SignXML: XML Signature in Python

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 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 (2.7-3.6+ 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:

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 python-defusedxml 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 python3-defusedxml 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`
OS X/Homebrew		`xcode-select --install; brew install openssl;` `export LDFLAGS="-L$(brew --prefix openssl)/lib" CFLAGS="-I$(brew --prefix openssl)/include"`

Synopsis

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(data_to_sign)
signed_root = XMLSigner().sign(root, key=key, cert=cert)
verified_data = XMLVerifier().verify(signed_root).signed_xml

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

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 XMLSigner: <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. 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.

Establish trust

If you do not supply any keyword arguments to verify(), the default behavior is to trust any valid XML signature generated using a valid X.509 certificate trusted by your system's CA store. This means anyone can get an SSL certificate and generate a signature that you will trust. To establish trust in the signer, use the x509_cert argument to specify a certificate that was pre-shared out-of-band (e.g. via SAML metadata, as shown in Verifying SAML assertions), or cert_subject_name to specify a subject name that must be in the signing X.509 certificate given by the signature (verified as if it were a domain name), or ca_pem_file/ca_path to give a custom CA.

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.