[Inf1mum]

Overview

NET-746: FTPClient#login(String, String) takes the password as a String, which cannot be reliably wiped after use. This PR adds a char[]-based login path so that sensitive data never has to be materialized into a String, and zeroes the password array once it has been transmitted to the server.

Changes

• FTPClient.login(String user, char[] password) (new): mirrors the existing login(String, String) flow, sends PASS through the new char[] path, and clears the password array with Arrays.fill(password, '\0') in a finally block once the password has been handed to the control connection (i.e. only when the password was actually transmitted; if the server never asks for a password, the array is left untouched).
• FTP.pass(char[]) (new): symmetric with the existing pass(String).
• FTP.sendCommand(String, char[]) and FTP.sendCommand(FTPCmd, char[]) (new): build and write the command as char[] end-to-end (buildMessage(String, char[]) and send(char[]) are new private helpers), so the sensitive arguments are never converted to a String. The ProtocolCommandEvent fired to command listeners reports the arguments as masked (PASS ******), unlike the String path, to keep secrets out of listener logs.
• FTPSClient: the CCC post-processing was extracted into a private postSendCCC(...) helper and a matching sendCommand(String, char[]) override was added so that FTPSClient behaves identically for the new overload (previously only sendCommand(String, String) restored the plain socket after CCC).
• Disambiguated existing bare-null call sites (sendCommand(command, null) in FTP) with (String) casts, since the new overloads otherwise make those calls ambiguous.

Tests

• New FTPClientLoginCharPasswordTest with a local scripted FTP server:
  • login(String, char[]) sends PASS <password> on the wire and clears the array afterwards (even on a 530 rejection);
  • the password is not leaked to ProtocolCommandListeners (they see PASS ******);
  • the array is left untouched when the server never requests a password;
  • FTP.pass(char[]) returns the expected reply code.
• FTPTest: CR/LF rejection tests for the new char[] argument path, symmetric with the existing String tests.

mvn test -Dtest=FTPTest,FTPClientTest,FTPClientLoginCharPasswordTest,FTPSClientTest passes.

Notes

• The existing String-based API is unchanged; the new overloads are additive (@since 3.13.1).
• changes.xml entry added under 3.13.1.

Thanks for considering this contribution!