Derived from https://github.com/cocagne/csrp
mbedtls-csrp is a minimal C implementation of the Secure Remote Password protocol. The project consists of a single C file and is intended for direct inclusion into utilizing programs. It's only dependency is mbedtls (https://github.com/ARMmbed/mbedtls).
SRP is a cryptographically strong authentication protocol for password-based, mutual authentication over an insecure network connection.
Unlike other common challenge-response autentication protocols, such as Kereros and SSL, SRP does not rely on an external infrastructure of trusted key servers or certificate management. Instead, SRP server applications use verification keys derived from each user's password to determine the authenticity of a network connection.
SRP provides mutual-authentication in that successful authentication requires both sides of the connection to have knowledge of the user's password. If the client side lacks the user's password or the server side lacks the proper verification key, the authentication will fail.
Unlike SSL, SRP does not directly encrypt all data flowing through the authenticated connection. However, successful authentication does result in a cryptographically strong shared key that can be used for symmetric-key encryption.
You need to take care of entropy to achieve cryptograhically sound random values. For real world use, you should change the implementation in init_random() to supply your own seed values. Also make sure to add entropy sources to your mbedtls port.