Possibility to give the expiration time on the JWT class

lib/jwt/dsl/encode.rb

@@ -13,6 +13,11 @@ def encode_payload(&block)
         @encode_payload
       end
 
+      def expiration(value = nil)
+        @expiration = value unless value.nil?
+        @expiration
+      end
+
       def encode!(payload, options = {})
         Internals.encode!(payload, options, self)
       end
@@ -29,7 +34,8 @@ def build_options(payload, options, context)
               key: options[:key] || context.signing_key,
               encode_payload_proc: context.encode_payload,
               headers: options[:headers],
-              algorithm: context.algorithm
+              algorithm: context.algorithm,
+              expiration: context.expiration
             }
 
             if opts[:algorithm].is_a?(String) && opts[:key].nil?

lib/jwt/encode.rb

@@ -29,7 +29,7 @@ def segments
     attr_reader :headers, :options, :algorithm, :alg
 
     def payload
-      options[:payload]
+      @payload ||= append_exp(options[:payload])
     end
 
     def key
@@ -84,6 +84,15 @@ def validate_claims!
       ClaimsValidator.new(payload).validate!
     end
 
+    def append_exp(payload)
+      return payload unless (expiration = options[:expiration])
+      return payload if payload.key?('exp') || payload.key?(:exp)
+
+      payload['exp'] = Time.now.to_i + expiration
+
+      payload
+    end
+
     class << self
       def encode(data)
         Base64.urlsafe_encode64(JWT::JSON.generate(data), padding: false)

spec/dsl/encode_spec.rb

@@ -63,5 +63,18 @@
         expect(jwt_class.encode!(payload)).to eq(::JWT.encode(payload, secret, 'HS256'))
       end
     end
+
+    context 'when expiration is set on the class and is negative' do
+      before do
+        jwt_class.algorithm('HS256')
+        jwt_class.expiration(-10)
+        jwt_class.signing_key(secret)
+      end
+
+      it 'will only generate expired tokens' do
+        token = jwt_class.encode!(payload)
+        expect { jwt_class.decode!(token) }.to raise_error(JWT::ExpiredSignature, 'Signature has expired')
+      end
+    end
   end
 end