diff --git a/tools/releasetools/check_target_files_signatures b/tools/releasetools/check_target_files_signatures
index ae372ba..45d30a6 100755
--- a/tools/releasetools/check_target_files_signatures
+++ b/tools/releasetools/check_target_files_signatures
@@ -135,7 +135,7 @@
 
     for i in to_load:
       f = open(i)
-      cert = ParseCertificate(f.read())
+      cert = common.ParseCertificate(f.read())
       f.close()
       name, _ = os.path.splitext(i)
       name, _ = os.path.splitext(name)
@@ -144,21 +144,6 @@
 ALL_CERTS = CertDB()
 
 
-def ParseCertificate(data):
-  """Parse a PEM-format certificate."""
-  cert = []
-  save = False
-  for line in data.split("\n"):
-    if "--END CERTIFICATE--" in line:
-      break
-    if save:
-      cert.append(line)
-    if "--BEGIN CERTIFICATE--" in line:
-      save = True
-  cert = "".join(cert).decode('base64')
-  return cert
-
-
 def CertFromPKCS7(data, filename):
   """Read the cert out of a PKCS#7-format file (which is what is
   stored in a signed .apk)."""
@@ -175,7 +160,7 @@
       AddProblem("error reading cert:\n" + err)
       return None
 
-    cert = ParseCertificate(out)
+    cert = common.ParseCertificate(out)
     if not cert:
       AddProblem("error parsing cert output")
       return None
diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index 58582ba..a3217dd 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -954,3 +954,18 @@
     return PARTITION_TYPES[fstab[mount_point].fs_type], fstab[mount_point].device
   else:
     return None
+
+
+def ParseCertificate(data):
+  """Parse a PEM-format certificate."""
+  cert = []
+  save = False
+  for line in data.split("\n"):
+    if "--END CERTIFICATE--" in line:
+      break
+    if save:
+      cert.append(line)
+    if "--BEGIN CERTIFICATE--" in line:
+      save = True
+  cert = "".join(cert).decode('base64')
+  return cert
diff --git a/tools/releasetools/sign_target_files_apks b/tools/releasetools/sign_target_files_apks
index 5556573..00693b8 100755
--- a/tools/releasetools/sign_target_files_apks
+++ b/tools/releasetools/sign_target_files_apks
@@ -71,8 +71,10 @@
   print >> sys.stderr, "Python 2.4 or newer is required."
   sys.exit(1)
 
+import base64
 import cStringIO
 import copy
+import errno
 import os
 import re
 import subprocess
@@ -161,11 +163,45 @@
       print "rewriting %s:" % (info.filename,)
       new_data = RewriteProps(data)
       output_tf_zip.writestr(out_info, new_data)
+    elif info.filename.endswith("mac_permissions.xml"):
+      print "rewriting %s with new keys." % (info.filename,)
+      new_data = ReplaceCerts(data)
+      output_tf_zip.writestr(out_info, new_data)
     else:
       # a non-APK file; copy it verbatim
       output_tf_zip.writestr(out_info, data)
 
 
+def ReplaceCerts(data):
+  """Given a string of data, replace all occurences of a set
+  of X509 certs with a newer set of X509 certs and return
+  the updated data string."""
+  for old, new in OPTIONS.key_map.iteritems():
+    try:
+      if OPTIONS.verbose:
+        print "    Replacing %s.x509.pem with %s.x509.pem" % (old, new)
+      f = open(old + ".x509.pem")
+      old_cert16 = base64.b16encode(common.ParseCertificate(f.read())).lower()
+      f.close()
+      f = open(new + ".x509.pem")
+      new_cert16 = base64.b16encode(common.ParseCertificate(f.read())).lower()
+      f.close()
+      # Only match entire certs.
+      pattern = "\\b"+old_cert16+"\\b"
+      (data, num) = re.subn(pattern, new_cert16, data, flags=re.IGNORECASE)
+      if OPTIONS.verbose:
+        print "    Replaced %d occurence(s) of %s.x509.pem with " \
+            "%s.x509.pem" % (num, old, new)
+    except IOError, e:
+      if (e.errno == errno.ENOENT and not OPTIONS.verbose):
+        continue
+
+      print "    Error accessing %s. %s. Skip replacing %s.x509.pem " \
+          "with %s.x509.pem." % (e.filename, e.strerror, old, new)
+
+  return data
+
+
 def EditTags(tags):
   """Given a string containing comma-separated tags, apply the edits
   specified in OPTIONS.tag_changes and return the updated string."""