diff --git a/lib/src/algorithms.dart b/lib/src/algorithms.dart
index 7631238..e00eca3 100644
--- a/lib/src/algorithms.dart
+++ b/lib/src/algorithms.dart
@@ -147,7 +147,12 @@ class HMACAlgorithm extends JWTAlgorithm {
     assert(key is SecretKey, 'key must be a SecretKey');
     final secretKey = key as SecretKey;
 
-    final hmac = Hmac(_getHash(name), utf8.encode(secretKey.key));
+    final hmac = Hmac(
+      _getHash(name),
+      secretKey.isBase64Encoded
+          ? base64Decode(secretKey.key)
+          : utf8.encode(secretKey.key),
+    );
 
     return Uint8List.fromList(hmac.convert(body).bytes);
   }
diff --git a/lib/src/keys.dart b/lib/src/keys.dart
index 058a166..71d4597 100644
--- a/lib/src/keys.dart
+++ b/lib/src/keys.dart
@@ -11,8 +11,9 @@ abstract class JWTKey {}
 /// For HMAC algorithms
 class SecretKey extends JWTKey {
   String key;
+  bool isBase64Encoded;
 
-  SecretKey(this.key);
+  SecretKey(this.key, {this.isBase64Encoded = false});
 }
 
 /// For RSA algorithm, in sign method