diff --git a/CMakeLists.txt b/CMakeLists.txt
index f86b762..e2f363a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -16,6 +16,7 @@ add_executable( grive
 	src/protocol/HTTP.cc
 	src/protocol/Json.cc
 	src/protocol/OAuth2.cc
+	src/util/Crypt.cc
 	src/util/DateTime.cc
 	src/util/OS.cc
 )
diff --git a/src/drive/Drive.cc b/src/drive/Drive.cc
index 37e4eb1..53f7829 100644
--- a/src/drive/Drive.cc
+++ b/src/drive/Drive.cc
@@ -22,19 +22,15 @@
 #include "protocol/HTTP.hh"
 #include "protocol/Json.hh"
 #include "protocol/OAuth2.hh"
+#include "util/Crypt.hh"
 #include "util/DateTime.hh"
 #include "util/OS.hh"
 
-// dependent libraries
-#include <openssl/evp.h>
-
 // standard C++ library
 #include <algorithm>
 #include <cassert>
 #include <fstream>
-#include <iomanip>
 #include <map>
-#include <sstream>
 
 // for debugging only
 #include <iostream>
@@ -43,30 +39,6 @@ namespace gr {
 
 const std::string root_url = "https://docs.google.com/feeds/default/private/full" ;
 
-std::string MD5( std::streambuf *file )
-{
-	char buf[64 * 1024] ;
-	EVP_MD_CTX	md ;
-	EVP_MD_CTX_init( &md );
-	EVP_DigestInit_ex( &md, EVP_md5(), 0 ) ;
-	
-	std::size_t count = 0 ;
-	while ( (count = file->sgetn( buf, sizeof(buf) )) > 0 )
-	{
-		EVP_DigestUpdate( &md, buf, count ) ;
-	}
-	
-	unsigned int md5_size = EVP_MAX_MD_SIZE ;
-	unsigned char md5[EVP_MAX_MD_SIZE] ;
-	EVP_DigestFinal_ex( &md, md5, &md5_size ) ;
-	
-	std::ostringstream ss ;
-	for ( unsigned int i = 0 ; i < md5_size ; i++ )
-		ss << std::hex << std::setw(2) << std::setfill('0') << static_cast<int>(md5[i]) ;
-	
-	return ss.str() ;
-}
-
 Drive::Drive( OAuth2& auth ) :
 	m_auth( auth ),
 	m_root( ".", "https://docs.google.com/feeds/default/private/full/folder%3Aroot" )
@@ -75,7 +47,7 @@ Drive::Drive( OAuth2& auth ) :
 	m_http_hdr.push_back( "GData-Version: 3.0" ) ;
 	
 	
-	Json resp = Json::Parse(HttpGet( root_url + "?alt=json&showfolders=true", m_http_hdr )) ;
+	Json resp = Json::Parse( http::Get( root_url + "?alt=json&showfolders=true", m_http_hdr )) ;
 	Json::Array entries = resp["feed"]["entry"].As<Json::Array>() ;
 	ConstructDirTree( entries ) ;
 	
@@ -182,8 +154,8 @@ void Drive::UpdateFile( const Json& entry )
 	if ( entry.Has( "docs$filename" ) )
 	{
 		// use title as the filename
-		std::string filename	= entry["docs$filename"]["$t"].As() ;
-		std::string url			= entry["content"]["src"].As() ;
+		std::string filename	= entry["docs$filename"]["$t"].Get() ;
+		std::string url			= entry["content"]["src"].Get() ;
 		std::string parent_href	= Parent( entry ) ;
 
 		bool changed = true ;
@@ -204,7 +176,7 @@ void Drive::UpdateFile( const Json& entry )
 		if ( ifile && entry.Has("docs$md5Checksum") )
 		{
 			std::string remote_md5	= entry["docs$md5Checksum"]["$t"].As<std::string>() ;
-			std::string local_md5	= MD5( ifile.rdbuf() ) ;
+			std::string local_md5	= crypt::MD5( ifile.rdbuf() ) ;
 			
 			std::transform( remote_md5.begin(), remote_md5.end(), remote_md5.begin(), tolower ) ;
 			if ( local_md5 == remote_md5 )
@@ -221,7 +193,7 @@ void Drive::UpdateFile( const Json& entry )
 			if ( remote > local )
 			{
 std::cout << "downloading " << path << std::endl ;
-				HttpGetFile( url, path, m_http_hdr ) ;
+				http::GetFile( url, path, m_http_hdr ) ;
 				os::SetFileTime( path, remote ) ;
 			}
 			else
@@ -241,7 +213,7 @@ void Drive::UploadFile( const Json& entry )
 		"http://schemas.google.com/g/2005#resumable-edit-media" )["href"] ;
 	std::cout << resume_link.As<std::string>() << std::endl ;
 
-	std::string resp = HttpPut( resume_link.As(), "" ) ;
+	std::string resp = http::Put( resume_link.Get(), "", m_http_hdr ) ;
 	
 	std::cout << "resp " << resp ;
 }
diff --git a/src/protocol/HTTP.cc b/src/protocol/HTTP.cc
index ed38cdb..452fddc 100644
--- a/src/protocol/HTTP.cc
+++ b/src/protocol/HTTP.cc
@@ -31,19 +31,9 @@
 #include <sstream>
 #include <streambuf>
 
-namespace gr {
+namespace {
 
-HttpException::HttpException( int curl_code, int http_code, const char *err_buf )
-	: runtime_error( Format( curl_code, http_code, err_buf ) )
-{
-}
-
-std::string HttpException::Format( int curl_code, int http_code, const char *err_buf )
-{
-	std::ostringstream ss ;
-	ss << "CURL code = " << curl_code << " HTTP code = " << http_code << " (" << err_buf << ")" ;
-	return ss.str() ;
-}
+using namespace gr::http ;
 
 // libcurl callback to append to a string
 std::size_t WriteCallback( char *data, size_t size, size_t nmemb, std::string *resp )
@@ -109,16 +99,32 @@ void DoCurl( CURL *curl )
 	
 	if ( curl_code != CURLE_OK )
 	{
-		throw HttpException( curl_code, http_code, error_buf ) ;
+		throw Exception( curl_code, http_code, error_buf ) ;
 	}
 	else if (http_code >= 400 )
 	{
 		std::cout << "http error " << http_code << std::endl ;
-		throw HttpException( curl_code, http_code, error_buf ) ;
+		throw Exception( curl_code, http_code, error_buf ) ;
 	}
 }
 
-std::string HttpGet( const std::string& url, const Headers& hdr )
+} // end of local namespace
+
+namespace gr { namespace http {
+
+Exception::Exception( int curl_code, int http_code, const char *err_buf )
+	: runtime_error( Format( curl_code, http_code, err_buf ) )
+{
+}
+
+std::string Exception::Format( int curl_code, int http_code, const char *err_buf )
+{
+	std::ostringstream ss ;
+	ss << "CURL code = " << curl_code << " HTTP code = " << http_code << " (" << err_buf << ")" ;
+	return ss.str() ;
+}
+
+std::string Get( const std::string& url, const Headers& hdr )
 {
 	std::string resp ;
 	CURL *curl = InitCurl( url, &resp, hdr ) ;
@@ -127,7 +133,7 @@ std::string HttpGet( const std::string& url, const Headers& hdr )
 	return resp;
 }
 
-void HttpGetFile(
+void GetFile(
 	const std::string&	url,
 	const std::string&	filename,
 	const Headers& 		hdr )
@@ -141,7 +147,7 @@ void HttpGetFile(
 	DoCurl( curl ) ;
 }
 
-void HttpGetFile(
+void GetFile(
 	const std::string&	url,
 	const std::string&	filename,
 	std::string&		md5sum,
@@ -149,7 +155,7 @@ void HttpGetFile(
 {
 }
 
-std::string HttpPostData( const std::string& url, const std::string& data, const Headers& hdr )
+std::string PostData( const std::string& url, const std::string& data, const Headers& hdr )
 {
 	std::string resp ;
 	CURL *curl = InitCurl( url, &resp, hdr ) ;
@@ -164,13 +170,13 @@ std::string HttpPostData( const std::string& url, const std::string& data, const
 	return resp;
 }
 
-std::string HttpPostFile( const std::string& url, const std::string& filename, const Headers& hdr )
+std::string PostFile( const std::string& url, const std::string& filename, const Headers& hdr )
 {
 	std::string resp ;
 	return resp;
 }
 
-std::string HttpPut(
+std::string Put(
 	const std::string&	url,
 	const std::string&	data,
 	const Headers&		hdr )
@@ -216,4 +222,4 @@ std::string Unescape( const std::string& str )
 	return result ;
 }
 
-}
+} } // end of namespace
diff --git a/src/protocol/HTTP.hh b/src/protocol/HTTP.hh
index 1b7ab05..669d57c 100644
--- a/src/protocol/HTTP.hh
+++ b/src/protocol/HTTP.hh
@@ -23,44 +23,46 @@
 #include <stdexcept>
 #include <vector>
 
-namespace gr
+namespace gr { namespace http
 {
 	typedef std::vector<std::string> Headers ;
 
-	std::string HttpGet( const std::string& url, const Headers& hdr = Headers() ) ;
-	void HttpGetFile(
+	std::string Get( const std::string& url, const Headers& hdr = Headers() ) ;
+	void GetFile(
 		const std::string&	url,
 		const std::string&	filename,
 		const Headers& 		hdr = Headers() ) ;
 	
-	void HttpGetFile(
+	void GetFile(
 		const std::string&	url,
 		const std::string&	filename,
 		std::string&		md5sum,
 		const Headers& 		hdr = Headers() ) ;
 	
-	std::string HttpPostData(
+	std::string PostData(
 		const std::string&	url,
 		const std::string&	data,
 		const Headers&		hdr = Headers() ) ;
-	std::string HttpPostFile(
+	std::string PostFile(
 		const std::string&	url,
 		const std::string& 	filename,
 		const Headers&		hdr = Headers() ) ;
 	
-	std::string HttpPut(
+	std::string Put(
 		const std::string&	url,
+		const std::string&	data,
 		const Headers&		hdr = Headers() ) ;
 	
 	std::string Escape( const std::string& str ) ;
 	std::string Unescape( const std::string& str ) ;
 	
-	class HttpException : public std::runtime_error
+	class Exception : public std::runtime_error
 	{
 	public :
-		HttpException( int curl_code, int http_code, const char *err_buf ) ;
+		Exception( int curl_code, int http_code, const char *err_buf ) ;
 	
 	private :
 		static std::string Format( int curl_code, int http_code, const char *err_buf ) ;
 	} ;
-}
\ No newline at end of file
+
+} } // end of namespace
diff --git a/src/protocol/Json.cc b/src/protocol/Json.cc
index 0fcc12a..8c691f4 100644
--- a/src/protocol/Json.cc
+++ b/src/protocol/Json.cc
@@ -138,9 +138,18 @@ void Json::Add( const std::string& key, const Json& json )
 	::json_object_object_add( m_json, key.c_str(), json.m_json ) ;
 }
 
-Json::Proxy Json::As() const
+template <>
+bool Json::As<bool>() const
 {
-	return Proxy( *this ) ;
+	assert( m_json != 0 ) ;
+	return ::json_object_get_boolean( m_json ) ;
+}
+
+template <>
+bool Json::Is<bool>() const
+{
+	assert( m_json != 0 ) ;
+	return ::json_object_is_type( m_json, json_type_boolean ) ;
 }
 
 template <>
@@ -248,4 +257,34 @@ bool Json::FindInArray( const std::string& key, const std::string& value, Json&
 	}
 }
 
+Json::Proxy Json::Get() const
+{
+	return Proxy( *this ) ;
+}
+
+Json::Proxy::operator bool() const
+{
+	return referring.As<bool>() ;
+}
+
+Json::Proxy::operator int() const
+{
+	return referring.As<int>() ;
+}
+
+Json::Proxy::operator Object() const
+{
+	return referring.As<Object>() ;
+}
+
+Json::Proxy::operator Array() const
+{
+	return referring.As<Array>() ;
+}
+
+Json::Proxy::operator std::string() const
+{
+	return referring.As<std::string>() ;
+}
+
 }
diff --git a/src/protocol/Json.hh b/src/protocol/Json.hh
index 5417526..3a9df2f 100644
--- a/src/protocol/Json.hh
+++ b/src/protocol/Json.hh
@@ -56,9 +56,13 @@ public :
 	{
 		const Json&	referring ;
 		explicit Proxy( const Json& j ) : referring( j ) { }
-		template <typename T> operator T() const { return referring.As<T>() ; }
+		operator bool() const ;
+		operator int() const ;
+		operator Object() const ;
+		operator Array() const ;
+		operator std::string() const ;
 	} ;
-	Proxy As() const ;
+	Proxy Get() const ;
 	
 	template <typename T>
 	bool Is() const ;
diff --git a/src/protocol/OAuth2.cc b/src/protocol/OAuth2.cc
index 7a77283..0e86db9 100644
--- a/src/protocol/OAuth2.cc
+++ b/src/protocol/OAuth2.cc
@@ -57,7 +57,7 @@ void OAuth2::Auth( const std::string&	auth_code )
 		"&redirect_uri="	+ "urn:ietf:wg:oauth:2.0:oob" +
 		"&grant_type=authorization_code" ;
 
-	Json resp = Json::Parse( HttpPostData( token_url, post ) ) ;
+	Json resp = Json::Parse( http::PostData( token_url, post ) ) ;
 	m_access	= resp["access_token"].As<std::string>() ;
 	m_refresh	= resp["refresh_token"].As<std::string>() ;
 }
@@ -66,6 +66,8 @@ std::string OAuth2::MakeAuthURL(
 	const std::string&	client_id,
 	const std::string&	state )
 {
+	using gr::http::Escape ;
+
 	return "https://accounts.google.com/o/oauth2/auth"
 		"?scope=" +
 			Escape( "https://www.googleapis.com/auth/userinfo.email" )		+ "+" + 
@@ -86,7 +88,7 @@ void OAuth2::Refresh( )
 		"&client_secret="	+ m_client_secret +
 		"&grant_type=refresh_token" ;
 
-	Json resp = Json::Parse( HttpPostData( token_url, post ) ) ;
+	Json resp	= Json::Parse( http::PostData( token_url, post ) ) ;
 	m_access	= resp["access_token"].As<std::string>() ;
 }
 
diff --git a/src/util/Crypt.cc b/src/util/Crypt.cc
new file mode 100644
index 0000000..35deeb0
--- /dev/null
+++ b/src/util/Crypt.cc
@@ -0,0 +1,55 @@
+/*
+	grive: an GPL program to sync a local directory with Google Drive
+	Copyright (C) 2012  Wan Wai Ho
+
+	This program is free software; you can redistribute it and/or
+	modify it under the terms of the GNU General Public License
+	as published by the Free Software Foundation version 2
+	of the License.
+
+	This program is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with this program; if not, write to the Free Software
+	Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+*/
+
+#include "Crypt.hh"
+
+#include <iomanip>
+#include <sstream>
+
+// dependent libraries
+#include <openssl/evp.h>
+
+namespace gr { namespace crypt {
+
+std::string MD5( std::streambuf *file )
+{
+	char buf[64 * 1024] ;
+	EVP_MD_CTX	md ;
+	EVP_MD_CTX_init( &md );
+	EVP_DigestInit_ex( &md, EVP_md5(), 0 ) ;
+	
+	std::size_t count = 0 ;
+	while ( (count = file->sgetn( buf, sizeof(buf) )) > 0 )
+	{
+		EVP_DigestUpdate( &md, buf, count ) ;
+	}
+	
+	unsigned int md5_size = EVP_MAX_MD_SIZE ;
+	unsigned char md5[EVP_MAX_MD_SIZE] ;
+	EVP_DigestFinal_ex( &md, md5, &md5_size ) ;
+	
+	// format the MD5 string
+	std::ostringstream ss ;
+	for ( unsigned int i = 0 ; i < md5_size ; i++ )
+		ss << std::hex << std::setw(2) << std::setfill('0') << static_cast<int>(md5[i]) ;
+	
+	return ss.str() ;
+}
+
+} } // end of namespaces
diff --git a/src/util/Crypt.hh b/src/util/Crypt.hh
new file mode 100644
index 0000000..e7f0293
--- /dev/null
+++ b/src/util/Crypt.hh
@@ -0,0 +1,32 @@
+/*
+	grive: an GPL program to sync a local directory with Google Drive
+	Copyright (C) 2012  Wan Wai Ho
+
+	This program is free software; you can redistribute it and/or
+	modify it under the terms of the GNU General Public License
+	as published by the Free Software Foundation version 2
+	of the License.
+
+	This program is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with this program; if not, write to the Free Software
+	Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+*/
+
+#pragma once
+
+#include <string>
+#include <iosfwd>
+
+namespace gr {
+
+namespace crypt
+{
+	std::string MD5( std::streambuf *file ) ;
+}
+
+} // end of namespace gr