Source

File: wp-admin/includes/class-wp-filesystem-ssh2.php

public function connect() {
		if ( ! $this->keys ) {
			$this->link = @ssh2_connect( $this->options['hostname'], $this->options['port'] );
		} else {
			$this->link = @ssh2_connect( $this->options['hostname'], $this->options['port'], $this->options['hostkey'] );
		}

		if ( ! $this->link ) {
			$this->errors->add(
				'connect',
				sprintf(
					/* translators: %s: hostname:port */
					__( 'Failed to connect to SSH2 Server %s' ),
					$this->options['hostname'] . ':' . $this->options['port']
				)
			);

			return false;
		}

		if ( ! $this->keys ) {
			if ( ! @ssh2_auth_password( $this->link, $this->options['username'], $this->options['password'] ) ) {
				$this->errors->add(
					'auth',
					sprintf(
						/* translators: %s: Username. */
						__( 'Username/Password incorrect for %s' ),
						$this->options['username']
					)
				);

				return false;
			}
		} else {
			if ( ! @ssh2_auth_pubkey_file( $this->link, $this->options['username'], $this->options['public_key'], $this->options['private_key'], $this->options['password'] ) ) {
				$this->errors->add(
					'auth',
					sprintf(
						/* translators: %s: Username. */
						__( 'Public and Private keys incorrect for %s' ),
						$this->options['username']
					)
				);

				return false;
			}
		}

		$this->sftp_link = ssh2_sftp( $this->link );

		if ( ! $this->sftp_link ) {
			$this->errors->add(
				'connect',
				sprintf(
					/* translators: %s: hostname:port */
					__( 'Failed to initialize a SFTP subsystem session with the SSH2 Server %s' ),
					$this->options['hostname'] . ':' . $this->options['port']
				)
			);

			return false;
		}

		return true;
	}