server/apps/files_external/js/public_key.js

$(document).ready(function() {

	OCA.External.Settings.mountConfig.whenSelectAuthMechanism(function($tr, authMechanism, scheme, onCompletion) {
		if (scheme === 'publickey' && authMechanism === 'publickey::rsa') {
			var config = $tr.find('.configuration');
			if ($(config).find('[name="public_key_generate"]').length === 0) {
				setupTableRow($tr, config);
				onCompletion.then(function() {
					// If there's no private key, build one
					if (0 === $(config).find('[data-parameter="private_key"]').val().length) {
						generateKeys($tr);
					}
				});
			}
		}
	});

	$('#externalStorage').on('click', '[name="public_key_generate"]', function(event) {
		event.preventDefault();
		var tr = $(this).parent().parent();
		generateKeys(tr);
	});

	function setupTableRow(tr, config) {
		var selectList = document.createElement('select');
		selectList.id = 'keyLength';

		var options = [1024, 2048, 4096];
		for (var i = 0; i < options.length; i++) {
			var option = document.createElement('option');
			option.value = options[i];
			option.text = options[i];
			selectList.appendChild(option);
		}

		$(config).append(selectList);

		$(config).append($(document.createElement('input'))
			.addClass('button auth-param')
			.attr('type', 'button')
			.attr('value', t('files_external', 'Generate keys'))
			.attr('name', 'public_key_generate')
		);
	}

	function generateKeys(tr) {
		var config = $(tr).find('.configuration');
		var keyLength = config.find('#keyLength').val();

		$.post(OC.filePath('files_external', 'ajax', 'public_key.php'), {
			keyLength: keyLength
		}, function(result) {
			if (result && result.status === 'success') {
				$(config).find('[data-parameter="public_key"]').val(result.data.public_key).keyup();
				$(config).find('[data-parameter="private_key"]').val(result.data.private_key);
				OCA.External.Settings.mountConfig.saveStorageConfig(tr, function() {
					// Nothing to do
				});
			} else {
				OC.dialogs.alert(result.data.message, t('files_external', 'Error generating key pair') );
			}
		});
	}
});