How to use a PublickeyAuthenticator with Apache MINA SSHD

SSHDServer.java

private static final String knownKey = "{SSH2.PUBLIC.KEY}";

public void start() {
   SshServer sshd = SshServer.setUpDefaultServer();
   sshd.setPort(22999);
		
   sshd.setKeyPairProvider(new SimpleGeneratorHostKeyProvider("hostkey.ser"));
   sshd.setPublickeyAuthenticator(new PublickeyAuthenticator() {
      public boolean authenticate(String username, PublicKey key, ServerSession session) {
         if(key instanceof RSAPublicKey) {
            String s1 = new String(encode((RSAPublicKey) key));
            String s2 = new String(Base64.decodeBase64(knownKey.getBytes()));					
            return s1.equals(s2); //Returns true if the key matches our known key, this allows auth to proceed.
         }
         return false; //Doesn't handle other key types currently.
      }
   });
}

//Converts a Java RSA PK to SSH2 Format.
public static byte[] encode(RSAPublicKey key) {
   try {
      ByteArrayOutputStream buf = new ByteArrayOutputStream();
      byte[] name = "ssh-rsa".getBytes("US-ASCII");
      write(name, buf);
      write(key.getPublicExponent().toByteArray(), buf);
      write(key.getModulus().toByteArray(), buf);
      return buf.toByteArray();
   }
   catch(Exception e) {
      e.printStackTrace();
   }
   return null;
}

private static void write(byte[] str, OutputStream os) throws IOException {
   for (int shift = 24; shift >= 0; shift -= 8)
      os.write((str.length >>> shift) & 0xFF);
   os.write(str);
}

Hi,
I have a question, whether the knownKey is the public keys in ~/.ssh/authorized_keys?
When client use ssh -i privateFile to connect the Server, how does the client find the publicKey and pass it to the server, i.e., the parameter of Public Key in the function: public boolean authenticate(String username, PublicKey key, ServerSession session), thanks very much!