Register AES256 with 3DES extension protocol, add tecnical preview note.

Author: mashhurs

docs/input-snmp.asciidoc

@@ -418,6 +418,12 @@ The `priv_pass` option specifies the SNMPv3 encryption password.
 
 The `priv_protocol` option specifies the SNMPv3 privacy/encryption protocol.
 
+.Technical Preview
+****
+The `aes256with3desKey` feature that allows using AES256 with 3DES key extension for `priv_protocol` is in Technical Preview.
+Configuration option and implementation details are subject to change in minor releases without being preceded by deprecation warnings.
+****
+
 [id="plugins-{type}s-{plugin}-security_level"]
 ===== `security_level`
 

src/main/java/org/logstash/snmp/SnmpClient.java

@@ -33,6 +33,7 @@
 import org.snmp4j.security.SecurityProtocols.SecurityProtocolSet;
 import org.snmp4j.security.TSM;
 import org.snmp4j.security.USM;
+import org.snmp4j.security.nonstandard.PrivAES256With3DESKeyExtension;
 import org.snmp4j.smi.Address;
 import org.snmp4j.smi.AssignableFromInteger;
 import org.snmp4j.smi.AssignableFromLong;
@@ -128,6 +129,7 @@ public static SnmpClientBuilder builder(MibManager mib, Set<String> protocols, i
         // global security models/protocols
         SecurityProtocols.getInstance().addPredefinedProtocolSet(SecurityProtocolSet.maxCompatibility);
         SecurityProtocols.getInstance().addPrivacyProtocol(new Priv3DES());
+        SecurityProtocols.getInstance().addPrivacyProtocol(new PrivAES256With3DESKeyExtension());
 
         if (supportedVersions.contains(SnmpConstants.version3)) {
             SecurityModels.getInstance().addSecurityModel(new TSM(localEngineId, false));