googleapis · aeitzman · Mar 25, 2025 · Mar 17, 2025 · Mar 19, 2025 · Mar 19, 2025
@@ -41,6 +41,7 @@
 except ImportError:  # pragma: NO COVER
    from collections import Mapping  # type: ignore
 import abc
+import base64
 import json
 import os
 from typing import NamedTuple
@@ -145,9 +146,88 @@ def get_subject_token(self, context, request):
 class _X509Supplier(SubjectTokenSupplier):
    """Internal supplier for X509 workload credentials. This class is used internally and always returns an empty string as the subject token."""

+    def __init__(self, trust_chain_path, leaf_cert_callback):
+        self._trust_chain_path = trust_chain_path
+        self._leaf_cert_callback = leaf_cert_callback
+
    @_helpers.copy_docstring(SubjectTokenSupplier)
    def get_subject_token(self, context, request):
-        return ""
+        # Import OpennSSL inline because it is an extra import only required by customers
+        # using mTLS.
+        from OpenSSL import crypto
+
+        leaf_cert = crypto.load_certificate(
+            crypto.FILETYPE_PEM, self._leaf_cert_callback()
+        )
+        trust_chain = self._read_trust_chain()
+        cert_chain = []
+
+        cert_chain.append(_X509Supplier._encode_cert(leaf_cert))
+
+        if trust_chain is None or len(trust_chain) == 0:
+            return json.dumps(cert_chain)
+
+        # Append the first cert if it is not the leaf cert.
+        first_cert = _X509Supplier._encode_cert(trust_chain[0])
+        if first_cert != cert_chain[0]:
+            cert_chain.append(first_cert)
+
+        for i in range(1, len(trust_chain)):
+            encoded = _X509Supplier._encode_cert(trust_chain[i])
+            # Check if the current cert is the leaf cert and raise an exception if it is.
+            if encoded == cert_chain[0]:
+                raise exceptions.RefreshError(
+                    "The leaf certificate must be at the top of the trust chain file"
+                )
+            else:
+                cert_chain.append(encoded)
+        return json.dumps(cert_chain)
+
+    def _read_trust_chain(self):
+        # Import OpennSSL inline because it is an extra import only required by customers
+        # using mTLS.
+        from OpenSSL import crypto
+
+        certificate_trust_chain = []
+        # If no trust chain path was provided, return an empty list.
+        if self._trust_chain_path is None or self._trust_chain_path == "":
+            return certificate_trust_chain
+        try:
+            # Open the trust chain file.
+            with open(self._trust_chain_path, "rb") as f:
+                trust_chain_data = f.read()
+                # Split PEM data into individual certificates.
+                cert_blocks = trust_chain_data.split(b"-----BEGIN CERTIFICATE-----")
+                for cert_block in cert_blocks:
+                    # Skip empty blocks.
+                    if cert_block.strip():
+                        cert_data = b"-----BEGIN CERTIFICATE-----" + cert_block
+                        try:
+                            # Load each certificate and add it to the trust chain.
+                            cert = crypto.load_certificate(
+                                crypto.FILETYPE_PEM, cert_data
+                            )
+                            certificate_trust_chain.append(cert)
+                        except Exception as e:
+                            raise exceptions.RefreshError(
+                                "Error loading PEM certificates from the trust chain file '{}'".format(
+                                    self._trust_chain_path
+                                )
+                            ) from e
+                return certificate_trust_chain
+        except FileNotFoundError:
+            raise exceptions.RefreshError(
+                "Trust chain file '{}' was not found.".format(self._trust_chain_path)
+            )
+
+    def _encode_cert(cert):
+        # Import OpennSSL inline because it is an extra import only required by customers
+        # using mTLS.
+        from OpenSSL import crypto
+
+        return base64.b64encode(
+            crypto.dump_certificate(crypto.FILETYPE_ASN1, cert)
+        ).decode("utf-8")


 def _parse_token_data(token_content, format_type="text", subject_token_field_name=None):
@@ -296,7 +376,9 @@ def __init__(
                    self._credential_source_headers,
                )
            else:  # self._credential_source_certificate
-                self._subject_token_supplier = _X509Supplier()
+                self._subject_token_supplier = _X509Supplier(
+                    self._trust_chain_path, self._get_cert_bytes
+                )

    @_helpers.copy_docstring(external_account.Credentials)
    def retrieve_subject_token(self, request):
@@ -314,6 +396,10 @@ def _get_mtls_cert_and_key_paths(self):
                self._certificate_config_location
            )

+    def _get_cert_bytes(self):
+        cert_path, _ = self._get_mtls_cert_and_key_paths()
+        return _mtls_helper._read_cert_file(cert_path)
+
    def _mtls_required(self):
        return self._credential_source_certificate is not None

@@ -350,6 +436,9 @@ def _validate_certificate_config(self):
        use_default = self._credential_source_certificate.get(
            "use_default_certificate_config"
        )
+        self._trust_chain_path = self._credential_source_certificate.get(
+            "trust_chain_path"
+        )
        if self._certificate_config_location and use_default:
            raise exceptions.MalformedError(
                "Invalid certificate configuration, certificate_config_location cannot be specified when use_default_certificate_config = true."

@@ -0,0 +1,52 @@
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----
@@ -0,0 +1,33 @@
+-----BEGIN CERTIFICATE-----
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@@ -0,0 +1,52 @@
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