Practical Implementation of STS Protocol

 Introduction

The Station To Station Protocol (STS) is a cryptographic key agreement scheme. The protocol is based on the classic Diffie-Hellman, which is not secure against a man-in-the-middle attack. This protocol assumes that the parties have signature keys which are used to sign messages, thereby providing security against man-in-the-middle attacks.

The following data must be generated before initiating the protocol.

• An asymmetric signature keypair for each party

• Key establishment parameters

Through the exchange of Diffie-Hellman (DH) parameters and signed certificates, both participants verify each other's identities and establish a shared session key for encrypted message exchange. This simulation covers the following components:

• Certificate Authority (CA): Responsible for generating and signing certificates.

• Participants (Alice and Bob): Engage in key exchange and secure communication using the STS protocol.

Code Structure

I have divided this simulation structure into two primary classes : CertificateAuthority and Participants.  Each class contains specific functions to manage keys, certificates, DH parameters, and secure communication.

1. Certificate Authority (CA) Class:

The __init__() method sets up the CA by checking for existing certificates; if none are found, it produces and stores new ones; otherwise, it loads the current credentials. The generate_ca_key_and_certificate() function generates the CA's RSA private key and a self-signed X.509 certificate with subject name and validity period. The save_ca_key_and_certificate() function saves these credentials to files (ca_private_key.pem and ca_certificate.pem), and load_ca_key_and_certificate() retrieves them as needed. The sign_certificate(csr) function validates a participant's Certificate Signing Request (CSR), signs it using the CA's private key, and returns an X.509 certificate with the given validity.

2. Participant (Alice & Bob) Class:

The __init__() function initializes participant characteristics, produces or loads Diffie-Hellman (DH) parameters if they are not previously stored, and creates folders to store keys and certificates. The generate_dh_parameters() method generates DH parameters (p and g) that are appropriate for secure communication. The generate_dh_key_pair() function produces the participant's DH private key and then derives the associated public key. generate_signing_key_pair() generates RSA keys required to sign communications. The create_certificate() method creates a CSR using the participant's public key, transmits it to the CA for signature, and saves the resulting certificate. The save_certificate_and_keys() function saves private keys and certificates to defined files in participant-specific folders, and load_certificate_and_keys() retrieves them, preserving data integrity.

For communication, send_message(recipient, message_type, data) creates, signs, and delivers a message to the receiver, whereas receive_message(sender, message) validates, parses, and processes a received message according to its type. Data encryption and decryption are handled by encrypt_data(data) and decrypt_data(encrypted_data), which employ AES-CTR with the session key, updating packet numbering for nonce uniqueness, and protecting against replay attacks. The derive_session_key(peer_public_key) method computes the shared secret using both the participant's private key and the peer's public key before generating a symmetric session key with HKDF. The construct_nonce() function generates a unique nonce for each message by combining the packet number, source MAC address, and priority byte.

Finally, verify_certificate(certificate) checks the peer's certificate against the CA's public key and validates its details, while verify_signature(data, signature, public_key) ensures data integrity by verifying the signature with the sender's public key.

Output

Practical Application

This simulation models a real-world secure communication system where identity verification, key exchange, and encrypted messaging are critical for secure interactions. In actual systems, similar principles protect secure websites, messaging apps, and other digital communications.

Conclusion

This STS protocol simulation shows the fundamental components of setting up a secure and authenticated communication channel between two parties (Alice & Bob). By incorporating essential cryptographic principles such as DH key exchange, RSA signatures, certificate authorities, and symmetric encryption, the simulation presents a full illustration of secure communication protocols.

Access Full Code Here: Github - Click Me To Get Code

References

1. Wikipedia contributors. (2024, March 29). Station-to-Station protocol. Wikipedia. https://en.wikipedia.org/wiki/Station-to-Station_protocol

2. Jcmorais. (n.d.). GitHub - jcmorais/Diffie-Hellman-Station-to-Station-Protocol: In public-key cryptography, the Station-to-Station (STS) protocol is a cryptographic key agreement scheme based on classic Diffie–Hellman that provides mutual key and entity authentication. GitHub. https://github.com/jcmorais/Diffie-Hellman-Station-to-Station-Protocol

3. STS Protocol. (n.d.). http://archive.dimacs.rutgers.edu/Workshops/Security/program2/boyd/node13.html

4. Diffie, W., Sun Microsystems, Van Oorschot, P. C., Wiener, M. J., & Bell-Northern Research. (1992). Authentication and Authenticated Key Exchanges. https://people.scs.carleton.ca/~paulv/papers/sts-final.pdf

5. www.naukri.com. (n.d.). Code 360 by Coding Ninjas. 2024 Naukri.com. https://www.naukri.com/code360/library/the-station-to-station-key-agreement-scheme

        6. Understanding Cryptography – From Established Symmetric and Asymmetric Ciphers to Post-                Quantum Algorithms. (n.d.). https://www.cryptography-textbook.com/