Simple class for storing Strings in the Keychain

KeychainStore.swift

import Foundation

final class KeychainStore {
	
	func string(for key: String) -> String? {
		return self.readItem(forKey: key)
	}
	
	func setString(_ string: String?, for key: String) {
		guard let string = string else {
			deleteItem(key: key)
			return
		}
		
		if self.readItem(forKey: key) == nil {
			createItem(string, forKey: key)
		} else {
			updateItem(string, forKey: key)
		}
	}
	
	// MARK: Keychain CRUD
	
	private func createItem(_ item: String, forKey key: String) {
		var newItem = self.query(key: key)
		newItem[kSecValueData as String] = item.data(using: .utf8)
		_ = SecItemAdd(newItem as CFDictionary, nil)
	}
	
	private func readItem(forKey key: String) -> String? {
		var query = self.query(key: key)
		query[kSecMatchLimit as String] = kSecMatchLimitOne
		query[kSecReturnAttributes as String] = kCFBooleanTrue
		query[kSecReturnData as String] = kCFBooleanTrue
		
		var queryResult: AnyObject?
		let status = withUnsafeMutablePointer(to: &queryResult) {
			SecItemCopyMatching(query as CFDictionary, UnsafeMutablePointer($0))
		}
		
		guard
			status == noErr,
			let existingItem = queryResult as? [String: Any],
			let stringData = existingItem[kSecValueData as String] as? Data,
			let string = String(data: stringData, encoding: .utf8)
			else {
				return nil
		}
		return string
	}
	
	private func updateItem(_ item: String, forKey key: String) {
		var attributesToUpdate = [String: Any]()
		attributesToUpdate[kSecValueData as String] = item.data(using: .utf8)
		let query = self.query(key: key)
		_ = SecItemUpdate(query as CFDictionary, attributesToUpdate as CFDictionary)
	}
	
	private func deleteItem(key: String) {
		let query = self.query(key: key)
		_ = SecItemDelete(query as CFDictionary)
	}
	
	private func query(key: String) -> [String: Any] {
		var query = [String: Any]()
		query[kSecClass as String] = kSecClassGenericPassword
		let serviceName = Bundle.main.bundleIdentifier ?? "DefaultServiceName"
		query[kSecAttrService as String] = serviceName
		
		let encodedKey: Data? = key.data(using: .utf8)
		query[kSecAttrGeneric as String] = encodedKey
		query[kSecAttrAccount as String] = encodedKey
		
		return query
	}
}