Hello sir. My profile might look empty as I have just joined Freelancer but I have integrated more than 20 payment gateways single-handedly from all over the world till date. Paypal, Braintree, Stripe, PayU, MPesa, Mula, Atom, Razorpay, SagePay, Syberpay, Paytm, Federal Bank, ICICI Bank, HDFC Bank, Juspay just to name a few. I integrate payment gateways day in day out. I have good idea of the different kinds of cryptography mechanisms as well as hashing techniques like mcrypt, bcrypt, sha, md5, base64 etc which are used by different payment gateways and follow best practices in terms of security. I have been working full time as a software architect and online payment in charge in a reputed educational erp provider. Most of my integrations have been on ruby, rails or php. I went through the PG link attached to the requirement and it is quite doable. I just need to know the framework on which your merchant app is built and the desired payment flow. Ideally it would take around 4 to 5 days to complete the integration with the PG sandbox. I promise, if given a chance, I will not disappoint. Good day.