SignXML is an implementation of the W3C XML Signature standard in Python. This standard (also known as XMLDSig and RFC 3275) is used to provide payload security in SAML 2.0, among other uses. Three versions of the standard exist (Version 1 "Second Edition", Version 1.1, and Version 2.0). SignXML implements all of the required components of the standard, and most recommended ones. Its features are:

• Use of defusedxml.lxml to defend against common XML-based attacks when verifying signatures
• Extensions to allow signing with and verifying X.509 certificate chains, including hostname/CN validation
• Support for exclusive XML canonicalization with inclusive prefixes (InclusiveNamespaces PrefixList, required to verify signatures generated by some SAML implementations)
• Modern Python compatibility (2.7-3.5+ and PyPy)
• Well-supported, portable, reliable dependencies: lxml, defusedxml, cryptography, eight, pyOpenSSL
• Comprehensive testing (including the XMLDSig interoperability suite) and continuous integration
• Simple interface with useful defaults
• Compactness, readability, and extensibility

pip install signxml

Note: SignXML depends on lxml and cryptography, which in turn depend on OpenSSL, LibXML, and Python tools to interface with them. You can install those as follows:

SignXML uses the ElementTree API (also supported by lxml) to work with XML data.

from signxml import XMLSigner, XMLVerifier

cert = open("example.pem").read()
key = open("example.key").read()
root = ElementTree.fromstring(signature_data)
signed_root = XMLSigner().sign(root, key=key, cert=cert)
verified_data = XMLVerifier().verify(signed_root).signed_xml

Assuming metadata.xml contains SAML metadata for the assertion source:

from lxml import etree
from base64 import b64decode
from signxml import XMLVerifier

with open("metadata.xml", "rb") as fh:
    cert = etree.parse(fh).find("//ds:X509Certificate").text

assertion_data = XMLVerifier().verify(b64decode(assertion_body), x509_cert=cert).signed_xml

The XML Signature specification defines three ways to compose a signature with the data being signed: enveloped, detached, and enveloping signature. Enveloped is the default method. To specify the type of signature that you want to generate, pass the method argument to sign():

signed_root = XMLSigner(method=signxml.methods.detached).sign(root, key=key, cert=cert)
verified_data = XMLVerifier().verify(signed_root).signed_xml

For detached signatures, the code above will use the Id or ID attribute of root to generate a relative URI (<Reference URI="#value"). You can also override the value of URI by passing a reference_uri argument to sign(). To verify a detached signature that refers to an external entity, pass a callable resolver in XMLVerifier().verify(data, uri_resolver=...).

See the API documentation for more.

• Andrey Kislyuk

• Project home page (GitHub)
• Documentation (Read the Docs)
• Package distribution (PyPI)
• Change log
• List of W3C XML Signature standards and drafts
• W3C Recommendation: XML Signature Syntax and Processing (Second Edition)
• W3C Recommendation: XML Signature Syntax and Processing Version 1.1
• W3C Working Group Note: XML Signature Syntax and Processing Version 2.0
• W3C Working Group Note: XML Security 2.0 Requirements and Design Considerations
• W3C Working Group Note: XML Signature Best Practices
• XML-Signature Interoperability
• W3C Working Group Note: Test Cases for C14N 1.1 and XMLDSig Interoperability
• XMLSec: Related links
• OWASP SAML Security Cheat Sheet

Please report bugs, issues, feature requests, etc. on GitHub.

Licensed under the terms of the Apache License, Version 2.0.