Re: [Cfrg] OPAQUE at Facebook
Bill Cox <waywardgeek@gmail.com> Sun, 08 September 2019 19:18 UTC
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From: Bill Cox <waywardgeek@gmail.com>
Date: Sun, 08 Sep 2019 12:18:09 -0700
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To: Kevin Lewi <klewi@cs.stanford.edu>
Cc: Kathleen Moriarty <kathleen.moriarty.ietf@gmail.com>, Stephen Farrell <stephen.farrell@cs.tcd.ie>, IRTF CFRG <cfrg@irtf.org>
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Subject: Re: [Cfrg] OPAQUE at Facebook
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On Wed, Aug 28, 2019 at 9:44 PM Kevin Lewi <klewi@cs.stanford.edu> wrote: > Although the primary motivation is to avoid logging of plaintext > passwords, we still want to use a login protocol that provides good > security guarantees and models the same user experience that exists > today. We want to allow users to log in with a password, as opposed to > a private key -- so I believe this would make HOBA a less suitable > option. I agree HOBA doesn't look like a near-term solution. Eliminating the problem of logging plaintext passwords can be done by encrypting the password to a backend key sent in the login form. For clients that cannot run Javascript, the plaintext password can be encrypted by the front-end machine (where where TLS terminates), minimizing the chance of accidental logging. IMO, this is a reasonable short-term solution for this problem. Various security upgrades are possible over this simple scheme. OPAQUE is interesting. In SCRAM, the salt is sent to the client upon the server's > first message, which would mean that an attacker could build an > offline dictionary based on this salt (a precomputation attack). > OPAQUE gets around this through the use of an oblivious PRF, which > allows the client to keep its password secret, and the server to keep > the client-specific salt secret as well. > It also has the potentially cool property that the OPRF server and password verification service can be in different security domains. > On the topic of client-side password hashing for OPAQUE: one advantage > this offers over the traditional plaintext passwords over TLS login > mechanism is that we can force the client to solve a challenge on each > password attempt. This may be useful in defending against "password > spraying" attacks, in which an attacker picks a single common password > and tries to log in to a large number of accounts using this password. > However, it is not clear if we can balance the thresholds for the > password hashing challenge to be significant enough to deter attackers > executing password spraying, while still allowing for a reasonable > user experience for the regular user attempting to log in. Thoughts? > If you switched initially to the simple scheme of encrypting the password to a public key in the login form, you also have the opportunity to do do client-side hashing. You can do rounds of hashing: Catena calls this "pepper". For example, the client could start with a 1MiB hash, then a 4MiB hash, and if it feels a 16MiB hash is too hard, the server could do the 16MiB hash for the client. The real salt should not be used: instead just a userID of some sort on the client. The final hash. after the required rounds of pepper, is done server-side with secret salt, maybe just HMAC(hardenedHash, secretSalt). With this sort of adaptive client-side hashing, you can begin to gather stats on what your client devices can do.
- [Cfrg] OPAQUE at Facebook Kevin Lewi
- Re: [Cfrg] OPAQUE at Facebook Bill Cox
- Re: [Cfrg] OPAQUE at Facebook Björn Haase
- Re: [Cfrg] OPAQUE at Facebook david wong
- Re: [Cfrg] OPAQUE at Facebook Stanislav V. Smyshlyaev
- Re: [Cfrg] OPAQUE at Facebook Neil Madden
- Re: [Cfrg] OPAQUE at Facebook Kathleen Moriarty
- Re: [Cfrg] OPAQUE at Facebook Bill Cox
- Re: [Cfrg] OPAQUE at Facebook Bill Cox
- Re: [Cfrg] OPAQUE at Facebook Kathleen Moriarty
- Re: [Cfrg] OPAQUE at Facebook Kevin Lewi
- Re: [Cfrg] OPAQUE at Facebook Kevin Lewi
- Re: [Cfrg] OPAQUE at Facebook Bill Cox