diff --git a/tools/releasetools/ota_from_target_files.py b/tools/releasetools/ota_from_target_files.py
index 1e7bb3a..dfcfb49 100755
--- a/tools/releasetools/ota_from_target_files.py
+++ b/tools/releasetools/ota_from_target_files.py
@@ -171,8 +171,16 @@
   --payload_signer_args <args>
       Specify the arguments needed for payload signer.
 
+  --payload_signer_maximum_signature_size <signature_size>
+      The maximum signature size (in bytes) that would be generated by the given
+      payload signer. Only meaningful when custom payload signer is specified
+      via '--payload_signer'.
+      If the signer uses a RSA key, this should be the number of bytes to
+      represent the modulus. If it uses an EC key, this is the size of a
+      DER-encoded ECDSA signature.
+
   --payload_signer_key_size <key_size>
-      Specify the key size in bytes of the payload signer.
+      Deprecated. Use the '--payload_signer_maximum_signature_size' instead.
 
   --skip_postinstall
       Skip the postinstall hooks when generating an A/B OTA package (default:
@@ -231,7 +239,7 @@
 OPTIONS.log_diff = None
 OPTIONS.payload_signer = None
 OPTIONS.payload_signer_args = []
-OPTIONS.payload_signer_key_size = None
+OPTIONS.payload_signer_maximum_signature_size = None
 OPTIONS.extracted_input = None
 OPTIONS.key_passwords = []
 OPTIONS.skip_postinstall = False
@@ -288,35 +296,31 @@
       self.signer = "openssl"
       self.signer_args = ["pkeyutl", "-sign", "-inkey", signing_key,
                           "-pkeyopt", "digest:sha256"]
-      self.key_size = self._GetKeySizeInBytes(signing_key)
+      self.maximum_signature_size = self._GetMaximumSignatureSizeInBytes(
+          signing_key)
     else:
       self.signer = OPTIONS.payload_signer
       self.signer_args = OPTIONS.payload_signer_args
-      if OPTIONS.payload_signer_key_size:
-        self.key_size = int(OPTIONS.payload_signer_key_size)
-        assert self.key_size == 256 or self.key_size == 512, \
-            "Unsupported key size {}".format(OPTIONS.payload_signer_key_size)
+      if OPTIONS.payload_signer_maximum_signature_size:
+        self.maximum_signature_size = int(
+            OPTIONS.payload_signer_maximum_signature_size)
       else:
-        self.key_size = 256
+        # The legacy config uses RSA2048 keys.
+        logger.warning("The maximum signature size for payload signer is not"
+                       " set, default to 256 bytes.")
+        self.maximum_signature_size = 256
 
   @staticmethod
-  def _GetKeySizeInBytes(signing_key):
-    modulus_file = common.MakeTempFile(prefix="modulus-")
-    cmd = ["openssl", "rsa", "-inform", "PEM", "-in", signing_key, "-modulus",
-           "-noout", "-out", modulus_file]
-    common.RunAndCheckOutput(cmd, verbose=False)
-
-    with open(modulus_file) as f:
-      modulus_string = f.read()
-    # The modulus string has the format "Modulus=$data", where $data is the
-    # concatenation of hex dump of the modulus.
-    MODULUS_PREFIX = "Modulus="
-    assert modulus_string.startswith(MODULUS_PREFIX)
-    modulus_string = modulus_string[len(MODULUS_PREFIX):]
-    key_size = len(modulus_string) // 2
-    assert key_size == 256 or key_size == 512, \
-        "Unsupported key size {}".format(key_size)
-    return key_size
+  def _GetMaximumSignatureSizeInBytes(signing_key):
+    out_signature_size_file = common.MakeTempFile("signature_size")
+    cmd = ["delta_generator", "--out_maximum_signature_size_file={}".format(
+        out_signature_size_file), "--private_key={}".format(signing_key)]
+    common.RunAndCheckOutput(cmd)
+    with open(out_signature_size_file) as f:
+      signature_size = f.read().rstrip()
+    logger.info("% outputs the maximum signature size: %", cmd[0],
+                signature_size)
+    return int(signature_size)
 
   def Sign(self, in_file):
     """Signs the given input file. Returns the output filename."""
@@ -396,7 +400,7 @@
     metadata_sig_file = common.MakeTempFile(prefix="sig-", suffix=".bin")
     cmd = ["brillo_update_payload", "hash",
            "--unsigned_payload", self.payload_file,
-           "--signature_size", str(payload_signer.key_size),
+           "--signature_size", str(payload_signer.maximum_signature_size),
            "--metadata_hash_file", metadata_sig_file,
            "--payload_hash_file", payload_sig_file]
     self._Run(cmd)
@@ -411,7 +415,7 @@
     cmd = ["brillo_update_payload", "sign",
            "--unsigned_payload", self.payload_file,
            "--payload", signed_payload_file,
-           "--signature_size", str(payload_signer.key_size),
+           "--signature_size", str(payload_signer.maximum_signature_size),
            "--metadata_signature_file", signed_metadata_sig_file,
            "--payload_signature_file", signed_payload_sig_file]
     self._Run(cmd)
@@ -2005,8 +2009,13 @@
       OPTIONS.payload_signer = a
     elif o == "--payload_signer_args":
       OPTIONS.payload_signer_args = shlex.split(a)
+    elif o == "--payload_signer_maximum_signature_size":
+      OPTIONS.payload_signer_maximum_signature_size = a
     elif o == "--payload_signer_key_size":
-      OPTIONS.payload_signer_key_size = a
+      # TODO(Xunchang) remove this option after cleaning up the callers.
+      logger.warning("The option '--payload_signer_key_size' is deprecated."
+                     " Use '--payload_signer_maximum_signature_size' instead.")
+      OPTIONS.payload_signer_maximum_signature_size = a
     elif o == "--extracted_input_target_files":
       OPTIONS.extracted_input = a
     elif o == "--skip_postinstall":
@@ -2047,6 +2056,7 @@
                                  "log_diff=",
                                  "payload_signer=",
                                  "payload_signer_args=",
+                                 "payload_signer_maximum_signature_size=",
                                  "payload_signer_key_size=",
                                  "extracted_input_target_files=",
                                  "skip_postinstall",
diff --git a/tools/releasetools/test_ota_from_target_files.py b/tools/releasetools/test_ota_from_target_files.py
index c3021a1..38faf64 100644
--- a/tools/releasetools/test_ota_from_target_files.py
+++ b/tools/releasetools/test_ota_from_target_files.py
@@ -885,10 +885,28 @@
       payload_offset, metadata_total = (
           property_files._GetPayloadMetadataOffsetAndSize(input_zip))
 
-    # Read in the metadata signature directly.
+    # The signature proto has the following format (details in
+    #  /platform/system/update_engine/update_metadata.proto):
+    #  message Signature {
+    #    optional uint32 version = 1;
+    #    optional bytes data = 2;
+    #    optional fixed32 unpadded_signature_size = 3;
+    #  }
+    #
+    # According to the protobuf encoding, the tail of the signature message will
+    # be [signature string(256 bytes) + encoding of the fixed32 number 256]. And
+    # 256 is encoded as 'x1d\x00\x01\x00\x00':
+    # [3 (field number) << 3 | 5 (type) + byte reverse of 0x100 (256)].
+    # Details in (https://developers.google.com/protocol-buffers/docs/encoding)
+    signature_tail_length = self.SIGNATURE_SIZE + 5
+    self.assertGreater(metadata_total, signature_tail_length)
     with open(output_file, 'rb') as verify_fp:
-      verify_fp.seek(payload_offset + metadata_total - self.SIGNATURE_SIZE)
-      metadata_signature = verify_fp.read(self.SIGNATURE_SIZE)
+      verify_fp.seek(payload_offset + metadata_total - signature_tail_length)
+      metadata_signature_proto_tail = verify_fp.read(signature_tail_length)
+
+    self.assertEqual(b'\x1d\x00\x01\x00\x00',
+                     metadata_signature_proto_tail[-5:])
+    metadata_signature = metadata_signature_proto_tail[:-5]
 
     # Now we extract the metadata hash via brillo_update_payload script, which
     # will serve as the oracle result.
@@ -1050,11 +1068,13 @@
     with open(file1, 'rb') as fp1, open(file2, 'rb') as fp2:
       self.assertEqual(fp1.read(), fp2.read())
 
+  @test_utils.SkipIfExternalToolsUnavailable()
   def test_init(self):
     payload_signer = PayloadSigner()
     self.assertEqual('openssl', payload_signer.signer)
-    self.assertEqual(256, payload_signer.key_size)
+    self.assertEqual(256, payload_signer.maximum_signature_size)
 
+  @test_utils.SkipIfExternalToolsUnavailable()
   def test_init_withPassword(self):
     common.OPTIONS.package_key = os.path.join(
         self.testdata_dir, 'testkey_with_passwd')
@@ -1067,18 +1087,27 @@
   def test_init_withExternalSigner(self):
     common.OPTIONS.payload_signer = 'abc'
     common.OPTIONS.payload_signer_args = ['arg1', 'arg2']
-    common.OPTIONS.payload_signer_key_size = '512'
+    common.OPTIONS.payload_signer_maximum_signature_size = '512'
     payload_signer = PayloadSigner()
     self.assertEqual('abc', payload_signer.signer)
     self.assertEqual(['arg1', 'arg2'], payload_signer.signer_args)
-    self.assertEqual(512, payload_signer.key_size)
+    self.assertEqual(512, payload_signer.maximum_signature_size)
 
-  def test_GetKeySizeInBytes_512Bytes(self):
+  @test_utils.SkipIfExternalToolsUnavailable()
+  def test_GetMaximumSignatureSizeInBytes_512Bytes(self):
     signing_key = os.path.join(self.testdata_dir, 'testkey_RSA4096.key')
     # pylint: disable=protected-access
-    key_size = PayloadSigner._GetKeySizeInBytes(signing_key)
-    self.assertEqual(512, key_size)
+    signature_size = PayloadSigner._GetMaximumSignatureSizeInBytes(signing_key)
+    self.assertEqual(512, signature_size)
 
+  @test_utils.SkipIfExternalToolsUnavailable()
+  def test_GetMaximumSignatureSizeInBytes_ECKey(self):
+    signing_key = os.path.join(self.testdata_dir, 'testkey_EC.key')
+    # pylint: disable=protected-access
+    signature_size = PayloadSigner._GetMaximumSignatureSizeInBytes(signing_key)
+    self.assertEqual(72, signature_size)
+
+  @test_utils.SkipIfExternalToolsUnavailable()
   def test_Sign(self):
     payload_signer = PayloadSigner()
     input_file = os.path.join(self.testdata_dir, self.SIGFILE)
diff --git a/tools/releasetools/testdata/testkey_EC.key b/tools/releasetools/testdata/testkey_EC.key
new file mode 100644
index 0000000..9e65a68
--- /dev/null
+++ b/tools/releasetools/testdata/testkey_EC.key
@@ -0,0 +1,5 @@
+-----BEGIN PRIVATE KEY-----
+MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgGaguGj8Yb1KkqKHd
+ISblUsjtOCbzAuVpX81i02sm8FWhRANCAARBnuotwKOsuvjH6iwTDhOAi7Q5pLWz
+xDkZjg2pcfbfi9FFTvLYETas7B2W6fx9PUezUmHTFTDV2JZuMYYFdZOw
+-----END PRIVATE KEY-----