use of "Content-Encoding:gzip" was causing the automatic login initiation to fail.

changelog.html

@@ -55,6 +55,9 @@ Upcoming changes</a>
 <!-- Record your changes in the trunk here. -->
 <div id="trunk" style="display:none"><!--=TRUNK-BEGIN=-->
 <ul class=image>
+  <li class=bug>
+    When accessing a page that requires authentication, redirection to start authentication results in a content decoding failure.
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-13625">issue 13625</a>)
   <li class=bug>
     Fixed a regression in untar operation in exotic platforms
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-13202">issue 13202</a>)

core/src/main/java/hudson/security/HudsonAuthenticationEntryPoint.java

@@ -34,10 +34,12 @@ import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
 import static javax.servlet.http.HttpServletResponse.SC_FORBIDDEN;
 import java.io.IOException;
+import java.io.OutputStream;
 import java.io.OutputStreamWriter;
 import java.io.PrintWriter;
 import java.net.URLEncoder;
 import java.text.MessageFormat;
+import java.util.zip.GZIPOutputStream;
 
 /**
  * For anonymous requests to pages that require authentication,
@@ -78,7 +80,13 @@ public class HudsonAuthenticationEntryPoint extends AuthenticationProcessingFilt
             rsp.setContentType("text/html;charset=UTF-8");
             PrintWriter out;
             try {
-                ServletOutputStream sout = rsp.getOutputStream();
+                OutputStream sout = rsp.getOutputStream();
+                if (rsp.containsHeader("Content-Encoding")) {
+                    // we serve Jelly pages with Content-Encoding:gzip.
+                    // ServletResponse doesn't provide means for us to check the value of the header,
+                    // so this is a hack.
+                    sout = new GZIPOutputStream(sout);
+                }
                 out = new PrintWriter(new OutputStreamWriter(sout));
             } catch (IllegalStateException e) {
                 out = rsp.getWriter();