diff --git a/tools/releasetools/ota_from_target_files.py b/tools/releasetools/ota_from_target_files.py
index d0636b6..a567760 100755
--- a/tools/releasetools/ota_from_target_files.py
+++ b/tools/releasetools/ota_from_target_files.py
@@ -109,6 +109,7 @@
 
 import multiprocessing
 import os
+import subprocess
 import tempfile
 import zipfile
 
@@ -1078,6 +1079,124 @@
   WriteMetadata(metadata, output_zip)
 
 
+def WriteABOTAPackageWithBrilloScript(target_file, output_file,
+                                      source_file=None):
+  """Generate an Android OTA package that has A/B update payload."""
+
+  # Setup signing keys.
+  if OPTIONS.package_key is None:
+    OPTIONS.package_key = OPTIONS.info_dict.get(
+        "default_system_dev_certificate",
+        "build/target/product/security/testkey")
+
+  # A/B updater expects key in RSA format.
+  cmd = ["openssl", "pkcs8",
+         "-in", OPTIONS.package_key + OPTIONS.private_key_suffix,
+         "-inform", "DER", "-nocrypt"]
+  rsa_key = common.MakeTempFile(prefix="key-", suffix=".key")
+  cmd.extend(["-out", rsa_key])
+  p1 = common.Run(cmd, stdout=subprocess.PIPE)
+  p1.wait()
+  assert p1.returncode == 0, "openssl pkcs8 failed"
+
+  # Stage the output zip package for signing.
+  temp_zip_file = tempfile.NamedTemporaryFile()
+  output_zip = zipfile.ZipFile(temp_zip_file, "w",
+                               compression=zipfile.ZIP_DEFLATED)
+
+  # Metadata to comply with Android OTA package format.
+  oem_props = OPTIONS.info_dict.get("oem_fingerprint_properties", None)
+  oem_dict = None
+  if oem_props:
+    if OPTIONS.oem_source is None:
+      raise common.ExternalError("OEM source required for this build")
+    oem_dict = common.LoadDictionaryFromLines(
+        open(OPTIONS.oem_source).readlines())
+
+  metadata = {
+      "post-build": CalculateFingerprint(oem_props, oem_dict,
+                                         OPTIONS.info_dict),
+      "pre-device": GetOemProperty("ro.product.device", oem_props, oem_dict,
+                                   OPTIONS.info_dict),
+      "post-timestamp": GetBuildProp("ro.build.date.utc", OPTIONS.info_dict),
+  }
+
+  if source_file is not None:
+    metadata["pre-build"] = CalculateFingerprint(oem_props, oem_dict,
+                                                 OPTIONS.source_info_dict)
+
+  # 1. Generate payload.
+  payload_file = common.MakeTempFile(prefix="payload-", suffix=".bin")
+  cmd = ["brillo_update_payload", "generate",
+         "--payload", payload_file,
+         "--target_image", target_file]
+  if source_file is not None:
+    cmd.extend(["--source_image", source_file])
+  p1 = common.Run(cmd, stdout=subprocess.PIPE)
+  p1.wait()
+  assert p1.returncode == 0, "brillo_update_payload generate failed"
+
+  # 2. Generate hashes of the payload and metadata files.
+  payload_sig_file = common.MakeTempFile(prefix="sig-", suffix=".bin")
+  metadata_sig_file = common.MakeTempFile(prefix="sig-", suffix=".bin")
+  cmd = ["brillo_update_payload", "hash",
+         "--unsigned_payload", payload_file,
+         "--signature_size", "256",
+         "--metadata_hash_file", metadata_sig_file,
+         "--payload_hash_file", payload_sig_file]
+  p1 = common.Run(cmd, stdout=subprocess.PIPE)
+  p1.wait()
+  assert p1.returncode == 0, "brillo_update_payload hash failed"
+
+  # 3. Sign the hashes and insert them back into the payload file.
+  signed_payload_sig_file = common.MakeTempFile(prefix="signed-sig-",
+                                                suffix=".bin")
+  signed_metadata_sig_file = common.MakeTempFile(prefix="signed-sig-",
+                                                 suffix=".bin")
+  # 3a. Sign the payload hash.
+  cmd = ["openssl", "pkeyutl", "-sign",
+         "-inkey", rsa_key,
+         "-pkeyopt", "digest:sha256",
+         "-in", payload_sig_file,
+         "-out", signed_payload_sig_file]
+  p1 = common.Run(cmd, stdout=subprocess.PIPE)
+  p1.wait()
+  assert p1.returncode == 0, "openssl sign payload failed"
+
+  # 3b. Sign the metadata hash.
+  cmd = ["openssl", "pkeyutl", "-sign",
+         "-inkey", rsa_key,
+         "-pkeyopt", "digest:sha256",
+         "-in", metadata_sig_file,
+         "-out", signed_metadata_sig_file]
+  p1 = common.Run(cmd, stdout=subprocess.PIPE)
+  p1.wait()
+  assert p1.returncode == 0, "openssl sign metadata failed"
+
+  # 3c. Insert the signatures back into the payload file.
+  signed_payload_file = common.MakeTempFile(prefix="signed-payload-",
+                                            suffix=".bin")
+  cmd = ["brillo_update_payload", "sign",
+         "--unsigned_payload", payload_file,
+         "--payload", signed_payload_file,
+         "--signature_size", "256",
+         "--metadata_signature_file", signed_metadata_sig_file,
+         "--payload_signature_file", signed_payload_sig_file]
+  p1 = common.Run(cmd, stdout=subprocess.PIPE)
+  p1.wait()
+  assert p1.returncode == 0, "brillo_update_payload sign failed"
+
+  # Add the signed payload file into the zip.
+  common.ZipWrite(output_zip, signed_payload_file, arcname="payload.bin",
+                  compress_type=zipfile.ZIP_STORED)
+  WriteMetadata(metadata, output_zip)
+
+  # Sign the whole package to comply with the Android OTA package format.
+  common.ZipClose(output_zip)
+  SignOutput(temp_zip_file.name, output_file)
+  temp_zip_file.close()
+
+
 class FileDifference(object):
   def __init__(self, partition, source_zip, target_zip, output_zip):
     self.deferred_patch_list = None
@@ -1683,6 +1802,37 @@
     common.Usage(__doc__)
     sys.exit(1)
 
+  # Load the dict file from the zip directly to have a peek at the OTA type.
+  # For packages using A/B update, unzipping is not needed.
+  input_zip = zipfile.ZipFile(args[0], "r")
+  OPTIONS.info_dict = common.LoadInfoDict(input_zip)
+  common.ZipClose(input_zip)
+
+  ab_update = OPTIONS.info_dict.get("ab_update") == "true"
+
+  if ab_update:
+    if OPTIONS.incremental_source is not None:
+      OPTIONS.target_info_dict = OPTIONS.info_dict
+      source_zip = zipfile.ZipFile(OPTIONS.incremental_source, "r")
+      OPTIONS.source_info_dict = common.LoadInfoDict(source_zip)
+      common.ZipClose(source_zip)
+
+    if OPTIONS.verbose:
+      print "--- target info ---"
+      common.DumpInfoDict(OPTIONS.info_dict)
+
+      if OPTIONS.incremental_source is not None:
+        print "--- source info ---"
+        common.DumpInfoDict(OPTIONS.source_info_dict)
+
+    WriteABOTAPackageWithBrilloScript(
+        target_file=args[0],
+        output_file=args[1],
+        source_file=OPTIONS.incremental_source)
+
+    print "done."
+    return
+
   if OPTIONS.extra_script is not None:
     OPTIONS.extra_script = open(OPTIONS.extra_script).read()
 
@@ -1714,9 +1864,7 @@
   if OPTIONS.device_specific is not None:
     OPTIONS.device_specific = os.path.abspath(OPTIONS.device_specific)
 
-  ab_update = OPTIONS.info_dict.get("ab_update") == "true"
-
-  if OPTIONS.info_dict.get("no_recovery") == "true" and not ab_update:
+  if OPTIONS.info_dict.get("no_recovery") == "true":
     raise common.ExternalError(
         "--- target build has specified no recovery ---")
 
@@ -1740,7 +1888,7 @@
 
   # Non A/B OTAs rely on /cache partition to store temporary files.
   cache_size = OPTIONS.info_dict.get("cache_size", None)
-  if cache_size is None and not ab_update:
+  if cache_size is None:
     print "--- can't determine the cache partition size ---"
   OPTIONS.cache_size = cache_size
 
@@ -1750,11 +1898,7 @@
 
   # Generate a full OTA.
   elif OPTIONS.incremental_source is None:
-    if ab_update:
-      # TODO: Pending for b/25715402.
-      pass
-    else:
-      WriteFullOTAPackage(input_zip, output_zip)
+    WriteFullOTAPackage(input_zip, output_zip)
 
   # Generate an incremental OTA. It will fall back to generate a full OTA on
   # failure unless no_fallback_to_full is specified.