Initial version.

LockItShared/Services/BluetoothLock/Crypto/AuthCryptoHelper.cs

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+using Serilog;
+using TINK.Model.Device;
+
+namespace TINK.Services.BluetoothLock.Crypto
+{
+    public class AuthCryptoHelper
+    {
+        private ICipher Cipher { get; }
+
+        /// <summary> Encrypted seed (random number) created inside ILOCKIT and passd to app.</summary>
+        private byte[] SeedLockEncrypted { get; }
+
+        /// <summary> Contstructs a auth crypto helper object.</summary>
+        /// <param name="seedLockEncrypted">Encrypted seed to deocode using <see cref="KeyCopri"/>.</param>
+        /// <param name="keyCopri">Key used to to decrypt <see cref="SeedLockEncrypted"/>.</param>
+        public AuthCryptoHelper(
+             byte[] seedLockEncrypted,
+             byte[] keyCopri,
+             ICipher cipher)
+        {
+            KeyCopri = keyCopri;
+            SeedLockEncrypted = seedLockEncrypted;
+            Cipher = cipher ?? new Cipher();
+        }
+
+
+        /// <summary> Public for testing purposes only.</summary>
+        public byte[] GetSeedLock()
+        {
+            byte[] seedLockDecrypted;
+            var seedLockEncrypted = SeedLockEncrypted;
+            var keyCopri = KeyCopri;
+            try
+            {
+                seedLockDecrypted = Cipher.Decrypt(
+                        keyCopri,
+                        seedLockEncrypted);
+            }
+            catch (System.Exception exception)
+            {
+                Log.ForContext<AuthCryptoHelper>().Error("Decrypting seed from lock failed. {Exception}", exception);
+                throw;
+            }
+
+            Log.ForContext<AuthCryptoHelper>().Verbose($"Lock random number decrypted from {string.Join(",", seedLockEncrypted)} to {string.Join(",", seedLockDecrypted)} using {string.Join(", ", keyCopri)}.");
+            return seedLockDecrypted;
+        }
+
+        public byte[] GetAccessKeyEncrypted()
+        {
+
+            var accessKey = GetSeedLock();
+
+            if (accessKey == null || accessKey.Length <= 0)
+            {
+                Log.ForContext<AuthCryptoHelper>().Error("Creating access key failed, Key must not be null or empty.");
+                throw new System.Exception();
+            }
+
+            accessKey[accessKey.Length - 1] += 1;
+            
+            var keyCopri = KeyCopri;
+            byte[] acccessKeyEncrypted;
+            try
+            {
+                acccessKeyEncrypted = Cipher.Encrypt(
+                    keyCopri,
+                    accessKey);
+            }
+            catch (System.Exception exception)
+            {
+                Log.ForContext<AuthCryptoHelper>().Error("Encrypting access key failed. {Exception}", exception);
+                throw;
+            }
+
+            Log.ForContext<AuthCryptoHelper>().Verbose($"Access key encrypted from {string.Join(",", accessKey)} to {string.Join(",", acccessKeyEncrypted)} using {string.Join(", ", keyCopri)}.");
+            return acccessKeyEncrypted;
+        }
+
+        public byte[] KeyCopri { get; }
+    }
+}

LockItShared/Services/BluetoothLock/Crypto/Cipher.cs

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+using System;
+using System.IO;
+using System.Security.Cryptography;
+using TINK.Model.Device;
+
+namespace TINK.Services.BluetoothLock.Crypto
+{
+    public class Cipher : ICipher
+    {
+        /// <summary> Decrypt data. </summary>
+        /// <remarks>
+        /// Further info see:
+        /// https://docs.microsoft.com/en-us/dotnet/api/system.security.cryptography.aes?view=netcore-3.1 for further info
+        //  https://docs.microsoft.com/en-us/dotnet/standard/security/cryptographic-services
+        //  https://stackoverflow.com/questions/24903575/how-to-return-byte-when-decrypt-using-cryptostream-descryptoserviceprovider/24903689
+
+        /// </remarks>
+        /// <param name="key">Key to decrypt data with.</param>
+        /// <param name="encrypted">Encrpyted data to decrypt.</param>
+        /// <returns>Decrypted data.</returns>
+        public byte[] Decrypt(byte[] key, byte[] encrypted)
+        {
+            // Check arguments.
+            if (encrypted == null || encrypted.Length <= 0)
+                throw new ArgumentNullException(nameof(encrypted));
+
+            if (key == null || key.Length <= 0)
+                throw new ArgumentNullException(nameof(key));
+
+            using (Aes aesAlg = Aes.Create())
+            {
+                aesAlg.KeySize = 192;
+                aesAlg.Mode = CipherMode.ECB;
+                aesAlg.Padding = PaddingMode.None;
+                aesAlg.Key = key;
+
+                // Create a decryptor to perform the stream transform.
+                ICryptoTransform decryptor = aesAlg.CreateDecryptor(aesAlg.Key, aesAlg.IV);
+
+                // Create the streams used for decryption.
+                using (var msDecrypt = new MemoryStream())
+                {
+                    using (var csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Write))
+                    {
+                        csDecrypt.Write(encrypted, 0, encrypted.Length);
+                        csDecrypt.FlushFinalBlock();
+                        return msDecrypt.ToArray();
+                    }
+                }
+            }
+        }
+
+        public byte[] Encrypt(byte[] key, byte[] clear)
+        {
+            // Check arguments.
+            if (clear == null || clear.Length <= 0)
+                throw new ArgumentNullException("plainText");
+
+            if (key == null || key.Length <= 0)
+                throw new ArgumentNullException("Key");
+
+            // Create an AesCryptoServiceProvider object
+            // with the specified key and IV.
+            using (AesCryptoServiceProvider aesAlg = new AesCryptoServiceProvider())
+            {
+                aesAlg.KeySize = 192;
+                aesAlg.Mode = CipherMode.ECB;
+                aesAlg.Padding = PaddingMode.None;
+                aesAlg.Key = key;
+
+                // Create an encryptor to perform the stream transform.
+                ICryptoTransform encryptor = aesAlg.CreateEncryptor(aesAlg.Key, aesAlg.IV);
+
+                // Create the streams used for encryption.
+                using (var msEncrypt = new MemoryStream())
+                {
+                    using (var csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
+                    {
+                        csEncrypt.Write(clear, 0, clear.Length);
+                        csEncrypt.FlushFinalBlock();
+                        return msEncrypt.ToArray();
+                    }
+                }
+            }
+
+        }
+    }
+}