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tl;dr to me seems: Since users can change devices, they'll need to reissue key material, this needs to be supported. WhatsApp reports key changing optionally, b

tl;dr to me seems: Since users can change devices, they'll need to reissue key material, this needs to be supported. WhatsApp reports key changing optionally, but doesn't tell the server that happened. If WhatsApp tries to backdoor a channel and one of the users has key change notification, they'll find out about it, and WhatsApp has no idea whether the warning was shown.

int_19h 10y ago
If I understand correctly, the default is no key change notification. So for the majority of users, MITM would go unnoticed here.
- Deregibus 10y ago
  I believe that you get a key change notification, but by default it doesn't require any sort of confirmation and will just continue to work with the new key.
  - FabHK 10y ago
    GP is correct: no notification of key changes by default. Even if you enable the key change notification, it is "non-blocking" in WhatsApp as outlined by the blog post: when you get the key change notification, the WhatsApp client will automatically without user intervention resend undelivered messages (unlike Signal, btw, from what I understand).
jMyles 10y ago
> If WhatsApp tries to backdoor a channel and one of the users has key change notification, they'll find out about it The problem is that the might not find about out retransmissions. You are trusting that the "double-tick" means that the message won't be resent, but presumably WhatsApp can indeed retransmit those messages with the new key under pressure from a state actor. They need to specifically address this point; it's the only thing worth talking about. The rest is just a discussion of implementation of key exchanges generally.
- Deregibus 10y ago
  That would require the client to be compromised though right? My understanding is that the client is making the decision whether to retransmit with the new key. Now it's fair to question whether you can trust the client, but if you can't then there's no limit to what they could do.
willvarfar 10y ago
Additionally, the WhatsApp notification is after the message is resent.
deleted 10y ago
[deleted]
dsp1234 10y ago
FTA: "WhatsApp server has no knowledge of whether users have enabled the change notifications, or whether users have verified safety numbers." If it's off by default, then the answer is likely to be near zero, as it often is with default options. This isn't an argument about the security of the app (which I have no background to know), more of a comment on relying on non-default behavior.

int_19h 10y ago

If I understand correctly, the default is no key change notification. So for the majority of users, MITM would go unnoticed here.

Deregibus 10y ago
I believe that you get a key change notification, but by default it doesn't require any sort of confirmation and will just continue to work with the new key.
- FabHK 10y ago
  GP is correct: no notification of key changes by default. Even if you enable the key change notification, it is "non-blocking" in WhatsApp as outlined by the blog post: when you get the key change notification, the WhatsApp client will automatically without user intervention resend undelivered messages (unlike Signal, btw, from what I understand).

jMyles 10y ago

> If WhatsApp tries to backdoor a channel and one of the users has key change notification, they'll find out about it The problem is that the might not find about out retransmissions. You are trusting that the "double-tick" means that the message won't be resent, but presumably WhatsApp can indeed retransmit those messages with the new key under pressure from a state actor. They need to specifically address this point; it's the only thing worth talking about. The rest is just a discussion of implementation of key exchanges generally.

Deregibus 10y ago
That would require the client to be compromised though right? My understanding is that the client is making the decision whether to retransmit with the new key. Now it's fair to question whether you can trust the client, but if you can't then there's no limit to what they could do.

willvarfar 10y ago

Additionally, the WhatsApp notification is after the message is resent.

deleted 10y ago

[deleted]

dsp1234 10y ago

FTA: "WhatsApp server has no knowledge of whether users have enabled the change notifications, or whether users have verified safety numbers." If it's off by default, then the answer is likely to be near zero, as it often is with default options. This isn't an argument about the security of the app (which I have no background to know), more of a comment on relying on non-default behavior.