CryptoKit: CryptoKit.sharedSecret.hkdfDerivedSymmetricKey

Hi there,

I am currently trying to implement AES-GCM encryption based on ECDH. So far, everything up to and including the generation of the shared secret on both machines (iOS 13 and NodeJS) works fine. However, when it comes to the key derivation, I am stuck:

On the NodeJS side I am currently using the package util-js-hkdf to generate they key from the shared secret and salt as follows:

let salt = crypto.randomBytes(16);
hkdf(secret, 32, {salt: salt, info: undefined, hash: 'SHA-256'});

if let slt = Data(base64Encoded: salt) {
     let key = (sharedSecret?.hkdfDerivedSymmetricKey(using: SHA256.self, salt: slt, sharedInfo: Data(), outputByteCount: 32))!;
     // print calculated key in hex 
     key.withUnsafeBytes { (bytes: UnsafeRawBufferPointer) -> Void in
          var key = Array(repeating: "", count: 32)
          for n in 0...31 {
               key[n] = String(format: "%02x", bytes[n])
          }
          print("KEY: \(key.joined())")
     };
}