Per discussion, implement the Omaha Client Update Protocol (CUP) in src/crypto.

crypto/cup.h

+// Copyright (c) 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CRYPTO_CUP_H_
+#define CRYPTO_CUP_H_
+
+#include <string>
+#include <vector>
+
+#include "base/basictypes.h"
+#include "base/strings/string_piece.h"
+#include "crypto/crypto_export.h"
+
+#if defined(USE_NSS) || defined(OS_WIN) || defined(OS_MACOSX)
+#include "crypto/scoped_nss_types.h"
+#endif
+
+namespace crypto {
+
+// Client Update Protocol, or CUP, is used by Google Update (Omaha) servers to
+// ensure freshness and authenticity of update checks over HTTP, without the
+// overhead of HTTPS -- namely, no PKI, no guarantee of privacy, and no request
+// replay protection (since update checks are idempotent).
+//
+// http://omaha.googlecode.com/svn/wiki/cup.html
+//
+// This implementation does not persist client proofs by design.
+class CRYPTO_EXPORT ClientUpdateProtocol {
+ public:
+  ~ClientUpdateProtocol();
+
+  // Initializes this instance of CUP with a versioned public key. |key_version|
+  // must be non-negative. |public_key| is expected to be a DER-encoded ASN.1
+  // Subject Public Key Info.  Returns NULL on failure.
+  static ClientUpdateProtocol* Create(int key_version,
+                                      const base::StringPiece& public_key);
+
+  // Returns a versioned encrypted secret (v|w) in a URL-safe Base64 encoding.
+  // Add to your URL before calling SignRequest().
+  std::string GetVersionedSecret() const;
+
+  // Generates freshness/authentication data for an outgoing update check.
+  // |url| contains the the URL that the request will be sent to; it should
+  // include GetVersionedSecret() in its query string.  This needs to be
+  // formatted in the way that the Omaha server expects: omit the scheme and
+  // any port number.  (e.g. "//tools.google.com/service/update2?w=1:abcdef")
+  // |request_body| contains the body of the update check request in UTF-8.
+  //
+  // On success, returns true, and |cp_out| receives a Base64-encoded client
+  // proof, which should be sent in the If-Match HTTP header.  On failure,
+  // returns false, and |cp_out| is not modified.
+  //
+  // This method will store internal state in this instance used by calls to
+  // ValidateResponse(); if you need to have multiple update checks in flight,
+  // initialize a separate CUP instance for each one.
+  bool SignRequest(const base::StringPiece& url,
+                   const base::StringPiece& request_body,
+                   std::string* client_proof_out);

[wtc, 2013/05/30 21:38:56]

Nit: it is uncommon to encode _in and _out in parameter names in the Chromium
source tree. The convention of using const references for inputs and non-const
pointers for outputs is usually enough.

[Ryan Myers (chromium), 2013/05/30 21:55:11]

Done.

+
+  // Validates a response to a update check request that was previously signed
+  // with SignRequest().  Returns true on success, false if it cannot validate.
+  // Uses internal state from the previous call to SignRequest().
+  bool ValidateResponse(const base::StringPiece& response_body,
+                        const base::StringPiece& cookie_in,
+                        const base::StringPiece& server_proof_in);

[wtc, 2013/05/30 21:38:56]

|cookie_in| and |server_proof_in| need to be documented.

Nit: with your naming convention, the first argument should be named
response_body_in. But I recommend dropping the _in suffix.

[Ryan Myers (chromium), 2013/05/30 21:55:11]

Done.

+
+ private:
+  friend class CupUnitTest;
+
+  // Constructor is private -- use one of the Create* methods above.
+  explicit ClientUpdateProtocol(int key_version);
+
+  // Decodes |public_key| into the appropriate internal structures.  Returns
+  // the length of the public key (modulus) in bytes, or 0 on failure.
+  size_t LoadPublicKey(const base::StringPiece& public_key);
+
+  // Helper function for BuildSharedKey() -- encrypts |key_source| (r) using
+  // the loaded public key, filling out |encrypted_key_source_| (w).
+  // Returns true on success.
+  bool EncryptKeySource(const std::vector<uint8>& key_source);
+
+  // Generates a key source (r) and uses that to fill out |shared_key_| (sk')
+  // and encrypted_key_source_ (w).  If |opt_key_source| is non-NULL, the key
+  // source is read from that location instead of being randomly generated.
+  // (Used for unit testing.)  Returns true on success.
+  bool BuildSharedKey(size_t public_key_length, const uint8* opt_key_source);
+
+  // The server keeps multiple private keys; a version must be sent so that
+  // the right private key is used to decode the versioned secret.  (The CUP
+  // specification calls this "v".)
+  int pub_key_version_;
+
+  // Holds the shared key, which is used to generate an HMAC signature for both
+  // the update check request and the update response. The client builds it
+  // locally, but sends the server an encrypted copy of the key source to
+  // synthesize it on its own. (The CUP specification calls this "sk'".)
+  std::vector<uint8> shared_key_;
+
+  // Holds the original contents of key_source_ that have been encrypted with
+  // the server's public key. The client sends this, along with the version of
+  // the keypair that was used, to the server. The server decrypts it using its
+  // private key to get the contents of key_source_, from which it recreates the
+  // shared key. (The CUP specification calls this "w".)
+  std::vector<uint8> encrypted_key_source_;
+
+  // Holds the hash of the update check request, the URL that it was sent to,
+  // and the versioned secret. This is filled out by a successful call to
+  // SignRequest(), and used by ValidateResponse() to confirm that the server
+  // has successfully decoded the versioned secret and signed the response using
+  // the same shared key as our own. (The CUP specification calls this "hw".)
+  std::vector<uint8> client_challenge_hash_;
+
+#if defined(USE_NSS) || defined(OS_WIN) || defined(OS_MACOSX)
+  // The public key used to encrypt the key source.  (The CUP specification
+  // calls this "pk[v]".)
+  ScopedSECKEYPublicKey public_key_;
+#endif
+
+  DISALLOW_COPY_AND_ASSIGN(ClientUpdateProtocol);
+};
+
+}  // namespace crypto
+
+#endif  // CRYPTO_CUP_H_
+