feat(wsse): pure-Python WS-Security signing (no xmlsec required)

[martincollignon]

Summary

Adds zeep.wsse.crypto — a pure-Python alternative to the existing xmlsec-based zeep.wsse.signature module. Uses the cryptography library instead of the C-based xmlsec, making installation straightforward on all platforms.

Motivation: We build Landbruget.dk, a Danish agricultural data transparency project. Integrating with Denmark's VetStat SOAP API for antibiotic usage data required WS-Security features that zeep's current xmlsec-based module doesn't support — and installing xmlsec across CI/CD and developer machines was a constant pain point. We ended up writing ~300 lines of manual XML signing. This PR extracts that into a clean, general-purpose module that benefits everyone.

What's new

Feature	Current (xmlsec)	New (crypto)
Sign Body + Timestamp	✅	✅
Sign UsernameToken	❌	✅
Sign BinarySecurityToken	❌	✅
Sign arbitrary elements	❌	✅ (extra_references)
Inclusive namespace prefixes	❌	✅ (per-reference)
PKCS#12 key loading	❌	✅
PEM key loading	✅	✅
Configurable digest algorithm	Partial	✅ (SHA1/SHA256/SHA384/SHA512)
Configurable signature algorithm	Partial	✅ (RSA-SHA1/SHA256/SHA384/SHA512)
No C library dependency	❌	✅
Signature verification	✅	✅

Usage

from zeep.wsse.crypto import CryptoSignature, CryptoBinarySignature

# Drop-in replacement for wsse.Signature
sig = CryptoSignature("key.pem", "cert.pem")

# BinarySecurityToken variant
sig = CryptoBinarySignature("key.pem", "cert.pem")

# PKCS#12 support
sig = CryptoBinarySignature.from_pkcs12("cert.p12", b"password")

# Sign extra elements + custom C14N prefixes (e.g. for government SOAP APIs)
sig = CryptoBinarySignature(
    "key.pem", "cert.pem",
    sign_username_token=True,
    sign_binary_security_token=True,
    digest_method="http://www.w3.org/2001/04/xmlenc#sha256",
    inclusive_ns_prefixes={"Body": ["wsse", "ds"]},
)

# Works with Compose
from zeep.wsse import Compose
from zeep.wsse.username import UsernameToken
wsse = Compose([UsernameToken("user", "pass"), sig])

Install: pip install zeep[crypto]

Related issues

• Closes 4.2.1: pytest errors cannot load crypto library for xmlsec. #1357 (xmlsec installation failures)
• Relates to WSSE signature plugin for SecurityTokenReference #1419 (SecurityTokenReference plugin)
• Relates to signed parts #1428 (signed parts)
• Relates to Example for Generating Binary Security Token Key Identifier Signatures #1363 (BinarySecurityToken examples)
• Relates to xmlsec version mismatch error #1318 (xmlsec version mismatch)

Test plan

• 33 new tests covering all features (signing, verification, algorithm combos, PKCS12, inclusive prefixes, Compose integration, edge cases)
• Full existing test suite passes (504 passed, 0 failures)
• No changes to existing signature.py — fully additive
• Graceful fallback if cryptography not installed (classes set to None in __init__.py)

🤖 Generated with Claude Code