SignXML: XML Signature in Python

SignXML is an implementation of the W3C XML Signature standard in Python (both "Second Edition" and Version 1.1). This standard (also known as XMLDSig and RFC 3275) is used to provide payload security in SAML 2.0, among other uses. 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. On Ubuntu, you can install those with:

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

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

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)
xmldsig(root).sign(key=key, cert=cert)
verified_data = xmldsig(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.

Detached signatures

The XML Signature specification requires support of detached signatures, where the signature document refers (in <Reference URI="...">) to an external document. SignXML does not support generating detached signatures. To verify a detached signature, pass a resolver callable to the xmldsig.verify() method.

See the API documentation for more.