add CompositeKey class with a hashing method

src/KeyKeeper/PasswordStore/Crypto/CompositeKey.cs

@@ -0,0 +1,34 @@
+using System;
+using System.Security.Cryptography;
+using System.Text;
+
+namespace KeyKeeper.PasswordStore.Crypto;
+
+public struct CompositeKey
+{
+    public string Password { get; }
+    public byte[] Salt { get; }
+
+    public CompositeKey(string password, byte[] salt)
+    {
+        if (password == null)
+            throw new ArgumentNullException("password");
+        Password = password;
+
+        if (salt == null || salt.Length < FileFormatConstants.MIN_MASTER_SALT_LEN ||
+            salt.Length > FileFormatConstants.MAX_MASTER_SALT_LEN)
+            throw new ArgumentException("salt");
+        Salt = salt;
+    }
+
+    public byte[] Hash()
+    {
+        byte[] passwordBytes = Encoding.UTF8.GetBytes(Password);
+        byte[] hashedString = new byte[passwordBytes.Length + Salt.Length * 2];
+        Salt.CopyTo(hashedString, 0);
+        passwordBytes.CopyTo(hashedString, Salt.Length);
+        Salt.CopyTo(hashedString, Salt.Length + passwordBytes.Length);
+
+        return SHA256.HashData(hashedString);
+    }
+}