Key Exchange with Diffie-Hellman Protocol and Composite Hash-Functions

For the secure transmission of symmetrically encrypted data, sharing not only the data but also its secret key is crucial. Therefore, an effective key exchange mechanism is essential for secure key transmission. The Diffie-Hellman key exchange protocol has traditionally served as a method for securely transmitting secret keys by generating session keys. It relays its security on the discrete logarithm problem and can be enhanced by combining it with other cryptographic tools. In this context, we propose a key exchange algorithm that incorporates composite Hash-functions. The shared key of the Diffie-Hellman protocol is used to construct 128 strings, each processed individually by a distinct composite SHA-512 function a secret number of times. Each number represents a byte of the key intended for transmission. Consequently, the encrypted key is formed by concatenating 128 strings of 512 bits each. The results of the encrypted key show entropy values close to 7.9, even when the secret key exhibits numerical patterns. The proposed algorithm utilizes 1024-bit numbers for the secret key, prime, and generator. It does not require sharing additional parameters beyond the public parameters of Diffie-Hellman and the encrypted key. Additionally, for enhanced security, private values can be changed in every communication, albeit at the cost of increased time, approximately 10 ms.