4 ms·

One thing I do not understand: how does web.whatsapp.com work even if my phone is turned off? The key must be somehow transported via Facebook servers to my PC.

m348e912 4y ago
Good question. Have you confirmed it works when your phone is off? I haven't tried.
psychphysic 4y ago
WhatsApp web generates a key which your phone signs and then sender's encrypt messages for all those keys. Presumably this means that the device knows how many devices it will be send to.
- bjt2n3904 4y ago
  If the private subkey is stored on their servers, then that means their servers are one of the "ends" in "end to end encryption", and they can read all your messages. Like putting a screen door on a submarine.
  - aaomidi 4y ago
    It’s stored in a local session on your computer. You won’t be able to start a new session. So no, not their servers.
emsixteen 4y ago
Is this a thing now? Never used to work for me, always got an error about needing my phone to be on.
- warp 4y ago
  Yes, this is part of their multi-device support, which was in beta at the end of 2021 and then rolled out to everyone some time later. https://www.makeuseof.com/use-whatsapp-multiple-devices/ https://www.makeuseof.com/use-whatsapp-multiple-devices/
pmlnr 4y ago
Read https://en.wikipedia.org/wiki/OMEMO https://en.wikipedia.org/wiki/OMEMO I'm guessing it's built on the same idea.
sigmar 4y ago
Just tested myself and you're right. Seems like very bad behavior. Someone with temporary access to your phone could setup whatsapp-web to transfer the key to a PC they control, then remove whatsapp web so there's no longer any devices listed in "linked devices," and still maintain a copy of the key. Doesn't seem like doing this forces any change in device keys. edit: Maybe I'm missing something in how the web device is provisioned (maybe treating it like a group chat with multiple keys?), but I don't see how it could decrypt messages intended for my phone without just getting a copy of the key
- aaomidi 4y ago
  It’s not a copy of the key, it’s a separate key
  - sigmar 4y ago
    If it is a different key, wouldn't there be a "keys have changed" notification in my friend's chat window when I add a new whatsapp-web login? If the keys haven't changed whatsapp-web must be capable of decryption of messages meant for my device, no? edit: Is there documentation somewhere? Makes no sense to me that my friend is encrypting with the same public key (before, during, and after whatsapp-web is provisioned), but somehow it is decrypted on a new device with a different key
    - gene91 4y ago
      Your primary key vouches for that secondary key. Therefore, the “keys have changed” dialog don’t pop up because the new key’s legitimacy is verified.
      - sigmar 4y ago
        AH, I see. Found it described pg 5-9 here too https://www.whatsapp.com/security/WhatsApp-Security-Whitepaper.pdf https://www.whatsapp.com/security/WhatsApp-Security-Whitepap...
nathan_phoenix 4y ago
Same way as how Signal desktop works with the phone being turned off. As another commenter noted, they just generate another key and when someone sends you a message they have to encrypt the same message twice (one for each key).
- Aachen 4y ago
  Signal exchanges keys in the QR code. It's not that you generate a random new key and people just start encrypting for that key without verification. (I don't use WhatsApp so I don't know if they do the same.)
aeyes 4y ago
Each device has a key, the message is sent multiple times with different keys. > WhatsApp multi-device uses a client-fanout approach, where the WhatsApp client sending the message encrypts and transmits it N number of times to N number of different devices — those in the sender and receiver’s device lists. https://engineering.fb.com/2021/07/14/security/whatsapp-multi-device/ https://engineering.fb.com/2021/07/14/security/whatsapp-mult...

m348e912 4y ago

Good question. Have you confirmed it works when your phone is off? I haven't tried.

psychphysic 4y ago

WhatsApp web generates a key which your phone signs and then sender's encrypt messages for all those keys. Presumably this means that the device knows how many devices it will be send to.

bjt2n3904 4y ago
If the private subkey is stored on their servers, then that means their servers are one of the "ends" in "end to end encryption", and they can read all your messages. Like putting a screen door on a submarine.
- aaomidi 4y ago
  It’s stored in a local session on your computer. You won’t be able to start a new session. So no, not their servers.

emsixteen 4y ago

Is this a thing now? Never used to work for me, always got an error about needing my phone to be on.

warp 4y ago
Yes, this is part of their multi-device support, which was in beta at the end of 2021 and then rolled out to everyone some time later. https://www.makeuseof.com/use-whatsapp-multiple-devices/ https://www.makeuseof.com/use-whatsapp-multiple-devices/

pmlnr 4y ago

Read https://en.wikipedia.org/wiki/OMEMO https://en.wikipedia.org/wiki/OMEMO I'm guessing it's built on the same idea.

sigmar 4y ago

Just tested myself and you're right. Seems like very bad behavior. Someone with temporary access to your phone could setup whatsapp-web to transfer the key to a PC they control, then remove whatsapp web so there's no longer any devices listed in "linked devices," and still maintain a copy of the key. Doesn't seem like doing this forces any change in device keys. edit: Maybe I'm missing something in how the web device is provisioned (maybe treating it like a group chat with multiple keys?), but I don't see how it could decrypt messages intended for my phone without just getting a copy of the key

aaomidi 4y ago
It’s not a copy of the key, it’s a separate key
- sigmar 4y ago
  If it is a different key, wouldn't there be a "keys have changed" notification in my friend's chat window when I add a new whatsapp-web login? If the keys haven't changed whatsapp-web must be capable of decryption of messages meant for my device, no? edit: Is there documentation somewhere? Makes no sense to me that my friend is encrypting with the same public key (before, during, and after whatsapp-web is provisioned), but somehow it is decrypted on a new device with a different key
  - gene91 4y ago
    Your primary key vouches for that secondary key. Therefore, the “keys have changed” dialog don’t pop up because the new key’s legitimacy is verified.
    - sigmar 4y ago
      AH, I see. Found it described pg 5-9 here too https://www.whatsapp.com/security/WhatsApp-Security-Whitepaper.pdf https://www.whatsapp.com/security/WhatsApp-Security-Whitepap...

nathan_phoenix 4y ago

Same way as how Signal desktop works with the phone being turned off. As another commenter noted, they just generate another key and when someone sends you a message they have to encrypt the same message twice (one for each key).

Aachen 4y ago
Signal exchanges keys in the QR code. It's not that you generate a random new key and people just start encrypting for that key without verification. (I don't use WhatsApp so I don't know if they do the same.)

aeyes 4y ago

Each device has a key, the message is sent multiple times with different keys. > WhatsApp multi-device uses a client-fanout approach, where the WhatsApp client sending the message encrypts and transmits it N number of times to N number of different devices — those in the sender and receiver’s device lists. https://engineering.fb.com/2021/07/14/security/whatsapp-multi-device/ https://engineering.fb.com/2021/07/14/security/whatsapp-mult...