[messaging] Passphrase-based key mobility (was: Peerio)

[Garrett Robinson]

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On 3/2/15 1:04 AM, Nadim Kobeissi wrote:
> We have decided to forego with user-chosen passphrases entirely,
> and to stick uniquely to the miniLock model of having a CSPRNG pick
> a high-entropy (112-bit) passphrase for users.

Cool! This has always been the SecureDrop model, although the entropy
of these passphrases is significantly higher than those of the default
length 7-word SecureDrop passphrases (~89.1 bits). We might want to
consider increasing the size of our dictionary (fairly small at 6800
words) or the default length of the passphrases; however, doing either
introduces usability tradeoffs that must be carefully considered.

The unfortunate downside is that to gain this security assurance,
you've lost the usability that was theoretically the motivator for
allowing users to pick their own passphrases in the first place.

Nadim, have you considered adding a feature to Peerio to help train
users to remember their long passphrases? Joe Bonneau (who commented
extensively on this thread) co-authored a paper [0] recently about
using spaced repetition to help people memorize high-entropy
passphrases (both more traditional "passwords" and Diceware
passphrases). It seems to me Peerio is at an interesting design
mid-point between high security and general-purpose usability that
might make this an interesting, novel, and potentially very helpful
approach.

Cheers,
Garrett

[0]
http://www.jbonneau.com/doc/BS14-USENIX-towards_memorizing_random_passwords.pdf
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