providers/saml: sign metadata when signing is enabled

authentik/providers/saml/processors/metadata.py

@@ -1,13 +1,16 @@
 """SAML Identity Provider Metadata Processor"""
 from typing import Iterator, Optional
 
+import xmlsec  # nosec
 from django.http import HttpRequest
 from django.shortcuts import reverse
 from lxml.etree import Element, SubElement, tostring  # nosec
 
 from authentik.providers.saml.models import SAMLProvider
+from authentik.providers.saml.utils import get_random_id
 from authentik.providers.saml.utils.encoding import strip_pem_header
 from authentik.sources.saml.processors.constants import (
+    DIGEST_ALGORITHM_TRANSLATION_MAP,
     NS_MAP,
     NS_SAML_METADATA,
     NS_SIGNATURE,
@@ -17,6 +20,7 @@ from authentik.sources.saml.processors.constants import (
     SAML_NAME_ID_FORMAT_PERSISTENT,
     SAML_NAME_ID_FORMAT_TRANSIENT,
     SAML_NAME_ID_FORMAT_X509,
+    SIGN_ALGORITHM_TRANSFORM_MAP,
 )
 
 
@@ -29,22 +33,21 @@ class MetadataProcessor:
     def __init__(self, provider: SAMLProvider, request: HttpRequest):
         self.provider = provider
         self.http_request = request
+        self.xml_id = get_random_id()
 
     def get_signing_key_descriptor(self) -> Optional[Element]:
         """Get Singing KeyDescriptor, if enabled for the provider"""
-        if self.provider.signing_kp:
+        if not self.provider.signing_kp:
+            return None
         key_descriptor = Element(f"{{{NS_SAML_METADATA}}}KeyDescriptor")
         key_descriptor.attrib["use"] = "signing"
         key_info = SubElement(key_descriptor, f"{{{NS_SIGNATURE}}}KeyInfo")
         x509_data = SubElement(key_info, f"{{{NS_SIGNATURE}}}X509Data")
-            x509_certificate = SubElement(
+        x509_certificate = SubElement(x509_data, f"{{{NS_SIGNATURE}}}X509Certificate")
-                x509_data, f"{{{NS_SIGNATURE}}}X509Certificate"
-            )
         x509_certificate.text = strip_pem_header(
             self.provider.signing_kp.certificate_data.replace("\r", "")
         )
         return key_descriptor
-        return None
 
     def get_name_id_formats(self) -> Iterator[Element]:
         """Get compatible NameID Formats"""
@@ -81,13 +84,64 @@ class MetadataProcessor:
             element.attrib["Location"] = url
             yield element
 
+    def _prepare_signature(self, entity_descriptor: Element):
+        sign_algorithm_transform = SIGN_ALGORITHM_TRANSFORM_MAP.get(
+            self.provider.signature_algorithm, xmlsec.constants.TransformRsaSha1
+        )
+        signature = xmlsec.template.create(
+            entity_descriptor,
+            xmlsec.constants.TransformExclC14N,
+            sign_algorithm_transform,
+            ns="ds",  # type: ignore
+        )
+        entity_descriptor.append(signature)
+
+    def _sign(self, entity_descriptor: Element):
+        digest_algorithm_transform = DIGEST_ALGORITHM_TRANSLATION_MAP.get(
+            self.provider.digest_algorithm, xmlsec.constants.TransformSha1
+        )
+        assertion = entity_descriptor.xpath("//md:EntityDescriptor", namespaces=NS_MAP)[
+            0
+        ]
+        xmlsec.tree.add_ids(assertion, ["ID"])
+        signature_node = xmlsec.tree.find_node(
+            assertion, xmlsec.constants.NodeSignature
+        )
+        ref = xmlsec.template.add_reference(
+            signature_node,
+            digest_algorithm_transform,
+            uri="#" + self.xml_id,
+        )
+        xmlsec.template.add_transform(ref, xmlsec.constants.TransformEnveloped)
+        xmlsec.template.add_transform(ref, xmlsec.constants.TransformExclC14N)
+        key_info = xmlsec.template.ensure_key_info(signature_node)
+        xmlsec.template.add_x509_data(key_info)
+
+        ctx = xmlsec.SignatureContext()
+
+        key = xmlsec.Key.from_memory(
+            self.provider.signing_kp.key_data,
+            xmlsec.constants.KeyDataFormatPem,
+            None,
+        )
+        key.load_cert_from_memory(
+            self.provider.signing_kp.certificate_data,
+            xmlsec.constants.KeyDataFormatCertPem,
+        )
+        ctx.key = key
+        ctx.sign(signature_node)
+
     def build_entity_descriptor(self) -> str:
         """Build full EntityDescriptor"""
         entity_descriptor = Element(
             f"{{{NS_SAML_METADATA}}}EntityDescriptor", nsmap=NS_MAP
         )
+        entity_descriptor.attrib["ID"] = self.xml_id
         entity_descriptor.attrib["entityID"] = self.provider.issuer
 
+        if self.provider.signing_kp:
+            self._prepare_signature(entity_descriptor)
+
         idp_sso_descriptor = SubElement(
             entity_descriptor, f"{{{NS_SAML_METADATA}}}IDPSSODescriptor"
         )
@@ -105,4 +159,7 @@ class MetadataProcessor:
         for binding in self.get_bindings():
             idp_sso_descriptor.append(binding)
 
+        if self.provider.signing_kp:
+            self._sign(entity_descriptor)
+
         return tostring(entity_descriptor, pretty_print=True).decode()