Make early protocol auth mechanism generic, and add SRP

Adds everything needed for SRP (and everything works too),
but still deactivated, as protocol v25 init packets aren't final yet.
Can be activated by changing the LATEST_PROTOCOL_VERSION header to 25
inside networkprotocol.h.

src/network/networkprotocol.h

@@ -120,11 +120,15 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 			permit translation
 		Add TOCLIENT_DELETE_PARTICLESPAWNER (0x53), fixing the u16 read and
 			reading u32
-		Add TOSERVER_INIT new opcode (0x02) for client presentation to server
-		Add TOSERVER_AUTH new opcode (0x03) for client authentication
+		Add new opcode TOSERVER_INIT for client presentation to server
+		Add new opcodes TOSERVER_FIRST_SRP, TOSERVER_SRP_BYTES_A,
+			TOSERVER_SRP_BYTES_M, TOCLIENT_SRP_BYTES_S_B
+			for the three supported auth mechanisms around srp
+		Add new opcodes TOCLIENT_ACCEPT_SUDO_MODE and TOCLIENT_DENY_SUDO_MODE
+			for sudo mode handling (auth mech generic way of changing password).
 		Add TOCLIENT_HELLO for presenting server to client after client
 			presentation
-		Add TOCLIENT_AUTH_ACCEPT to accept connexion from client
+		Add TOCLIENT_AUTH_ACCEPT to accept connection from client
 */
 
 #define LATEST_PROTOCOL_VERSION 24
@@ -151,14 +155,31 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 enum ToClientCommand
 {
 	TOCLIENT_HELLO = 0x02,
-	TOCLIENT_AUTH_ACCEPT = 0x03,
-	TOCLIENT_ACCESS_DENIED = 0x0A,
 	/*
-		u16 command
-		u16 reason_length
-		wstring reason
-	*/
+		Sent after TOSERVER_INIT.
 
+		u8 deployed version
+		u32 supported auth methods
+		std::string username that should be used for legacy hash (for proper casing)
+	*/
+	TOCLIENT_AUTH_ACCEPT = 0x03,
+	/*
+		Message from server to accept auth.
+
+		v3s16 player's position + v3f(0,BS/2,0) floatToInt'd
+		u64 map seed
+		f1000 recommended send interval
+		u32 : supported auth methods for sudo mode
+		      (where the user can change their password)
+	*/
+	TOCLIENT_ACCEPT_SUDO_MODE = 0x04,
+	/*
+		Sent to client to show it is in sudo mode now.
+	*/
+	TOCLIENT_DENY_SUDO_MODE = 0x05,
+	/*
+		Signals client that sudo mode auth failed.
+	*/
 	TOCLIENT_INIT_LEGACY = 0x10,
 	/*
 		Server's reply to TOSERVER_INIT.
@@ -173,7 +194,11 @@ enum ToClientCommand
 		NOTE: The position in here is deprecated; position is
 		      explicitly sent afterwards
 	*/
-
+	TOCLIENT_ACCESS_DENIED = 0x0A,
+	/*
+		u8 reason
+		std::string custom reason (if reason == SERVER_ACCESSDENIED_CUSTOM_STRING)
+	*/
 	TOCLIENT_BLOCKDATA = 0x20, //TODO: Multiple blocks
 	TOCLIENT_ADDNODE = 0x21,
 	/*
@@ -589,7 +614,16 @@ enum ToClientCommand
 		u32 id
 	*/
 
-	TOCLIENT_NUM_MSG_TYPES = 0x54,
+	TOCLIENT_SRP_BYTES_S_B = 0x60,
+	/*
+		Belonging to AUTH_MECHANISM_LEGACY_PASSWORD and AUTH_MECHANISM_SRP.
+
+		u16 command
+		std::string bytes_s
+		std::string bytes_B
+	*/
+
+	TOCLIENT_NUM_MSG_TYPES = 0x61,
 };
 
 enum ToServerCommand
@@ -598,18 +632,11 @@ enum ToServerCommand
 	/*
 		Sent first after connected.
 
-		[0] u16 TOSERVER_INIT
 		[2] u8 SER_FMT_VER_HIGHEST_READ
 		[3] u8 compression_modes
-	*/
-
-	TOSERVER_AUTH = 0x03,
-	/*
-		Sent first after presentation (INIT).
-		[0] std::string player_name
-		[0+*] std::string password (new in some version)
-		[0+*+*] u16 minimum supported network protocol version (added sometime)
-		[0+*+*+2] u16 maximum supported network protocol version (added later than the previous one)
+		[4] u16 minimum supported network protocol version
+		[6] u16 maximum supported network protocol version
+		[8] std::string player name
 	*/
 
 	TOSERVER_INIT_LEGACY = 0x10,
@@ -817,15 +844,6 @@ enum ToServerCommand
 			u8[len] field value
 	*/
 
-	TOSERVER_PASSWORD = 0x3d,
-	/*
-		Sent to change password.
-
-		[0] u16 TOSERVER_PASSWORD
-		[2] std::string old password
-		[2+*] std::string new password
-	*/
-
 	TOSERVER_REQUEST_MEDIA = 0x40,
 	/*
 		u16 command
@@ -857,7 +875,49 @@ enum ToServerCommand
 		u8[len] full_version_string
 	*/
 
-	TOSERVER_NUM_MSG_TYPES = 0x44,
+	TOSERVER_FIRST_SRP = 0x50,
+	/*
+		Belonging to AUTH_MECHANISM_FIRST_SRP.
+
+		std::string srp salt
+		std::string srp verification key
+		u8 is_empty (=1 if password is empty, 0 otherwise)
+	*/
+
+	TOSERVER_SRP_BYTES_A = 0x51,
+	/*
+		Belonging to AUTH_MECHANISM_LEGACY_PASSWORD and AUTH_MECHANISM_SRP,
+			depending on current_login_based_on.
+
+		std::string bytes_A
+		u8 current_login_based_on : on which version of the password's
+		                            hash this login is based on (0 legacy hash,
+		                            or 1 directly the password)
+	*/
+
+	TOSERVER_SRP_BYTES_M = 0x52,
+	/*
+		Belonging to AUTH_MECHANISM_LEGACY_PASSWORD and AUTH_MECHANISM_SRP.
+
+		std::string bytes_M
+	*/
+
+	TOSERVER_NUM_MSG_TYPES = 0x53,
+};
+
+enum AuthMechanism
+{
+	// reserved
+	AUTH_MECHANISM_NONE = 0,
+
+	// SRP based on the legacy hash
+	AUTH_MECHANISM_LEGACY_PASSWORD = 1 << 0,
+
+	// SRP based on the srp verification key
+	AUTH_MECHANISM_SRP = 1 << 1,
+
+	// Establishes a srp verification key, for first login and password changing
+	AUTH_MECHANISM_FIRST_SRP = 1 << 2,
 };
 
 enum AccessDeniedCode {