Allow alg header to be given

CHANGELOG.md

@@ -15,6 +15,7 @@ Take a look at the [upgrade guide](UPGRADING.md) for more details.
 
 **Features:**
 - JWT::EncodedToken#verify! method that bundles signature and claim validation [#647](https://github.com/jwt/ruby-jwt/pull/647) ([@anakinj](https://github.com/anakinj))
+- Do not override the alg header if already given [#659](https://github.com/jwt/ruby-jwt/pull/659) ([@anakinj](https://github.com/anakinj))
 - Your contribution here
 
 **Fixes and enhancements:**

lib/jwt/token.rb

@@ -95,7 +95,7 @@ def sign!(algorithm:, key:)
       raise ::JWT::EncodeError, 'Token already signed' if @signature
 
       JWA.resolve(algorithm).tap do |algo|
-        header.merge!(algo.header)
+        header.merge!(algo.header) { |_key, old, _new| old }
         @signature = algo.sign(data: signing_input, signing_key: key)
       end
 

spec/jwt/encoded_token_spec.rb

@@ -103,6 +103,15 @@
       end
     end
 
+    context 'when header has invalid alg value' do
+      let(:header) { { 'alg' => 'HS123' } }
+
+      it 'does not raise' do
+        expect(token.header).to eq(header)
+        expect(token.verify_signature!(algorithm: 'HS256', key: 'secret')).to eq(nil)
+      end
+    end
+
     context 'when payload is detached' do
       let(:encoded_token) { detached_payload_token.jwt }
 

spec/jwt/jwt_spec.rb

@@ -586,9 +586,9 @@
   end
 
   context 'when the alg value is given as a header parameter' do
-    it 'does not override the actual algorithm used' do
+    it 'overrides the actual algorithm used' do
       headers = JSON.parse(JWT::Base64.url_decode(JWT.encode('Hello World', 'secret', 'HS256', { alg: 'HS123' }).split('.').first))
-      expect(headers['alg']).to eq('HS256')
+      expect(headers['alg']).to eq('HS123')
     end
 
     it 'should generate the same token' do

spec/jwt/token_spec.rb

@@ -41,6 +41,18 @@
         expect { token.jwt }.to raise_error(JWT::EncodeError)
       end
     end
+
+    context 'when alg is given in header' do
+      let(:header) { { 'alg' => 'HS123' } }
+
+      before do
+        token.sign!(algorithm: 'HS256', key: 'secret')
+      end
+
+      it 'returns a signed and encoded token' do
+        expect(JWT::EncodedToken.new(token.jwt).header).to eq({ 'alg' => 'HS123' })
+      end
+    end
   end
 
   describe '#detach_payload!' do