If a passive adversary is someone who can steal a hard-disk and inspect whatever data they find, an active adversary is someone who has followed a programming course (perhaps a cryptography course too!) and is going to change the code run by their own server to exploit any vulnerabilities in the protocol you are running with them. In this case you might need a protocol which is secure against active adversaries, or in other words adversaries that might arbitrarily deviate from the protocol specification. Thus, the security guarantees provided by passive security might not be enough. This is true both for passive security and for the other levels of security.Īctive Security In practice, if you are performing secure computation with a group of servers over the internet, it is safe to assume that you don’t really trust the people you are interacting with, and you might be worried that they won’t follow the protocol as instructed. In other words, it is common practice to assume that there is a single, “monolithic” adversary that controls all corrupt parties or that the multiple adversaries are willing to cooperate among themselves.

Multiple Adversaries? If several participants are corrupt at the same time, we always assume that the adversary has access to the state of all of them. Looking back at the protocols from the previous blog, we can see that the information gathered by any single server isn’t sufficient to reconstruct any of the secrets. This kind of adversary is also referred to as honest-but-curious (to stress that the adversary follows the protocol but might try to infer more information, analyzing all information that they might be able to gather) or semi-honest. Passive Security One way of describing a passive adversary in an MPC protocol is by considering someone that is capable of breaking into the machines of participants of the MPC protocol and looking at their internal state, without the ability to replace the code they are running in the protocol. Since this blog series is about secure multiparty computation, this post will focus on some of the most common taxonomy for security levels in multiparty computation, with a particular focus on active security. However, we have not yet defined what secure means in this context. Informally, we argued that using this technique, no single participant will ever learn any of the secrets they are computing on. Last week’s blog titled “Secret Sharing” explained how to securely compute on secret data using a special technique known as “replicated secret-sharing”.

Mpc 2. extra