$ gem install active_merchant_first_data

require "active_merchant_first_data"

gateway = ActiveMerchant::Billing::FirstDataGateway.new(
  pem:  File.read("1234567_keystore.pem"),
  pem_password: "5x64jq8n234c"
)

# Authorize for 10 euros (1000 euro cents)
response = gateway.authorize(1000, client_ip_addr: '127.0.0.1')

# Use this url to enter credit card data
gateway.redirect_url(response.authorization)

# Capture the money
gateway.capture(1000, response.authorization, client_ip_addr: '127.0.0.1')

1. Generate a new certificate

   $ openssl req -newkey rsa:2048 -keyout spec/certs/1234567_key.pem -out spec/certs/1234567_req.pem -subj "/C=lv/O=example.com/CN=1234567" -outform PEM
   Enter PEM pass phrase: 81f174259v45

2. Request your certificate using 1234567_req.pem

3. Copy the 3 files you received in e-mail to spec/certs/:

   1234567.pem
   1234567_certificate_chain.p7.pem
   ECOMM.pem
   

4. Convert the certificates and keys to 1234567_keystore.pem

   $ openssl pkcs12 -export -in spec/certs/1234567.pem -out spec/certs/1234567_keystore.p12 -certfile spec/certs/ECOMM.pem -inkey spec/certs/1234567_key.pem
   Enter pass phrase for 1234567_key.pem: 81f174259v45
   Enter Export Password: <empty>

   $ openssl pkcs12 -in spec/certs/1234567_keystore.p12 > spec/certs/1234567_keystore.pem
   Enter Import Password: <empty>
   Enter PEM pass phrase: 5x64jq8n234c

5. Set your WAN IP address

Perhaps the best way to mock responses in tests is to disallow remote connections altogether and then control what responses specific requests receive.

For example, to mock response to the #purchase request with WebMock you'd:

before do
  body = {
    "TRANSACTION_ID" => "e+oClP4em8uBDozaZ4CBBbipEcM="
  }.merge(options).map{ |k, v| "#{k}: #{v}" }.join("\n")

  WebMock.stub_request(:post, %r'firstdata.lv').
    with(body: hash_including("command" => "v")).
    to_return(body: body)
end

Peruse first_data_gateway.rb to find out which command letters map to which methods.
Additionally, have a look at spec/cassettes for response body examples.