Argon2i

Argon2i is a key derivation function designed to be resistant to side-channel attacks. It was the winner of the Password Hashing Competition in 2015. Argon2i is one of the three variants of Argon2, the others being Argon2d and Argon2id. The 'i' in Argon2i stands for "interactive," indicating its resistance to parallel computation attacks.

The primary goal of Argon2i is to make it computationally expensive and difficult for attackers to crack passwords, even if they possess significant computing resources. It achieves this by incorporating several parameters that can be tuned to increase security. These parameters include memory cost, time cost, and the parallelism degree. A higher memory cost requires more RAM, a higher time cost requires more CPU cycles, and a higher parallelism degree allows for more threads to be used in the hashing process.

Argon2i achieves its resistance to side-channel attacks by accessing memory in an irregular pattern. This means that memory accesses are not sequential, making it harder for an attacker to infer information about the password by monitoring memory access patterns. This is in contrast to Argon2d, which is optimized for resistance to GPU cracking but is more vulnerable to side-channel attacks.

Argon2id is a hybrid variant that combines the strengths of both Argon2i and Argon2d. It uses Argon2i's data-independent memory access for the first half of its computation and Argon2d's data-dependent memory access for the second half, offering a balance of resistance against both types of attacks. Argon2i is generally recommended when side-channel resistance is a primary concern.