A trusted server stores all portfolio images for each user. Since each image is derived directly from the seed, the server only needs to store the seed and not the entire image. In our prototype implementation, the seed is 8 bytes long, hence the storage overhead for each portfolio is small. For each authentication challenge, the server creates a challenge set, which consists of portfolio and decoy images. If the user correctly identifies all portfolio images, she is authenticated.
In general, a weakness of this system is that the server needs to store the seeds of the portfolio images of each user in cleartext. Tricks similar to the hashed passwords in the /etc/passwd file do not work in this case, because the server needs to present the portfolio to the user, hidden within the decoy images. For this reason, we assume the server to be secure and trusted, similar to Kerberos [SNS88]. To reduce the trust required from each server, the portfolio can be to split among multiple servers, and each server can contribute a part of the challenge set for each authentication.