Simple String Key Value Access to iOS Keychain

Keychain.swift

import Foundation
import Security

private let SecClass: String = kSecClass as String
private let SecAttrService: String = kSecAttrService as String
private let SecAttrAccessible: String = kSecAttrAccessible as String
private let SecAttrGeneric: String = kSecAttrGeneric as String
private let SecAttrAccount: String = kSecAttrAccount as String
private let SecMatchLimit: String = kSecMatchLimit as String
private let SecReturnData: String = kSecReturnData as String
private let SecValueData: String! = kSecValueData as String

private let KeyChainAccessibility = kSecAttrAccessibleWhenUnlocked

class Keychain {

    @discardableResult static func save(key: String, value: String) -> Bool {
        var query = basicKeychainQuery(with: key)
        
        let encodedValue = value.data(using: .utf8)!
        query[SecValueData] = encodedValue
        
        let status: OSStatus = SecItemAdd(query as CFDictionary, nil)
        
        if status == errSecSuccess {
            return true
        } else if status == errSecDuplicateItem {
           return update(key: key, value: value)
        } else {
            return false
        }
    }
    
    static func load(key: String) -> String? {
        guard let data = keychainData(with: key) else { return nil }
        
        return String(data: data, encoding: .utf8) as String?
    }
    
    @discardableResult static func delete(key: String) -> Bool {
        let query = basicKeychainQuery(with: key)
        
        let status = SecItemDelete(query as CFDictionary)
        
        if status == errSecSuccess {
            return true
        } else {
            return false
        }
    }
}

private func keychainData(with key: String) -> Data? {
    var query = basicKeychainQuery(with: key)
    
    query[SecMatchLimit] = kSecMatchLimitOne
    query[SecReturnData] = kCFBooleanTrue
    
    var result: AnyObject?
    let status = withUnsafeMutablePointer(to: &result) {
        SecItemCopyMatching(query as CFDictionary, UnsafeMutablePointer($0))
    }
    
    if status == noErr {
        return result as? Data
    } else {
        return nil
    }
}

private func update(key: String, value: String) -> Bool {
    let query = basicKeychainQuery(with: key)
    
    let encodedValue = value.data(using: .utf8)!
    let update = [SecValueData: encodedValue]
    
    let status = SecItemUpdate(query as CFDictionary, update as CFDictionary)
    
    if status == errSecSuccess {
        return true
    } else {
        return false
    }
}

private func basicKeychainQuery(with key: String) -> [String:Any] {
    let encodedKey = key.data(using: .utf8)!
    return [SecClass: kSecClassGenericPassword,
            SecAttrService: Bundle.main.bundleIdentifier!,
            SecAttrAccessible: KeyChainAccessibility,
            SecAttrGeneric: encodedKey,
            SecAttrAccount: encodedKey]
}