Add an option to verify the signature on decode

lib/jwt.rb

@@ -12,6 +12,7 @@ module JWT
   class DecodeError < StandardError; end
   class VerificationError < DecodeError; end
   class ExpiredSignature < DecodeError; end
+  class IncorrectAlgorithm < DecodeError; end
   class ImmatureSignature < DecodeError; end
   class InvalidIssuerError < DecodeError; end
   class InvalidIatError < DecodeError; end
@@ -122,6 +123,9 @@ def decode(jwt, key=nil, verify=true, options={}, &keyfinder)
 
     if verify
       algo, key = signature_algorithm_and_key(header, key, &keyfinder)
+      if options[:algorithm] && algo != options[:algorithm]
+        raise JWT::IncorrectAlgorithm.new('Expected a different algorithm')
+      end
       verify_signature(algo, key, signing_input, signature)
     end
 

spec/jwt_spec.rb

@@ -191,6 +191,16 @@
     expect { JWT.encode(@payload, 'secret', 'HS1024') }.to raise_error(NotImplementedError)
   end
 
+  it 'raises exception when decoded with a different algorithm than it was encoded with' do
+    jwt = JWT.encode(@payload, 'foo', 'HS384')
+    expect { JWT.decode(jwt, 'foo', true, algorithm: 'HS512') }.to raise_error(JWT::IncorrectAlgorithm)
+  end
+
+  it 'does not raise exception when encoded with the expected algorithm' do
+    jwt = JWT.encode(@payload, 'foo', 'HS512')
+    JWT.decode(jwt, 'foo', true, algorithm: 'HS512')
+  end
+
   it 'encodes and decodes plaintext JWTs' do
     jwt = JWT.encode(@payload, nil, nil)
     expect(jwt.split('.').length).to eq(2)