gem install ritm   

1. Write your interception handlers
   

   require 'ritm'
   
   # A single answer for all your google searches
   Ritm.on_request do |req|
     if req.request_uri.host.start_with? 'www.google.'
       new_query_string = req.request_uri.query.gsub(/(?<=^q=|&q=)(((?!&|$).)*)(?=&|$)/, 'RubyInTheMiddle')
       req.request_uri.query = new_query_string
     end
   end
   
   my_picture = File.read('i_am_famous.jpg')
   
   # Replaces every picture on the web with my pretty face
   Ritm.on_response do |_req, res|
     if res.header['content-type'] && res.header['content-type'].start_with?('image/')
       res.header['content-type'] = 'image/jpeg'
       res.body = my_picture
     end
   end

2. Start the proxy server
   

   proxy = Ritm::Proxy::Launcher.new
   proxy.start
   
   puts 'Hit enter to finish'
   gets
   
   proxy.shutdown

3. Configure your browser
   Or whatever HTTP client you want to intercept traffic from, to connect to the proxy in localhost:8080
   
4. Browse the web!
   For the examples above, search anything in google and also visit your favorite newspaper website.
   

Client <--- SSL session ---> RITM <--- SSL session ---> Server

1. Generate a Certificate Authority PEM and Private Key files
   You can use OpenSSL or RITM to generate these two files. With OpenSSL:
   

   openssl req -new -nodes -x509 -days 365 -extensions v3_ca -keyout insecure_ca.key -out insecure_ca.crt

   Or with RITM:
   

   require 'ritm/certs/ca'
   
   ca = Ritm::CA.create common_name: 'InsecureCA'
   
   File.write('insecure_ca.crt', ca.pem)
   File.write('insecure_ca.key', ca.private_key.to_s)

2. Repeat step 2 from the previous example, this time indicating what CA should be used to sign certificates
   

   proxy = Ritm::Proxy::Launcher.new(ca_crt_path: 'path/to/insecure_ca.crt',
                                     ca_key_path: 'path/to/insecure_ca.key')
   proxy.start
   
   puts 'Hit enter to finish'
   gets
   
   proxy.shutdown

3. Trust the CA certificate into your browser or client
   I'll leave it to you to figure out how this is done in your browser or client.
   
4. Surf the web!
5. When you are done Remove the CA from your trusted authorities!
   Or take really good care of the CA private key since anyone in possession of that key will be capable of decrypting all your traffic! Also notice that when using the proxy every server will be automatically trusted even if the end server certificate is not valid.