signxml

Python XML Signature library

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Project description

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 and WS-Security, among other uses. Two versions of the standard exist (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 a libxml2-based XML parser configured to defend against common XML 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 (2.7-3.8+ and PyPy)
Well-supported, portable, reliable dependencies: lxml, cryptography, eight, pyOpenSSL
Comprehensive testing (including the XMLDSig interoperability suite) and continuous integration
Simple interface with useful defaults
Compactness, readability, and extensibility

Installation

pip3 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:

OS	Python	Command
Ubuntu	Python 2	apt-get install python-dev python-cffi libxml2-dev libxslt1-dev libssl-dev libffi-dev python-lxml python-cryptography python-openssl python-certifi build-essential
Ubuntu	Python 3	apt-get install python3-dev python3-cffi libxml2-dev libxslt1-dev libssl-dev libffi-dev python3-lxml python3-cryptography python3-openssl python3-certifi build-essential
Red Hat	Python 2	yum install python-devel python-cffi libxml2-devel libxslt1-devel openssl-devel
Red Hat	Python 3	yum install python3-devel python3-cffi libxml2-devel libxslt1-devel openssl-devel
Mac OS	Homebrew Python	Install Homebrew, then run brew install python.

Synopsis

SignXML uses the lxml ElementTree API to work with XML data.

from lxml import etree
from signxml import XMLSigner, XMLVerifier

data_to_sign = "<Test/>"
cert = open("example.pem", "rb").read()
key = open("example.key", "rb").read()
root = etree.fromstring(data_to_sign)
signed_root = XMLSigner().sign(root, key=key, cert=cert)
verified_data = XMLVerifier().verify(signed_root).signed_xml

To make this example self-sufficient for test purposes:

Generate a test certificate and key using openssl req -x509 -sha256 -nodes -subj "/CN=test" -days 1 -newkey rsa:2048 -keyout example.key -out example.pem.
Pass the x509_cert=cert keyword argument to XMLVerifier.verify(). (In production, ensure this is replaced with the correct configuration for the trusted CA or certificate - this determines which signatures your software trusts.)

Verifying SAML assertions

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

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. Failure to follow this rule can lead to vulnerability against attacks like SAML signature wrapping.

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 signed_xml attribute of the return value is the XML node or string that was signed.

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 = 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.

XML parsing security and compatibility with xml.etree.ElementTree

SignXML uses the lxml ElementTree library, not the ElementTree from Python’s standard library, to work with XML. lxml is used due to its superior resistance to XML attacks, as well as XML canonicalization and namespace organization features. It is recommended that you pass XML string input directly to signxml before further parsing, and use lxml to work with untrusted XML input in general. If you do pass trusted xml.etree.ElementTree objects to SignXML, you should be aware of differences in XML namespace handling between the two libraries. See the following references for more information:

Authors

Andrey Kislyuk

License

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

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4.0.2

Sep 11, 2024

4.0.1

Aug 30, 2024

4.0.0

Aug 22, 2024

3.2.2

Jan 29, 2024

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Aug 6, 2023

3.2.0

Apr 13, 2023

3.1.1

Apr 8, 2023

3.1.0

Jan 4, 2023

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Nov 29, 2022

3.0.1

Nov 27, 2022

3.0.0

Nov 14, 2022

2.10.1

Sep 9, 2022

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Aug 21, 2022

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Oct 8, 2021

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May 14, 2021

2.8.1

Oct 29, 2020

2.8.0

Jun 21, 2020

2.7.3

Jun 10, 2020

2.7.2

Dec 2, 2019

2.7.1

Nov 30, 2019

This version

2.7.0

Nov 30, 2019

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0.1.5

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0.1.4

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0.1.3

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0.1.2

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0.1.1

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0.1.0

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0.0.1

Sep 3, 2014

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signxml 2.7.0

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Verifying SAML assertions

XML signature methods: enveloped, detached, enveloping

XML parsing security and compatibility with xml.etree.ElementTree

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