From 49dd52a1fbfafb38995be5e3f1d2afccfc34cd11 Mon Sep 17 00:00:00 2001
From: Morgan Pretty <morgan.t.pretty@gmail.com>
Date: Tue, 8 Feb 2022 17:01:37 +1100
Subject: [PATCH] Added code to support generating a derived key for id
 blinding

---
 .../Utilities/Sodium+Conversion.swift         | 26 +++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/SessionMessagingKit/Utilities/Sodium+Conversion.swift b/SessionMessagingKit/Utilities/Sodium+Conversion.swift
index c522bdf92..9ad03dc67 100644
--- a/SessionMessagingKit/Utilities/Sodium+Conversion.swift
+++ b/SessionMessagingKit/Utilities/Sodium+Conversion.swift
@@ -39,3 +39,29 @@ extension Sign {
         return x25519SecretKey
     }
 }
+
+extension Sodium {
+    public typealias SOGSDerivedKey = Data
+    
+    private static let publicKeyBytes: Int = Int(crypto_scalarmult_bytes())
+    private static let sharedSecretBytes: Int = Int(crypto_scalarmult_bytes())
+    
+    public func derivedKey(serverPublicKeyBytes: [UInt8], userKeyBytes: [UInt8]) -> SOGSDerivedKey? {
+        guard serverPublicKeyBytes.count == Sodium.publicKeyBytes && userKeyBytes.count == Sodium.publicKeyBytes else { return nil }
+        
+        let sharedSecretPtr: UnsafeMutablePointer<UInt8> = UnsafeMutablePointer<UInt8>.allocate(capacity: Sodium.sharedSecretBytes)
+        let result = userKeyBytes.withUnsafeBytes { (userPublicKeyPtr: UnsafeRawBufferPointer) in
+            return serverPublicKeyBytes.withUnsafeBytes { (serverPublicKeyPtr: UnsafeRawBufferPointer) -> Int32 in
+                guard let serverKeyBaseAddress: UnsafePointer<UInt8> = serverPublicKeyPtr.baseAddress?.assumingMemoryBound(to: UInt8.self), let userKeyBaseAddress: UnsafePointer<UInt8> = userPublicKeyPtr.baseAddress?.assumingMemoryBound(to: UInt8.self) else {
+                    return -1
+                }
+                
+                return crypto_scalarmult(sharedSecretPtr, serverKeyBaseAddress, userKeyBaseAddress)
+            }
+        }
+        
+        guard result == 0 else { return nil }
+        
+        return Data(bytes: sharedSecretPtr, count: Sodium.sharedSecretBytes)
+    }
+}