Create Apple Watch App

Sesame/SymmetricKey+Extensions.swift

@@ -46,3 +46,26 @@ extension String {
         return results.map { String($0) }
     }
 }
+
+let protocolSalt = "CryptoKit Playgrounds Putting It Together".data(using: .utf8)!
+
+/// Generates an ephemeral key agreement key and performs key agreement to get the shared secret and derive the symmetric encryption key.
+func encrypt(_ data: Data, to theirEncryptionKey: Curve25519.KeyAgreement.PublicKey, signedBy ourSigningKey: Curve25519.Signing.PrivateKey) throws ->
+    (ephemeralPublicKeyData: Data, ciphertext: Data, signature: Data) {
+        let ephemeralKey = Curve25519.KeyAgreement.PrivateKey()
+        let ephemeralPublicKey = ephemeralKey.publicKey.rawRepresentation
+        
+        let sharedSecret = try ephemeralKey.sharedSecretFromKeyAgreement(with: theirEncryptionKey)
+        
+        let symmetricKey = sharedSecret.hkdfDerivedSymmetricKey(using: SHA256.self,
+                                                                salt: protocolSalt,
+                                                                sharedInfo: ephemeralPublicKey +
+                                                                    theirEncryptionKey.rawRepresentation +
+                                                                    ourSigningKey.publicKey.rawRepresentation,
+                                                                outputByteCount: 32)
+        
+        let ciphertext = try ChaChaPoly.seal(data, using: symmetricKey).combined
+        let signature = try ourSigningKey.signature(for: ciphertext + ephemeralPublicKey + theirEncryptionKey.rawRepresentation)
+        
+        return (ephemeralPublicKey, ciphertext, signature)
+}