diff --git a/payload_verifier.cc b/payload_verifier.cc
new file mode 100644
index 0000000..fa23d4f
--- /dev/null
+++ b/payload_verifier.cc
@@ -0,0 +1,222 @@
+// Copyright 2014 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "update_engine/payload_verifier.h"
+
+#include <base/logging.h>
+#include <openssl/pem.h>
+
+#include "update_engine/delta_performer.h"
+#include "update_engine/omaha_hash_calculator.h"
+#include "update_engine/payload_generator/delta_diff_generator.h"
+#include "update_engine/update_metadata.pb.h"
+#include "update_engine/utils.h"
+
+using std::string;
+using std::vector;
+
+namespace chromeos_update_engine {
+
+const uint32_t kSignatureMessageOriginalVersion = 1;
+const uint32_t kSignatureMessageCurrentVersion = 1;
+
+namespace {
+
+// The following is a standard PKCS1-v1_5 padding for SHA256 signatures, as
+// defined in RFC3447. It is prepended to the actual signature (32 bytes) to
+// form a sequence of 256 bytes (2048 bits) that is amenable to RSA signing. The
+// padded hash will look as follows:
+//
+//    0x00 0x01 0xff ... 0xff 0x00  ASN1HEADER  SHA256HASH
+//   |--------------205-----------||----19----||----32----|
+//
+// where ASN1HEADER is the ASN.1 description of the signed data. The complete 51
+// bytes of actual data (i.e. the ASN.1 header complete with the hash) are
+// packed as follows:
+//
+//  SEQUENCE(2+49) {
+//   SEQUENCE(2+13) {
+//    OBJECT(2+9) id-sha256
+//    NULL(2+0)
+//   }
+//   OCTET STRING(2+32) <actual signature bytes...>
+//  }
+const unsigned char kRSA2048SHA256Padding[] = {
+  // PKCS1-v1_5 padding
+  0x00, 0x01, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+  0xff, 0xff, 0xff, 0xff, 0x00,
+  // ASN.1 header
+  0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
+  0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05,
+  0x00, 0x04, 0x20,
+};
+
+}  // namespace
+
+bool PayloadVerifier::LoadPayload(const string& payload_path,
+                                  vector<char>* out_payload,
+                                  DeltaArchiveManifest* out_manifest,
+                                  uint64_t* out_metadata_size) {
+  vector<char> payload;
+  // Loads the payload and parses the manifest.
+  TEST_AND_RETURN_FALSE(utils::ReadFile(payload_path, &payload));
+  LOG(INFO) << "Payload size: " << payload.size();
+  ErrorCode error = ErrorCode::kSuccess;
+  InstallPlan install_plan;
+  DeltaPerformer delta_performer(NULL, NULL, &install_plan);
+  TEST_AND_RETURN_FALSE(
+      delta_performer.ParsePayloadMetadata(payload, &error) ==
+      DeltaPerformer::kMetadataParseSuccess);
+  TEST_AND_RETURN_FALSE(delta_performer.GetManifest(out_manifest));
+  *out_metadata_size = delta_performer.GetMetadataSize();
+  LOG(INFO) << "Metadata size: " << *out_metadata_size;
+  out_payload->swap(payload);
+  return true;
+}
+
+bool PayloadVerifier::VerifySignature(const std::vector<char>& signature_blob,
+                                      const std::string& public_key_path,
+                                      std::vector<char>* out_hash_data) {
+  return VerifySignatureBlob(signature_blob, public_key_path,
+                             kSignatureMessageCurrentVersion, out_hash_data);
+}
+
+bool PayloadVerifier::VerifySignatureBlob(
+    const std::vector<char>& signature_blob,
+    const std::string& public_key_path,
+    uint32_t client_version,
+    std::vector<char>* out_hash_data) {
+  TEST_AND_RETURN_FALSE(!public_key_path.empty());
+
+  Signatures signatures;
+  LOG(INFO) << "signature size = " <<  signature_blob.size();
+  TEST_AND_RETURN_FALSE(signatures.ParseFromArray(&signature_blob[0],
+                                                  signature_blob.size()));
+
+  // Finds a signature that matches the current version.
+  int sig_index = 0;
+  for (; sig_index < signatures.signatures_size(); sig_index++) {
+    const Signatures_Signature& signature = signatures.signatures(sig_index);
+    if (signature.has_version() &&
+        signature.version() == client_version) {
+      break;
+    }
+  }
+  TEST_AND_RETURN_FALSE(sig_index < signatures.signatures_size());
+
+  const Signatures_Signature& signature = signatures.signatures(sig_index);
+  vector<char> sig_data(signature.data().begin(), signature.data().end());
+
+  return GetRawHashFromSignature(sig_data, public_key_path, out_hash_data);
+}
+
+
+bool PayloadVerifier::GetRawHashFromSignature(
+    const std::vector<char>& sig_data,
+    const std::string& public_key_path,
+    std::vector<char>* out_hash_data) {
+  TEST_AND_RETURN_FALSE(!public_key_path.empty());
+
+  // The code below executes the equivalent of:
+  //
+  // openssl rsautl -verify -pubin -inkey |public_key_path|
+  //   -in |sig_data| -out |out_hash_data|
+
+  // Loads the public key.
+  FILE* fpubkey = fopen(public_key_path.c_str(), "rb");
+  if (!fpubkey) {
+    LOG(ERROR) << "Unable to open public key file: " << public_key_path;
+    return false;
+  }
+
+  char dummy_password[] = { ' ', 0 };  // Ensure no password is read from stdin.
+  RSA* rsa = PEM_read_RSA_PUBKEY(fpubkey, NULL, NULL, dummy_password);
+  fclose(fpubkey);
+  TEST_AND_RETURN_FALSE(rsa != NULL);
+  unsigned int keysize = RSA_size(rsa);
+  if (sig_data.size() > 2 * keysize) {
+    LOG(ERROR) << "Signature size is too big for public key size.";
+    RSA_free(rsa);
+    return false;
+  }
+
+  // Decrypts the signature.
+  vector<char> hash_data(keysize);
+  int decrypt_size = RSA_public_decrypt(
+      sig_data.size(),
+      reinterpret_cast<const unsigned char*>(sig_data.data()),
+      reinterpret_cast<unsigned char*>(hash_data.data()),
+      rsa,
+      RSA_NO_PADDING);
+  RSA_free(rsa);
+  TEST_AND_RETURN_FALSE(decrypt_size > 0 &&
+                        decrypt_size <= static_cast<int>(hash_data.size()));
+  hash_data.resize(decrypt_size);
+  out_hash_data->swap(hash_data);
+  return true;
+}
+
+bool PayloadVerifier::VerifySignedPayload(const std::string& payload_path,
+                                          const std::string& public_key_path,
+                                          uint32_t client_key_check_version) {
+  vector<char> payload;
+  DeltaArchiveManifest manifest;
+  uint64_t metadata_size;
+  TEST_AND_RETURN_FALSE(LoadPayload(
+      payload_path, &payload, &manifest, &metadata_size));
+  TEST_AND_RETURN_FALSE(manifest.has_signatures_offset() &&
+                        manifest.has_signatures_size());
+  CHECK_EQ(payload.size(),
+           metadata_size + manifest.signatures_offset() +
+           manifest.signatures_size());
+  vector<char> signature_blob(
+      payload.begin() + metadata_size + manifest.signatures_offset(),
+      payload.end());
+  vector<char> signed_hash;
+  TEST_AND_RETURN_FALSE(VerifySignatureBlob(
+      signature_blob, public_key_path, client_key_check_version, &signed_hash));
+  TEST_AND_RETURN_FALSE(!signed_hash.empty());
+  vector<char> hash;
+  TEST_AND_RETURN_FALSE(OmahaHashCalculator::RawHashOfBytes(
+      payload.data(), metadata_size + manifest.signatures_offset(), &hash));
+  PadRSA2048SHA256Hash(&hash);
+  TEST_AND_RETURN_FALSE(hash == signed_hash);
+  return true;
+}
+
+bool PayloadVerifier::PadRSA2048SHA256Hash(std::vector<char>* hash) {
+  TEST_AND_RETURN_FALSE(hash->size() == 32);
+  hash->insert(hash->begin(),
+               reinterpret_cast<const char*>(kRSA2048SHA256Padding),
+               reinterpret_cast<const char*>(kRSA2048SHA256Padding +
+                                             sizeof(kRSA2048SHA256Padding)));
+  TEST_AND_RETURN_FALSE(hash->size() == 256);
+  return true;
+}
+
+}  // namespace chromeos_update_engine