Hello, I know the answer is too late for you but I am leaving it in case that someone finds it useful. What you've mentioned so far is correct you do combine multiple keys to create 3 or 4 shared secrets (depending on if you have the Onte time pre-keys or not). DH or Diffie-Hellman is a key agreement. And you should treat it as such by using the KeyAgreement of an appropriate Public-key cryptography curve like (P256, Curve25519,...). This will get you 3 or 4 shared secrets that you will concatenate together in one string. Here is a rough outline: let s1 = try self?.myIdKeyPair?.privateKey.sharedSecretFromKeyAgreement(with: otherSignedPubKey) let s2 = try self?.oneTimeKey?.privateKey.sharedSecretFromKeyAgreement(with: otherIdPub) let s3 = try self?.oneTimeKey?.privateKey.sharedSecretFromKeyAgreement(with: otherSignedPubKey) let s4 = try self?.oneTimeKey?.privateKey.sharedSecretFromKeyAgreement(with: otherOneTimeKey)           let computedSharedSecretString = s1!.description.split(separator: " ")[1] +                         s2!.description.split(separator: " ")[1] +                         s3!.description.split(separator: " ")[1] +                         s4!.description.split(separator: " ")[1] You'd want to use this computedSharedSecretString and run it through a KDF function like HKDF and hash the output afterwards (SHA256) func hkdf<H: HashFunction>(_ seed: Data, salt: Data, info: Data, outputSize: Int = 32, hashFunction: H) -> Data? {   let hashByteCount = H.Digest.byteCount   let iterations = UInt8(ceil(Double(outputSize) / Double(hashByteCount)))   guard iterations <= 255 else {     return nil   }       let prk = HMAC<H>.authenticationCode(for: seed, using: SymmetricKey(data: salt))   let key = SymmetricKey(data: prk)   var hkdf = Data()   var value = Data()       for i in 1...iterations {     value.append(info)     value.append(i)           let code = HMAC<H>.authenticationCode(for: value, using: key)     hkdf.append(contentsOf: code)           value = Data(code)   }   return hkdf.prefix(outputSize) }