Share between an iOS extension and its containing app with the keychain?

Question

I understand I can share data between my share extension and its containing app by enabling app groups and using NSUserDefaults (see Sharing data between an iOS 8 share extension and main app).

However, the data I am storing is sensitive, so I hoped to use the keychain. So the user would enter account information in the containing app, and then the share extension would read that data to perform the intended sharing action.

Does anyone know if this is possible? My first crack at it suggests that the extension and the containing app have separate keychains (saving the data with a key in the containing app returns null when attempting to return data for that key in the extension).

Thanks!

P.S. Using Lockbox for Keychain access, but I could ditch it if it's too much of an abstraction to make it work. https://github.com/granoff/Lockbox

Answer 1

To make the Keychain shared in Xcode 8.

1) In your App target in Capabilities find and turn on "Keychain Sharing", add a Keychain Group key (a reverse-domain style string like com.myappdomain.myappname)

2) Do exactly the same for the extension target. Make sure the Keychain Group key is the same for both - the app and the extension.

Add and retrieve data from Keychain in your usual way, no special changes required in the code. For example, here's how I put data into Keychain in the main app (a little old-fashioned but still works in Swift 3):

let login = loginString
let domain = domainString
let passwordData: Data = passwordString.data(using: String.Encoding.utf8, allowLossyConversion: false)!
let keychainQuery: [NSString: NSObject] = [
    kSecClass: kSecClassGenericPassword,
    kSecAttrAccount: login as NSObject,  // login and domain strings help identify
    kSecAttrService: domain as NSObject, // the required record in the Keychain
    kSecValueData: passwordData as NSObject]
SecItemDelete(keychainQuery as CFDictionary) //Deletes the item just in case it already exists
let keychainSaveStatus: OSStatus = SecItemAdd(keychainQuery as CFDictionary, nil)

And then retrieve it in the extension:

let keychainQuery: [NSString: NSObject] = [
    kSecClass: kSecClassGenericPassword,
    kSecAttrAccount: login as NSObject,
    kSecAttrService: domain as NSObject,
    kSecReturnData: kCFBooleanTrue,
    kSecMatchLimit: kSecMatchLimitOne]
var rawResult: AnyObject?
let keychain_get_status: OSStatus = SecItemCopyMatching(keychainQuery as CFDictionary, &rawResult)

if (keychain_get_status == errSecSuccess) {
    if let retrievedData = rawResult as? Data,
        let password = String(data: retrievedData, encoding: String.Encoding.utf8) {
       // "password" contains the password string now
    }
}

Note that you will still need to pass "login" and "domain" over to the extension in order to identify the correct record. This can be done via NSUserDefaults. See this answer on how to do this.