code: public GSSCredential get(GSSCredential credential,
                               String username,
                               String passphrase,
                               int lifetime)
          throws MyProxyException {
          GetParams request = new GetParams();
          request.setUserName(username);
          request.setPassphrase(passphrase);
          request.setLifetime(lifetime);
          return get(credential, request);
      }
docs: Retrieves delegated credentials from the MyProxy server.
  
  Notes: Performs simple verification of private/public keys of
  the delegated credential. Should be improved later.
  And only checks for RSA keys.
  
  @param  credential
  The local GSI credentials to use for authentication.
  Can be set to null if no local credentials.
  @param  username
  The username of the credentials to retrieve.
  @param  passphrase
  The passphrase of the credentials to retrieve.
  @param  lifetime
  The requested lifetime of the retrieved credential (in seconds).
  @return GSSCredential
  The retrieved delegated credentials.
  @exception MyProxyException
  If an error occurred during the operation.