Elliptic Curve Diffie Hellman in ios/swift - ios

Question

Does iOS expose API for key generation, and secret key derivation using ECDH?
From what I see, apple are using it (and specifically x25519) internally but I don't see it exposed as public API by common crypto or otherwise.
Thanks,
Z

Answer 1

Done in playground with Xcode 8.3.3, generates a private/public key using EC for Alice, Bob, then calculating the shared secret for Alice using Alice's private and Bob's public, and share secret for Bob using Bob's private and Alice's public and finally asserting that they're equal.
import Security
import UIKit
let attributes: [String: Any] =
[kSecAttrKeySizeInBits as String: 256,
kSecAttrKeyType as String: kSecAttrKeyTypeEC,
kSecPrivateKeyAttrs as String:
[kSecAttrIsPermanent as String: false]
]
var error: Unmanaged<CFError>?
if #available(iOS 10.0, *) {
// generate a key for alice
guard let privateKey1 = SecKeyCreateRandomKey(attributes as CFDictionary, &error) else {
throw error!.takeRetainedValue() as Error
}
let publicKey1 = SecKeyCopyPublicKey(privateKey1)
// generate a key for bob
guard let privateKey2 = SecKeyCreateRandomKey(attributes as CFDictionary, &error) else {
throw error!.takeRetainedValue() as Error
}
let publicKey2 = SecKeyCopyPublicKey(privateKey2)
let dict: [String: Any] = [:]
// alice is calculating the shared secret
guard let shared1 = SecKeyCopyKeyExchangeResult(privateKey1, SecKeyAlgorithm.ecdhKeyExchangeStandardX963SHA256, publicKey2!, dict as CFDictionary, &error) else {
throw error!.takeRetainedValue() as Error
}
// bob is calculating the shared secret
guard let shared2 = SecKeyCopyKeyExchangeResult(privateKey2, SecKeyAlgorithm.ecdhKeyExchangeStandardX963SHA256, publicKey1!, dict as CFDictionary, &error) else {
throw error!.takeRetainedValue() as Error
}
print(shared1==shared2)
} else {
// Fallback on earlier versions
print("unsupported")
}
Thanks #Mats for sending me in the right direction..3

Answer 2

Here is the latest code with swift 5 and changes in parameters.
import Security
import UIKit
var error: Unmanaged<CFError>?
let keyPairAttr:[String : Any] = [kSecAttrKeySizeInBits as String: 256,
SecKeyKeyExchangeParameter.requestedSize.rawValue as String: 32,
kSecAttrKeyType as String: kSecAttrKeyTypeECSECPrimeRandom,
kSecPrivateKeyAttrs as String: [kSecAttrIsPermanent as String: false],
kSecPublicKeyAttrs as String:[kSecAttrIsPermanent as String: false]]
let algorithm:SecKeyAlgorithm = SecKeyAlgorithm.ecdhKeyExchangeStandardX963SHA256//ecdhKeyExchangeStandardX963SHA256
do {
guard let privateKey = SecKeyCreateRandomKey(keyPairAttr as CFDictionary, &error) else {
throw error!.takeRetainedValue() as Error
}
let publicKey = SecKeyCopyPublicKey(privateKey)
print("public ky1: \(String(describing: publicKey)),\n private key: \(privateKey)\n\n")
guard let privateKey2 = SecKeyCreateRandomKey(keyPairAttr as CFDictionary, &error) else {
throw error!.takeRetainedValue() as Error
}
let publicKey2 = SecKeyCopyPublicKey(privateKey2)
print("public ky2: \(String(describing: publicKey2)),\n private key2: \(privateKey2)\n\n")
let shared:CFData? = SecKeyCopyKeyExchangeResult(privateKey, algorithm, publicKey2!, keyPairAttr as CFDictionary, &error)
let sharedData:Data = shared! as Data
print("shared Secret key: \(sharedData.hexEncodedString())\n\n")
let shared2:CFData? = SecKeyCopyKeyExchangeResult(privateKey2, algorithm, publicKey!, keyPairAttr as CFDictionary, &error)
let sharedData2:Data = shared2! as Data
print("shared Secret key 2: \(sharedData2.hexEncodedString())\n\n")
// shared secret key and shared secret key 2 should be same
} catch let error as NSError {
print("error: \(error)")
} catch {
print("unknown error")
}
extension Data {
struct HexEncodingOptions: OptionSet {
let rawValue: Int
static let upperCase = HexEncodingOptions(rawValue: 1 << 0)
}
func hexEncodedString(options: HexEncodingOptions = []) -> String {
let format = options.contains(.upperCase) ? "%02hhX" : "%02hhx"
return self.map { String(format: format, $0) }.joined()
}
}

Answer 3

Circa - 2020 and iOS13. In Zohar's code snippet below, also specify a key size in the dictionary before attempting to get the shared secrets.
let dict: [String: Any] = [SecKeyKeyExchangeParameter.requestedSize.rawValue as String: 32]
Or there would be an error saying this.
kSecKeyKeyExchangeParameterRequestedSize is missing