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let bob_shared_secret: NSData = generateSharedKey_ecdh(publicKey: alicePublicKey, privateKey: bobPrivateKey)!
let alice_shared_secret: NSData = generateSharedKey_ecdh(publicKey: bobPublicKey, privateKey: alicePrivateKey)!

print("equals? \(bob_shared_secret == alice_shared_secret)!") //true

let cipherTextData: Data? = SecKeyCreateEncryptedData(alicePublicKey, algorithm,
                                                              clearTextData as CFData,
                                                              &error) as Data?

let clearTextData = SecKeyCreateDecryptedData(???? as SecKey, //what to put here??
                                                          algorithm,
                                                          cipherTextData as CFData,
                                                          &error) as Data?

private func generateSharedKey_ecdh(publicKey: SecKey, privateKey: SecKey) -> NSData?
    {
        var error: Unmanaged<CFError>?
        
        let keyPairAttr:[String : Any] = [
            kSecAttrKeySizeInBits as String: 256, //retro compatibility
            kSecAttrKeyType as String: kSecAttrKeyTypeECSECPrimeRandom, //Elliptic curve algorithm.
            kSecPrivateKeyAttrs as String: [kSecAttrIsPermanent as String: false],
            kSecPublicKeyAttrs as String:[kSecAttrIsPermanent as String: false],
            SecKeyKeyExchangeParameter.requestedSize.rawValue as String: 256
        ]
        let algorithm:SecKeyAlgorithm = SecKeyAlgorithm.ecdhKeyExchangeStandardX963SHA256
        
        let shared:CFData? = SecKeyCopyKeyExchangeResult(privateKey, algorithm, publicKey, keyPairAttr as CFDictionary, &error)
        
        return shared
    }