ln.snmp/endpoint/USMEndpoint.cs

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

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

    public class USMEndpoint : SnmpEndpoint
    {
        public OctetString RemoteEngineID { get; set; }
        public Integer CacheAuthoritativeEngineBoots { get; set; }
        public Integer CacheAuthoritativeEngineTime { get; set; }

        public string Username { get; set; }

        public byte[] AuthKey { get; set; }
        public byte[] PrivKey { get; set; }

        public byte[] LocalAuthKey { get; set; }
        public byte[] LocalPrivKey { 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);

            USMMessage replyUSM = InternalRequest(request);

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

            return responsePDU;
        }

        private USMMessage InternalRequest(USMMessage request)
        {
            SnmpMessage reply = SNMPEngine.SNMPRequest(RemoteEndpoint, request, SNMPEngine.Timeout);
            USMMessage usmReply = reply as USMMessage;

            CacheAuthoritativeEngineBoots = usmReply.SecurityParameters.msgAuthoritativeEngineBoots;
            CacheAuthoritativeEngineTime = usmReply.SecurityParameters.msgAuthoritativeEngineTime;

            return usmReply;
        }

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

            ScopedPDU scopedPDU = new ScopedPDU();

            queryMessage.msgData = scopedPDU;

            queryMessage.Dump();

            USMMessage usmReply = InternalRequest(queryMessage);

            RemoteEngineID = usmReply.SecurityParameters.msgAuthoritativeEngineID;
            //RemoteEngineID.Bytes = new byte[] { 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x02 };

            LocalizeKeys();
        }

        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];
            message.SecurityParameters.msgAuthoritativeEngineBoots = CacheAuthoritativeEngineBoots != null ? CacheAuthoritativeEngineBoots : new Integer();
            message.SecurityParameters.msgAuthoritativeEngineTime = CacheAuthoritativeEngineTime != null ? CacheAuthoritativeEngineTime : new Integer(); 


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

            byte[] extendedAuthKey = new byte[64];
            byte[] authKey = LocalAuthKey;

            Array.Copy(authKey, extendedAuthKey, authKey.Length);

            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]);

            using (HashAlgorithm hash = CreateAuthHashAlgorithm())
            {
                byte[] b1 = new byte[K1.Length + wholeMsg.Length];

                Array.Copy(K1, b1, K1.Length);
                Array.Copy(wholeMsg, 0, b1, K1.Length, wholeMsg.Length);

                byte[] intermediate = hash.ComputeHash(b1);
                byte[] inter2 = new byte[K2.Length + intermediate.Length];

                Array.Copy(K2, inter2, K2.Length);
                Array.Copy(intermediate, 0, inter2, K2.Length, intermediate.Length);

                byte[] mac = hash.ComputeHash(inter2);

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

            return true;
        }

        private byte[] DeriveKey(string passphrase,HashAlgorithm hash)
        {
            using (hash)
            {
                byte[] pass = Encoding.ASCII.GetBytes(passphrase);
                byte[] block = new byte[64];

                for (int n = 0; n < 1048576; n += 64)
                {
                    for (int p = 0; p < block.Length; p++)
                    {
                        block[p] = pass[(p + n) % pass.Length];
                    }
                    hash.TransformBlock(block, 0, block.Length, block, 0);
                }

                hash.TransformFinalBlock(new byte[0], 0, 0);

                byte[] result = hash.Hash;
                return result;
            }
        }

        private HashAlgorithm CreateAuthHashAlgorithm()
        {
            return AuthMethod == SnmpV3AuthMethod.SHA ? (HashAlgorithm)SHA1.Create() : (HashAlgorithm)MD5.Create();
        }

        private void LocalizeKeys()
        {
            if ((AuthKey != null) && (RemoteEngineID != null))
            {
                using (HashAlgorithm hash = CreateAuthHashAlgorithm())
                {
                    byte[] engineID = (byte[])RemoteEngineID.Bytes.Clone();

                    hash.TransformBlock(AuthKey, 0, AuthKey.Length, AuthKey, 0);
                    hash.TransformBlock(engineID, 0, engineID.Length, engineID, 0);
                    hash.TransformBlock(AuthKey, 0, AuthKey.Length, AuthKey, 0);

                    hash.TransformFinalBlock(new byte[0], 0, 0);

                    LocalAuthKey = hash.Hash;

                    Logging.Log(LogLevel.DEBUG, "Derived LocalAuthKey from AuthKey: {0}", BitConverter.ToString(LocalAuthKey));
                }
            }
        }

        public string AuthKeyPhrase
        {
            get => throw new NotSupportedException();
            set
            {
                AuthKey = DeriveKey(
                                value,
                                CreateAuthHashAlgorithm()
                                );

                Logging.Log(LogLevel.DEBUG, "Derived AuthKey from Phrase: {0}",BitConverter.ToString(AuthKey));

                if (RemoteEngineID != null)
                    LocalizeKeys();
            }
        }
        public string PrivKeyPhrase { get; set; }




        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;
        }

    }
}