diff --git a/src/crypto/crypto.cc b/src/crypto/crypto.cc
index d6c27aa..6bcb7c7 100644
--- a/src/crypto/crypto.cc
+++ b/src/crypto/crypto.cc
@@ -184,14 +184,14 @@ Nonce::Nonce( uint64_t val )
   memcpy( bytes + 4, &val_net, 8 );
 }
 
-uint64_t Nonce::val( void )
+uint64_t Nonce::val( void ) const
 {
   uint64_t ret;
   memcpy( &ret, bytes + 4, 8 );
   return be64toh( ret );
 }
 
-Nonce::Nonce( char *s_bytes, size_t len )
+Nonce::Nonce( const char *s_bytes, size_t len )
 {
   if ( len != 8 ) {
     throw CryptoException( "Nonce representation must be 8 octets long." );
@@ -201,18 +201,18 @@ Nonce::Nonce( char *s_bytes, size_t len )
   memcpy( bytes + 4, s_bytes, 8 );
 }
 
-Message::Message( char *nonce_bytes, size_t nonce_len,
-		  char *text_bytes, size_t text_len )
+Message::Message( const char *nonce_bytes, size_t nonce_len,
+		  const char *text_bytes, size_t text_len )
   : nonce( nonce_bytes, nonce_len ),
-    text( (char *)text_bytes, text_len )
+    text( text_bytes, text_len )
 {}
 
-Message::Message( Nonce s_nonce, string s_text )
+Message::Message( const Nonce & s_nonce, const string & s_text )
   : nonce( s_nonce ),
     text( s_text )
 {}
 
-string Session::encrypt( Message plaintext )
+const string Session::encrypt( const Message & plaintext )
 {
   const size_t pt_len = plaintext.text.size();
   const int ciphertext_len = pt_len + 16;
@@ -262,13 +262,13 @@ string Session::encrypt( Message plaintext )
   return plaintext.nonce.cc_str() + text;
 }
 
-Message Session::decrypt( string ciphertext )
+const Message Session::decrypt( const string & ciphertext )
 {
   if ( ciphertext.size() < 24 ) {
     throw CryptoException( "Ciphertext must contain nonce and tag." );
   }
 
-  char *str = (char *)ciphertext.data();
+  const char *str = ciphertext.data();
 
   int body_len = ciphertext.size() - 8;
   int pt_len = body_len - 16;
@@ -297,7 +297,7 @@ Message Session::decrypt( string ciphertext )
     throw CryptoException( "Packet failed integrity check." );
   }
 
-  Message ret( nonce, string( plaintext_buffer.data(), pt_len ) );
+  const Message ret( nonce, string( plaintext_buffer.data(), pt_len ) );
 
   return ret;
 }
diff --git a/src/crypto/crypto.h b/src/crypto/crypto.h
index 472850a..ae51155 100644
--- a/src/crypto/crypto.h
+++ b/src/crypto/crypto.h
@@ -109,21 +109,21 @@ namespace Crypto {
 
   public:
     Nonce( uint64_t val );
-    Nonce( char *s_bytes, size_t len );
+    Nonce( const char *s_bytes, size_t len );
     
     string cc_str( void ) const { return string( bytes + 4, 8 ); }
     const char *data( void ) const { return bytes; }
-    uint64_t val( void );
+    uint64_t val( void ) const;
   };
   
   class Message {
   public:
-    Nonce nonce;
-    string text;
+    const Nonce nonce;
+    const string text;
     
-    Message( char *nonce_bytes, size_t nonce_len,
-	     char *text_bytes, size_t text_len );
-    Message( Nonce s_nonce, string s_text );
+    Message( const char *nonce_bytes, size_t nonce_len,
+	     const char *text_bytes, size_t text_len );
+    Message( const Nonce & s_nonce, const string & s_text );
   };
   
   class Session {
@@ -145,8 +145,8 @@ namespace Crypto {
     Session( Base64Key s_key );
     ~Session();
     
-    string encrypt( Message plaintext );
-    Message decrypt( string ciphertext );
+    const string encrypt( const Message & plaintext );
+    const Message decrypt( const string & ciphertext );
     
     Session( const Session & );
     Session & operator=( const Session & );
diff --git a/src/network/network.cc b/src/network/network.cc
index e920a5d..347bde4 100644
--- a/src/network/network.cc
+++ b/src/network/network.cc
@@ -96,7 +96,7 @@ string Packet::tostring( Session *session )
   return session->encrypt( Message( Nonce( direction_seq ), timestamps + payload ) );
 }
 
-Packet Connection::new_packet( string &s_payload )
+Packet Connection::new_packet( const string &s_payload )
 {
   uint16_t outgoing_timestamp_reply = -1;
 
@@ -391,7 +391,7 @@ Connection::Connection( const char *key_str, const char *ip, const char *port )
   set_MTU( remote_addr.sa.sa_family );
 }
 
-void Connection::send( string s )
+void Connection::send( const string & s )
 {
   if ( !has_remote_addr ) {
     return;
diff --git a/src/network/network.h b/src/network/network.h
index 677d0d0..c5f743c 100644
--- a/src/network/network.h
+++ b/src/network/network.h
@@ -82,7 +82,7 @@ namespace Network {
     string payload;
     
     Packet( Direction s_direction,
-	    uint16_t s_timestamp, uint16_t s_timestamp_reply, string s_payload )
+	    uint16_t s_timestamp, uint16_t s_timestamp_reply, const string & s_payload )
       : seq( Crypto::unique() ), direction( s_direction ),
 	timestamp( s_timestamp ), timestamp_reply( s_timestamp_reply ), payload( s_payload )
     {}
@@ -190,7 +190,7 @@ namespace Network {
     bool have_send_exception;
     NetworkException send_exception;
 
-    Packet new_packet( string &s_payload );
+    Packet new_packet( const string &s_payload );
 
     void hop_port( void );
 
@@ -209,7 +209,7 @@ namespace Network {
     Connection( const char *desired_ip, const char *desired_port ); /* server */
     Connection( const char *key_str, const char *ip, const char *port ); /* client */
 
-    void send( string s );
+    void send( const string & s );
     string recv( void );
     const std::vector< int > fds( void ) const;
     int get_MTU( void ) const { return MTU; }
diff --git a/src/network/transportfragment.cc b/src/network/transportfragment.cc
index ad52187..181d63e 100644
--- a/src/network/transportfragment.cc
+++ b/src/network/transportfragment.cc
@@ -72,7 +72,7 @@ string Fragment::tostring( void )
   return ret;
 }
 
-Fragment::Fragment( string &x )
+Fragment::Fragment( const string &x )
   : id( -1 ), fragment_num( -1 ), final( false ), initialized( true ),
     contents()
 {
diff --git a/src/network/transportfragment.h b/src/network/transportfragment.h
index 9fda5a8..e5559f8 100644
--- a/src/network/transportfragment.h
+++ b/src/network/transportfragment.h
@@ -61,12 +61,12 @@ namespace Network {
       : id( -1 ), fragment_num( -1 ), final( false ), initialized( false ), contents()
     {}
 
-    Fragment( uint64_t s_id, uint16_t s_fragment_num, bool s_final, string s_contents )
+    Fragment( uint64_t s_id, uint16_t s_fragment_num, bool s_final, const string & s_contents )
       : id( s_id ), fragment_num( s_fragment_num ), final( s_final ), initialized( true ),
 	contents( s_contents )
     {}
 
-    Fragment( string &x );
+    Fragment( const string &x );
 
     string tostring( void );
 
diff --git a/src/network/transportsender.cc b/src/network/transportsender.cc
index 01f3105..7e4d58c 100644
--- a/src/network/transportsender.cc
+++ b/src/network/transportsender.cc
@@ -229,7 +229,7 @@ void TransportSender<MyState>::add_sent_state( uint64_t the_timestamp, uint64_t
 }
 
 template <class MyState>
-void TransportSender<MyState>::send_to_receiver( string diff )
+void TransportSender<MyState>::send_to_receiver( const string & diff )
 {
   uint64_t new_num;
   if ( current_state == sent_states.back().state ) { /* previously sent */
@@ -310,7 +310,7 @@ const string TransportSender<MyState>::make_chaff( void )
 }
 
 template <class MyState>
-void TransportSender<MyState>::send_in_fragments( string diff, uint64_t new_num )
+void TransportSender<MyState>::send_in_fragments( const string & diff, uint64_t new_num )
 {
   Instruction inst;
 
diff --git a/src/network/transportsender.h b/src/network/transportsender.h
index 9ad3848..ba9423c 100644
--- a/src/network/transportsender.h
+++ b/src/network/transportsender.h
@@ -64,9 +64,9 @@ namespace Network {
     void update_assumed_receiver_state( void );
     void attempt_prospective_resend_optimization( string &proposed_diff );
     void rationalize_states( void );
-    void send_to_receiver( string diff );
+    void send_to_receiver( const string & diff );
     void send_empty_ack( void );
-    void send_in_fragments( string diff, uint64_t new_num );
+    void send_in_fragments( const string & diff, uint64_t new_num );
     void add_sent_state( uint64_t the_timestamp, uint64_t num, MyState &state );
 
     /* state of sender */
diff --git a/src/network/transportstate.h b/src/network/transportstate.h
index efe3c59..9c52b69 100644
--- a/src/network/transportstate.h
+++ b/src/network/transportstate.h
@@ -42,7 +42,7 @@ namespace Network {
     uint64_t num;
     State state;
     
-    TimestampedState( uint64_t s_timestamp, uint64_t s_num, State &s_state )
+    TimestampedState( uint64_t s_timestamp, uint64_t s_num, const State &s_state )
       : timestamp( s_timestamp ), num( s_num ), state( s_state )
     {}
 
diff --git a/src/statesync/completeterminal.cc b/src/statesync/completeterminal.cc
index c22c6d0..64211ff 100644
--- a/src/statesync/completeterminal.cc
+++ b/src/statesync/completeterminal.cc
@@ -102,7 +102,7 @@ string Complete::init_diff( void ) const
   return diff_from( Complete( get_fb().ds.get_width(), get_fb().ds.get_height() ));
 }
 
-void Complete::apply_string( string diff )
+void Complete::apply_string( const string & diff )
 {
   HostBuffers::HostMessage input;
   fatal_assert( input.ParseFromString( diff ) );
diff --git a/src/statesync/completeterminal.h b/src/statesync/completeterminal.h
index cbcaa12..26e89a7 100644
--- a/src/statesync/completeterminal.h
+++ b/src/statesync/completeterminal.h
@@ -77,7 +77,7 @@ namespace Terminal {
     void subtract( const Complete * ) const {}
     std::string diff_from( const Complete &existing ) const;
     std::string init_diff( void ) const;
-    void apply_string( std::string diff );
+    void apply_string( const std::string & diff );
     bool operator==( const Complete &x ) const;
 
     bool compare( const Complete &other ) const;
diff --git a/src/statesync/user.cc b/src/statesync/user.cc
index 763e860..77dab8e 100644
--- a/src/statesync/user.cc
+++ b/src/statesync/user.cc
@@ -105,7 +105,7 @@ string UserStream::diff_from( const UserStream &existing ) const
   return output.SerializeAsString();
 }
 
-void UserStream::apply_string( string diff )
+void UserStream::apply_string( const string &diff )
 {
   ClientBuffers::UserMessage input;
   fatal_assert( input.ParseFromString( diff ) );
diff --git a/src/statesync/user.h b/src/statesync/user.h
index 5c5a295..04a3ad5 100644
--- a/src/statesync/user.h
+++ b/src/statesync/user.h
@@ -57,8 +57,8 @@ namespace Network {
     Parser::UserByte userbyte;
     Parser::Resize resize;
 
-    UserEvent( Parser::UserByte s_userbyte ) : type( UserByteType ), userbyte( s_userbyte ), resize( -1, -1 ) {}
-    UserEvent( Parser::Resize s_resize ) : type( ResizeType ), userbyte( 0 ), resize( s_resize ) {}
+    UserEvent( const Parser::UserByte & s_userbyte ) : type( UserByteType ), userbyte( s_userbyte ), resize( -1, -1 ) {}
+    UserEvent( const Parser::Resize & s_resize ) : type( ResizeType ), userbyte( 0 ), resize( s_resize ) {}
 
     UserEvent() /* default constructor required by C++11 STL */
       : type( UserByteType ),
@@ -79,8 +79,8 @@ namespace Network {
   public:
     UserStream() : actions() {}
     
-    void push_back( Parser::UserByte s_userbyte ) { actions.push_back( UserEvent( s_userbyte ) ); }
-    void push_back( Parser::Resize s_resize ) { actions.push_back( UserEvent( s_resize ) ); }
+    void push_back( const Parser::UserByte & s_userbyte ) { actions.push_back( UserEvent( s_userbyte ) ); }
+    void push_back( const Parser::Resize & s_resize ) { actions.push_back( UserEvent( s_resize ) ); }
     
     bool empty( void ) const { return actions.empty(); }
     size_t size( void ) const { return actions.size(); }
@@ -90,7 +90,7 @@ namespace Network {
     void subtract( const UserStream *prefix );
     string diff_from( const UserStream &existing ) const;
     string init_diff( void ) const { assert( false ); return string(); };
-    void apply_string( string diff );
+    void apply_string( const string &diff );
     bool operator==( const UserStream &x ) const { return actions == x.actions; }
 
     bool compare( const UserStream & ) { return false; }
diff --git a/src/terminal/terminaldispatcher.cc b/src/terminal/terminaldispatcher.cc
index 2c901a6..b7c0ee9 100644
--- a/src/terminal/terminaldispatcher.cc
+++ b/src/terminal/terminaldispatcher.cc
@@ -170,7 +170,7 @@ DispatchRegistry & Terminal::get_global_dispatch_registry( void )
 }
 
 static void register_function( Function_Type type,
-			       std::string dispatch_chars,
+			       const std::string & dispatch_chars,
 			       Function f )
 {
   switch ( type ) {
@@ -186,7 +186,7 @@ static void register_function( Function_Type type,
   }
 }
 
-Function::Function( Function_Type type, std::string dispatch_chars,
+Function::Function( Function_Type type, const std::string & dispatch_chars,
 		    void (*s_function)( Framebuffer *, Dispatcher * ),
 		    bool s_clears_wrap_state )
   : function( s_function ), clears_wrap_state( s_clears_wrap_state )
diff --git a/src/terminal/terminaldispatcher.h b/src/terminal/terminaldispatcher.h
index 5bdc479..cc92c2e 100644
--- a/src/terminal/terminaldispatcher.h
+++ b/src/terminal/terminaldispatcher.h
@@ -59,7 +59,7 @@ namespace Terminal {
   class Function {
   public:
     Function() : function( NULL ), clears_wrap_state( true ) {}
-    Function( Function_Type type, std::string dispatch_chars,
+    Function( Function_Type type, const std::string & dispatch_chars,
 	      void (*s_function)( Framebuffer *, Dispatcher * ),
 	      bool s_clears_wrap_state = true );
     void (*function)( Framebuffer *, Dispatcher * );
diff --git a/src/terminal/terminaldisplay.h b/src/terminal/terminaldisplay.h
index 887c93d..00b3f59 100644
--- a/src/terminal/terminaldisplay.h
+++ b/src/terminal/terminaldisplay.h
@@ -49,9 +49,9 @@ namespace Terminal {
 
     FrameState( const Framebuffer &s_last );
 
-    void append( const char c ) { str.append( 1, c ); }
-    void append( const size_t s, const char c ) { str.append( s, c ); }
-    void append( const wchar_t wc ) { Cell::append_to_str( str, wc ); }
+    void append( char c ) { str.append( 1, c ); }
+    void append( size_t s, char c ) { str.append( s, c ); }
+    void append( wchar_t wc ) { Cell::append_to_str( str, wc ); }
     void append( const char * s ) { str.append( s ); }
     void append_string( const std::string &append ) { str.append(append); }