CBOR.dec(encoded)

# Fuzzing
rand_obj = RandCBORObject()._fix()

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