feat: implement static key provider

src/StevanFreeborn.Extensions.Configuration.Secure/Cryptography/StaticKeyProvider.cs

@@ -0,0 +1,37 @@
+namespace StevanFreeborn.Extensions.Configuration.Secure.Cryptography;
+
+internal sealed class StaticKeyProvider : IEncryptionKeyProvider
+{
+  private readonly byte[] _key;
+
+  public StaticKeyProvider(string base64Key)
+  {
+    if (string.IsNullOrWhiteSpace(base64Key))
+    {
+      throw new ArgumentNullException(nameof(base64Key));
+    }
+
+    byte[] decodedKey;
+
+    try
+    {
+      decodedKey = Convert.FromBase64String(base64Key);
+    }
+    catch (FormatException ex)
+    {
+      throw new ArgumentException("The provided key is not a valid Base64 string.", nameof(base64Key), ex);
+    }
+
+    if (decodedKey.Length != 32)
+    {
+      throw new ArgumentException("The encryption key must be exactly 32 bytes (256 bits) for AES-256 encryption.", nameof(base64Key));
+    }
+
+    _key = decodedKey;
+  }
+
+  public byte[] GetKey()
+  {
+    return _key;
+  }
+}