signxml 0.5.0

Python XML Signature library

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 (2.7-3.4+ 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

Installation

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:

Ubuntu, Python 2:

apt-get install python-dev python-cffi libxml2-dev libxslt1-dev libssl-dev

Ubuntu, Python 3:

apt-get install python3-dev python3-cffi libxml2-dev libxslt1-dev libssl-dev

Ubuntu 12.04: python-cffi is not available on 12.04. Use apt-get install libffi-dev followed by pip install cffi.

OS X: Use Homebrew:

brew install libxml2 libxslt
brew link --force libxml2 libxslt

Synopsis

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

from signxml import xmldsig

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

Verifying SAML assertions

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

from lxml import etree
from base64 import b64decode
from signxml import xmldsig

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

assertion_data = xmldsig(b64decode(assertion_body)).verify(x509_cert=cert)

Signing SAML assertions

The SAML assertion schema specifies a location for the enveloped XML signature (between <Issuer> and <Subject>). To sign a SAML assertion in a schema-compliant way, insert a signature placeholder tag at that location before calling xmldsig: <ds:Signature Id="placeholder"></ds:Signature>.

See what is signed

It is important to understand and follow the best practice rule of “See what is signed” when verifying XML signatures. The gist of this rule is: if your application neglects to verify that the information it trusts is what was actually signed, the attacker can supply a valid signature but point you to malicious data that wasn’t signed by that signature.

In SignXML, you can ensure that the information signed is what you expect to be signed by only trusting the data returned by the verify() method. The return value is the XML node or string that was signed.

Recommended reading: http://www.w3.org/TR/xmldsig-bestpractices/#practices-applications

XML signature methods: enveloped, detached, enveloping

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

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 xmldsig.verify(uri_resolver=...).

See the API documentation for more.

Authors

• Andrey Kislyuk

Links

• 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 Signature Best Practices

• XML-Signature Interoperability

• W3C Working Group Note: Test Cases for C14N 1.1 and XMLDSig Interoperability

• XMLSec: Related links

Bugs

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

License

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