Swift Keychain Class

## KeychainService.swift
import UIKit
import Security

let serviceIdentifier = "com.company"
let accessGroup = "com.company.app"

let kSecClassValue = kSecClass as NSString
let kSecAttrAccountValue = kSecAttrAccount as NSString
let kSecValueDataValue = kSecValueData as NSString
let kSecClassGenericPasswordValue = kSecClassGenericPassword as NSString
let kSecAttrServiceValue = kSecAttrService as NSString
let kSecMatchLimitValue = kSecMatchLimit as NSString
let kSecReturnDataValue = kSecReturnData as NSString
let kSecMatchLimitOneValue = kSecMatchLimitOne as NSString

class KeychainService: NSObject {
    class func setString(value: NSString, forKey: String) {
        self.save(serviceIdentifier, key: forKey, data: value)
    }

    class func stringForKey(key: String) -> NSString? {
        var token = self.load(serviceIdentifier, key: key)

        return token
    }

    class func removeItemForKey(key: String) {
        self.save(serviceIdentifier, key: key, data: "")
    }

    class func save(service: NSString, key: String, data: NSString) {
        var dataFromString: NSData = data.dataUsingEncoding(NSUTF8StringEncoding, allowLossyConversion: false)!
        // Instantiate a new default keychain query
        var keychainQuery: NSMutableDictionary = NSMutableDictionary(objects: [kSecClassGenericPasswordValue, service, key, dataFromString], forKeys: [kSecClassValue, kSecAttrServiceValue, kSecAttrAccountValue, kSecValueDataValue])

        // Delete any existing items
        SecItemDelete(keychainQuery as CFDictionaryRef)

        if data == "" { return }

        // Add the new keychain item
        var status: OSStatus = SecItemAdd(keychainQuery as CFDictionaryRef, nil)
    }

    class func load(service: NSString, key: String) -> NSString? {
        // Instantiate a new default keychain query
        // Tell the query to return a result
        // Limit our results to one item
        var keychainQuery: NSMutableDictionary = NSMutableDictionary(objects: [kSecClassGenericPasswordValue, service, key, kCFBooleanTrue, kSecMatchLimitOneValue], forKeys: [kSecClassValue, kSecAttrServiceValue, kSecAttrAccountValue, kSecReturnDataValue, kSecMatchLimitValue])

        var dataTypeRef :Unmanaged<AnyObject>?

        // Search for the keychain items
        let status: OSStatus = SecItemCopyMatching(keychainQuery, &dataTypeRef)

        let opaque = dataTypeRef?.toOpaque()

        var contentsOfKeychain: NSString?

        if let op = opaque? {
            let retrievedData = Unmanaged<NSData>.fromOpaque(op).takeUnretainedValue()

            // Convert the data retrieved from the keychain into a string
            contentsOfKeychain = NSString(data: retrievedData, encoding: NSUTF8StringEncoding)
        } else {
            return nil
        }

        return contentsOfKeychain
    }
}