Sold Need: Secure authentication system using rsa w/ nonce

I am looking for someone capable of developing me a secure authentication system from my client to my server that uses RSA with a nonce to prevent mitm and replay attacks. There is already an authentication system in place that needs to be secured further.

You must have experience with both C# and PHP

Budget $50-75

Post here or pm me to apply. Looking to get this done ASAP!