feat: Support transport and binding-enforcement MDS parameters.

oauth2_http/java/com/google/auth/oauth2/ComputeEngineCredentials.java

@@ -109,6 +109,40 @@ public class ComputeEngineCredentials extends GoogleCredentials
   static final int MAX_COMPUTE_PING_TRIES = 3;
   static final int COMPUTE_PING_CONNECTION_TIMEOUT_MS = 500;
 
+  public enum GoogleAuthTransport {
+    // Authenticating to Google APIs via DirectPath
+    ALTS("alts"),
+    // Authenticating to Google APIs via GFE
+    MTLS("mtls");
+
+    private final String label;
+
+    private GoogleAuthTransport(String label) {
+      this.label = label;
+    }
+
+    public String getLabel() {
+      return label;
+    }
+  }
+
+  public enum BindingEnforcement {
+    // Binding enforcement will always happen, irrespective of the IAM policy.
+    ON("on"),
+    // Binding enforcement will depend on IAM policy.
+    IAM_POLICY("iam-policy");
+
+    private final String label;
+
+    private BindingEnforcement(String label) {
+      this.label = label;
+    }
+
+    public String getLabel() {
+      return label;
+    }
+  }
+
   private static final String METADATA_FLAVOR = "Metadata-Flavor";
   private static final String GOOGLE = "Google";
   private static final String WINDOWS = "windows";
@@ -122,6 +156,9 @@ public class ComputeEngineCredentials extends GoogleCredentials
 
   private final Collection<String> scopes;
 
+  private final GoogleAuthTransport transport;
+  private final BindingEnforcement bindingEnforcement;
+
   private transient HttpTransportFactory transportFactory;
   private transient String serviceAccountEmail;
 
@@ -152,6 +189,8 @@ private ComputeEngineCredentials(ComputeEngineCredentials.Builder builder) {
       scopeList.removeAll(Arrays.asList("", null));
       this.scopes = ImmutableSet.<String>copyOf(scopeList);
     }
+    this.transport = builder.getGoogleAuthTransport();
+    this.bindingEnforcement = builder.getBindingEnforcement();
   }
 
   @Override
@@ -191,7 +230,10 @@ public final Collection<String> getScopes() {
   }
 
   /**
-   * If scopes is specified, add "?scopes=comma-separated-list-of-scopes" to the token url.
+   * If scopes is specified, add "?scopes=comma-separated-list-of-scopes" to the token url. If
+   * transport is specified, add "?transport=xyz" to the token url; xyz is one of "alts" or "mtls".
+   * If bindingEnforcement is specified, add "?binding-enforcement=xyz" to the token url; xyz is one
+   * of "iam-policy" or "on".
    *
    * @return token url with the given scopes
    */
@@ -200,6 +242,12 @@ String createTokenUrlWithScopes() {
     if (!scopes.isEmpty()) {
       tokenUrl.set("scopes", Joiner.on(',').join(scopes));
     }
+    if (transport != null) {
+      tokenUrl.set("transport", transport.getLabel());
+    }
+    if (bindingEnforcement != null) {
+      tokenUrl.set("binding-enforcement", bindingEnforcement.getLabel());
+    }
     return tokenUrl.toString();
   }
 
@@ -643,6 +691,9 @@ public static class Builder extends GoogleCredentials.Builder {
     private Collection<String> scopes;
     private Collection<String> defaultScopes;
 
+    private GoogleAuthTransport transport;
+    private BindingEnforcement bindingEnforcement;
+
     protected Builder() {
       setRefreshMargin(COMPUTE_REFRESH_MARGIN);
       setExpirationMargin(COMPUTE_EXPIRATION_MARGIN);
@@ -684,6 +735,28 @@ public Builder setQuotaProjectId(String quotaProjectId) {
       return this;
     }
 
+    /**
+     * Set the {@code GoogleAuthTransport} type.
+     *
+     * @param transport the transport type over which to authenticate to Google APIs
+     */
+    @CanIgnoreReturnValue
+    public Builder setGoogleAuthTransport(GoogleAuthTransport transport) {
+      this.transport = transport;
+      return this;
+    }
+
+    /**
+     * Set the {@code BindingEnforcement} type.
+     *
+     * @param bindingEnforcement the token binding enforcement policy.
+     */
+    @CanIgnoreReturnValue
+    public Builder setBindingEnforcement(BindingEnforcement bindingEnforcement) {
+      this.bindingEnforcement = bindingEnforcement;
+      return this;
+    }
+
     public HttpTransportFactory getHttpTransportFactory() {
       return transportFactory;
     }
@@ -696,6 +769,24 @@ public Collection<String> getDefaultScopes() {
       return defaultScopes;
     }
 
+    /**
+     * Get the {@code GoogleAuthTransport} type.
+     *
+     * @return the transport type over which to authenticate to Google APIs
+     */
+    public GoogleAuthTransport getGoogleAuthTransport() {
+      return transport;
+    }
+
+    /**
+     * Get the {@code BindingEnforcement} type.
+     *
+     * @return the token binding enforcement policy.
+     */
+    public BindingEnforcement getBindingEnforcement() {
+      return bindingEnforcement;
+    }
+
     @Override
     public ComputeEngineCredentials build() {
       return new ComputeEngineCredentials(this);

oauth2_http/javatests/com/google/auth/oauth2/ComputeEngineCredentialsTest.java

@@ -190,6 +190,110 @@ public void buildTokenUrlWithScopes_defaultScopes() {
     assertEquals("bar", scopes.toArray()[1]);
   }
 
+  @Test
+  public void buildTokenUrl_nullTransport() {
+    ComputeEngineCredentials credentials =
+        ComputeEngineCredentials.newBuilder()
+            .setGoogleAuthTransport(null)
+            .setBindingEnforcement(ComputeEngineCredentials.BindingEnforcement.ON)
+            .build();
+    String softBoundTokenUrl = credentials.createTokenUrlWithScopes();
+
+    assertEquals(TOKEN_URL + "?binding-enforcement=on", softBoundTokenUrl);
+  }
+
+  @Test
+  public void buildTokenUrl_nullBindingEnforcement() {
+    ComputeEngineCredentials credentials =
+        ComputeEngineCredentials.newBuilder()
+            .setGoogleAuthTransport(ComputeEngineCredentials.GoogleAuthTransport.MTLS)
+            .setBindingEnforcement(null)
+            .build();
+    String softBoundTokenUrl = credentials.createTokenUrlWithScopes();
+
+    assertEquals(TOKEN_URL + "?transport=mtls", softBoundTokenUrl);
+  }
+
+  @Test
+  public void buildTokenUrl_nullTransport_nullBindingEnforcement() {
+    ComputeEngineCredentials credentials =
+        ComputeEngineCredentials.newBuilder()
+            .setGoogleAuthTransport(null)
+            .setBindingEnforcement(null)
+            .build();
+    String softBoundTokenUrl = credentials.createTokenUrlWithScopes();
+
+    assertEquals(TOKEN_URL, softBoundTokenUrl);
+  }
+
+  @Test
+  public void buildTokenUrlSoftMtlsBound_mtls_transport() {
+    ComputeEngineCredentials credentials =
+        ComputeEngineCredentials.newBuilder()
+            .setGoogleAuthTransport(ComputeEngineCredentials.GoogleAuthTransport.MTLS)
+            .build();
+    String softBoundTokenUrl = credentials.createTokenUrlWithScopes();
+
+    assertEquals(TOKEN_URL + "?transport=mtls", softBoundTokenUrl);
+  }
+
+  @Test
+  public void buildTokenUrlSoftMtlsBound_iam_enforcement() {
+    ComputeEngineCredentials credentials =
+        ComputeEngineCredentials.newBuilder()
+            .setBindingEnforcement(ComputeEngineCredentials.BindingEnforcement.IAM_POLICY)
+            .build();
+    String softBoundTokenUrl = credentials.createTokenUrlWithScopes();
+
+    assertEquals(TOKEN_URL + "?binding-enforcement=iam-policy", softBoundTokenUrl);
+  }
+
+  @Test
+  public void buildTokenUrlSoftMtlsBound_mtls_transport_iam_enforcement() {
+    ComputeEngineCredentials credentials =
+        ComputeEngineCredentials.newBuilder()
+            .setGoogleAuthTransport(ComputeEngineCredentials.GoogleAuthTransport.MTLS)
+            .setBindingEnforcement(ComputeEngineCredentials.BindingEnforcement.IAM_POLICY)
+            .build();
+    String softBoundTokenUrl = credentials.createTokenUrlWithScopes();
+
+    assertEquals(TOKEN_URL + "?transport=mtls&binding-enforcement=iam-policy", softBoundTokenUrl);
+  }
+
+  @Test
+  public void buildTokenUrlHardMtlsBound_always_enforced() {
+    ComputeEngineCredentials credentials =
+        ComputeEngineCredentials.newBuilder()
+            .setBindingEnforcement(ComputeEngineCredentials.BindingEnforcement.ON)
+            .build();
+    String softBoundTokenUrl = credentials.createTokenUrlWithScopes();
+
+    assertEquals(TOKEN_URL + "?binding-enforcement=on", softBoundTokenUrl);
+  }
+
+  @Test
+  public void buildTokenUrlHardMtlsBound_mtls_transport_always_enforced() {
+    ComputeEngineCredentials credentials =
+        ComputeEngineCredentials.newBuilder()
+            .setGoogleAuthTransport(ComputeEngineCredentials.GoogleAuthTransport.MTLS)
+            .setBindingEnforcement(ComputeEngineCredentials.BindingEnforcement.ON)
+            .build();
+    String softBoundTokenUrl = credentials.createTokenUrlWithScopes();
+
+    assertEquals(TOKEN_URL + "?transport=mtls&binding-enforcement=on", softBoundTokenUrl);
+  }
+
+  @Test
+  public void buildTokenUrlHardDirectPathBound_alts_transport() {
+    ComputeEngineCredentials credentials =
+        ComputeEngineCredentials.newBuilder()
+            .setGoogleAuthTransport(ComputeEngineCredentials.GoogleAuthTransport.ALTS)
+            .build();
+    String softBoundTokenUrl = credentials.createTokenUrlWithScopes();
+
+    assertEquals(TOKEN_URL + "?transport=alts", softBoundTokenUrl);
+  }
+
   @Test
   public void buildScoped_scopesPresent() throws IOException {
     ComputeEngineCredentials credentials =