using System;
using System.Net;
using ln.snmp.types;
using System.Text;
using System.Security.Cryptography;
using ln.snmp.asn1;
using System.Linq;

namespace ln.snmp.channel
{
    public enum SnmpV3AuthMethod { MD5, SHA }
    public enum SnmpV3PrivMethod { DES }
    public enum SnmpAuthLevel : int { noAuthNoPriv = 0, authNoPriv = 0x01, authPriv = 0x03 }

    public class USMEndpoint : SNMPInterface
    {
        public OctetString RemoteEngineID { get; set; }

        public string Username { get; set; }
        public string AuthKey { get; set; }
        public string PrivKey { get; set; }

        public SnmpV3AuthMethod AuthMethod { get; set; }
        public SnmpV3PrivMethod PrivMethod { get; set; }

        public override SnmpVersion SnmpVersion => SnmpVersion.V3;

        public USMEndpoint(SNMPEngine snmpEngine,IPEndPoint remoteEndpoint)
            :base(snmpEngine,remoteEndpoint)
        {
        }

        public SnmpAuthLevel AuthLevel
        {
            get
            {
                if (PrivKey != null)
                    return SnmpAuthLevel.authPriv;
                if (AuthKey != null)
                    return SnmpAuthLevel.authNoPriv;

                return SnmpAuthLevel.noAuthNoPriv;
            }
        }

        public override PDU snmpRequest(PDU pdu)
        {
            if ((RemoteEngineID == null)||(RemoteEngineID.Bytes.Length == 0))
            {
                QueryEngineID();
            }

            USMMessage request = new USMMessage();
            ScopedPDU scopedPDU = new ScopedPDU();

            request.MessageID = SNMPEngine.NextMessageID;
            request.msgData = scopedPDU;

            scopedPDU.contextEngineID = RemoteEngineID;
            scopedPDU.PDU = pdu;

            AuthenticateMessage(request);


            SnmpMessage reply = SNMPEngine.SNMPRequest(RemoteEndpoint, request, SNMPEngine.Timeout);
            USMMessage replyUSM = reply as USMMessage;

            PDU responsePDU = (replyUSM.msgData as ScopedPDU).PDU;

            return responsePDU;
        }

        public void QueryEngineID()
        {
            USMMessage queryMessage = new USMMessage();
            queryMessage.MessageID = SNMPEngine.NextMessageID;

            ScopedPDU scopedPDU = new ScopedPDU();

            queryMessage.msgData = scopedPDU;

            queryMessage.Dump();


            SnmpMessage reply = SNMPEngine.SNMPRequest(RemoteEndpoint, queryMessage, SNMPEngine.Timeout);
            USMMessage usmReply = reply as USMMessage;

            RemoteEngineID = usmReply.SecurityParameters.msgAuthoritativeEngineID;
        }

        public bool AuthenticateMessage(USMMessage message)
        {
            message.msgGlobalData.msgFlags.Bytes = new byte[] { (byte)(int)AuthLevel };
            message.SecurityParameters.msgUserName.StringValue = Username;
            message.SecurityParameters.msgAuthoritativeEngineID = RemoteEngineID;
            message.SecurityParameters.msgAuthenticationParameters.Bytes = new byte[12];

            byte[] wholeMsg = ((ASN1Value)message).AsByteArray;

            byte[] extendedAuthKey = new byte[64];
            byte[] authKey = Encoding.ASCII.GetBytes(AuthKey);

            //Array.Copy(authKey, extendedAuthKey, 16);

            //byte[] K1 = new byte[64];
            //for (int n = 0; n < 64; n++)
            //    K1[n] = (byte)(extendedAuthKey[n] ^ IPAD[n]);

            //byte[] K2 = new byte[64];
            //for (int n = 0; n < 64; n++)
                //K2[n] = (byte)(extendedAuthKey[n] ^ OPAD[n]);

            HMAC hmac = null;

            switch (AuthMethod)
            {
                case SnmpV3AuthMethod.MD5:
                    hmac = HMACMD5.Create();
                    break;
                case SnmpV3AuthMethod.SHA:
                    hmac = HMACSHA1.Create();
                    break;
            }

            hmac.Key = authKey;
            byte[] mac = hmac.ComputeHash(wholeMsg);

            message.SecurityParameters.msgAuthenticationParameters.Bytes = mac.Take(12).ToArray();

            return true;
        }

        static byte[] IPAD = __PAD(0x36);
        static byte[] OPAD = __PAD(0x5C);

        public static byte[] __PAD(byte pad)
        {
            byte[] ipad = new byte[0x40];
            for (int n = 0; n < ipad.Length; n++)
                ipad[n] = pad;
            return ipad;
        }

    }
}