fix PlainAuthenticationHandlerFactory base64 decode behaviour (#143)

src/main/java/org/subethamail/smtp/auth/PlainAuthenticationHandlerFactory.java

@@ -84,10 +84,17 @@ public Optional<String> auth(String clientInput, MessageContext context) throws
 				}
 			}
 
-			byte[] decodedSecret = Base64.getDecoder().decode(secret);
-			if (decodedSecret == null)
+			byte[] decodedSecret;
+			try {
+			    decodedSecret = Base64.getDecoder().decode(secret);
+			} catch (IllegalArgumentException e) {
+			    decodedSecret = null;
+			}
+
+			if (decodedSecret == null) {
 				throw new RejectException(501, /*5.5.4*/
 						"Invalid command argument, not a valid Base64 string");
+			}
 
 			/*
 			 * RFC4616: The client presents the authorization identity (identity

src/test/java/org/subethamail/smtp/PlainAuthenticationHandlerFactoryTest.java

@@ -0,0 +1,31 @@
+package org.subethamail.smtp;
+
+import static org.junit.Assert.assertEquals;
+
+import org.junit.Assert;
+import org.junit.Test;
+import org.mockito.Mockito;
+import org.subethamail.smtp.auth.LoginFailedException;
+import org.subethamail.smtp.auth.PlainAuthenticationHandlerFactory;
+import org.subethamail.smtp.auth.UsernamePasswordValidator;
+
+public class PlainAuthenticationHandlerFactoryTest {
+
+    @Test
+    public void testBadBase64String() throws RejectException {
+        UsernamePasswordValidator validator = (username, password, c) -> {
+            if (!username.equals("fred") || !password.equals("blah")) {
+                throw new LoginFailedException();
+            }
+        };
+        AuthenticationHandler auth = new PlainAuthenticationHandlerFactory(validator).create();
+        MessageContext context = Mockito.mock(MessageContext.class);
+        try {
+            auth.auth("AUTH PLAIN b", context);
+            Assert.fail();
+        } catch (RejectException e) {
+            assertEquals("Invalid command argument, not a valid Base64 string", e.getMessage());
+        }
+    }
+
+}