This is because they don’t retain your (encrypted) messages on their servers right? Is this for storage reasons, or more just security philosophy of not being able to access past chats when you login from elsewhere?
This is not entirely correct. Messages are stored on their servers temporarily (last I saw, for up to 30 days), so that even if your device is offline for a while, you still get all your messages.
In theory, you could have messages waiting in your queue for device A, when you add device B, but device B will still not get the messages, even though the encrypted message is still on their servers.
This is because messages are encrypted per device, rather than per user. So if you have a friend who uses a phone and computer, and you also use a phone and computer, the client sending the message encrypts it three times, and sends each encrypted copy to the server. Each client then pulls its copy, and decrypts it. If a device does not exist when the message is encrypted and sent, it is never encrypted for that device, so that new device cannot pull the message down and decrypt it.
Yes, as long as you set up the desktop client before sending the message.
Messages sent with Signal are encrypted per device, not per user, so if your desktop client doesn’t exist when the message is sent, it is never encrypted and sent for that device.
When you set up a new client, you will only see new messages.
The chat continues on all linked devices from the point in time that they are linked.
Imagine two people having a face-to-face conversation, then a third person walks up and joins in. The third person doesn’t know what was said before they joined the conversation, but all three continue the conversation from that point on.
Linked devices are like the above example, if two of those people were married and tell each other every conversation they’ve had since their wedding.
Message logs doesn’t break forward secrecy in a cryptographic sense, retaining original asymmetric decryption keys (or method to recreate them) does. Making history editable would help against that too.
What Signal actually intends is to limit privacy leaks, it only allows history transfer when you transfer the entire account to another device and “deactivate” the account on the first one, so you can’t silently get access to all of somebody’s history
You’re describing something very different - you already have the messages, and you already have them decrypted. You can transfer them without the keys. If someone gets your device, they have them, too.
Whether Signal keeps the encrypted the messages or not, a new device has no way of getting the old messages from the server.
I run a cryptography forum, I know the exact definition of these terms. Message logs in plaintext is very distinct from forward secrecy. What forward secrecy means in particular is that captured network traffic can’t be decrypted later even if you at a later point can steal the user’s keys (because the session used session keys that were later deleted). Retrieving local logs with no means of verifying authenticity is nothing more than a classical security breach.
You can transfer messages as a part of an account transfer on Signal (at least on Android). This deactivates the app on the old device (so you can’t do it silently to somebody’s device)
I would argue that it is not limited to network traffic, it is the general concept that historical information is not compromised, even if current (including long-term) secrets are compromised.
From my comment earlier:
There is no sharing of messages between linked devices - that would break forward secrecy
This describes devices linked to an account, where each is retrieving messages from the server - not a point-to-point transfer, which is how data is transferred from one Android device to another. If a new device could retrieve and decrypt old messages on the server, that would be a breach of the forward security concept.
Does it work phone to phone? I was under the impression that a backup restore was needed if you wanted previous messages. It’s really an unnecessary security risk to have previous message sync. Someone gets your phone in their hand for 20 seconds, links your device and they get every message you have ever sent? No bueno.
I haven’t actually synced a new phone to Signal, does everything just carry over? I assumed you needed to transfer your account from phone to phone, not just link a new device.
Okay, but can’t it be an optional feature? I’d like it if a new device could download message history from an old device by having both online at the same time.
Optional how so? It’s a rotating key. Unless you have all of those keys to export into your computer, then you’ll be stuck with the current synced key.
I don’t see why the current key can’t encrypt old messages and send those. I admit I might be missing something obvious though. Maybe something like not wanting to accidentally leak old messages? As in it’s less attack surface or something?
It means that if you have chats on one device and install Signal on another one, the chats don’t transfer to it. After you link new device, new chats do sync perfectly fine.
The inability to continue chat from phone is a feature.
New messages will show on all your devices, but yes, it is intentional that old messages are not available to new devices.
This is because they don’t retain your (encrypted) messages on their servers right? Is this for storage reasons, or more just security philosophy of not being able to access past chats when you login from elsewhere?
This is not entirely correct. Messages are stored on their servers temporarily (last I saw, for up to 30 days), so that even if your device is offline for a while, you still get all your messages.
In theory, you could have messages waiting in your queue for device A, when you add device B, but device B will still not get the messages, even though the encrypted message is still on their servers.
This is because messages are encrypted per device, rather than per user. So if you have a friend who uses a phone and computer, and you also use a phone and computer, the client sending the message encrypts it three times, and sends each encrypted copy to the server. Each client then pulls its copy, and decrypts it. If a device does not exist when the message is encrypted and sent, it is never encrypted for that device, so that new device cannot pull the message down and decrypt it.
For more details: https://signal.org/docs/specifications/sesame/
That’s for your insightful comment. I’m now going down the rabbit hole of the signal spec :)
Correct
But if I reply on the phone will it populate the desktop chat and vice versa?
Yes, as long as you set up the desktop client before sending the message.
Messages sent with Signal are encrypted per device, not per user, so if your desktop client doesn’t exist when the message is sent, it is never encrypted and sent for that device.
When you set up a new client, you will only see new messages.
See https://signal.org/docs/specifications/sesame/ for details.
Cool, could I recover a backup to te desktop to have access to the historical ones?
From http://support.signal.org/hc/en-us/articles/360007059752-Backup-and-Restore-Messages#desktop_restore:
The chat continues on all linked devices from the point in time that they are linked.
Imagine two people having a face-to-face conversation, then a third person walks up and joins in. The third person doesn’t know what was said before they joined the conversation, but all three continue the conversation from that point on.
Linked devices are like the above example, if two of those people were married and tell each other every conversation they’ve had since their wedding.
There is no sharing of messages between linked devices - that would break forward secrecy, which prevents a successful attacker from getting historical messages. See the first bullet of: https://support.signal.org/hc/en-us/articles/360007320551-Linked-Devices
Messages are encrypted per device, not per user (https://signal.org/docs/specifications/sesame/), and forward secrecy is preserved (https://en.m.wikipedia.org/wiki/Forward_secrecy, for the concept in general, and https://signal.org/docs/specifications/doubleratchet/ for Signal’s specific approach).
Message logs doesn’t break forward secrecy in a cryptographic sense, retaining original asymmetric decryption keys (or method to recreate them) does. Making history editable would help against that too.
What Signal actually intends is to limit privacy leaks, it only allows history transfer when you transfer the entire account to another device and “deactivate” the account on the first one, so you can’t silently get access to all of somebody’s history
You’re describing something very different - you already have the messages, and you already have them decrypted. You can transfer them without the keys. If someone gets your device, they have them, too.
Whether Signal keeps the encrypted the messages or not, a new device has no way of getting the old messages from the server.
I run a cryptography forum, I know the exact definition of these terms. Message logs in plaintext is very distinct from forward secrecy. What forward secrecy means in particular is that captured network traffic can’t be decrypted later even if you at a later point can steal the user’s keys (because the session used session keys that were later deleted). Retrieving local logs with no means of verifying authenticity is nothing more than a classical security breach.
You can transfer messages as a part of an account transfer on Signal (at least on Android). This deactivates the app on the old device (so you can’t do it silently to somebody’s device)
I would argue that it is not limited to network traffic, it is the general concept that historical information is not compromised, even if current (including long-term) secrets are compromised.
From my comment earlier:
This describes devices linked to an account, where each is retrieving messages from the server - not a point-to-point transfer, which is how data is transferred from one Android device to another. If a new device could retrieve and decrypt old messages on the server, that would be a breach of the forward security concept.
There is no reason why the message sync that works from phone to phone could not be implemented on the desktop client as well.
Does it work phone to phone? I was under the impression that a backup restore was needed if you wanted previous messages. It’s really an unnecessary security risk to have previous message sync. Someone gets your phone in their hand for 20 seconds, links your device and they get every message you have ever sent? No bueno.
You can sync messages from phone to phone. https://support.signal.org/hc/en-us/articles/360007059752-Backup-and-Restore-Messages#android_transfer
Any new client doesn’t get old messages. Phone only allows the possibility of transferring a backup, which desktop doesn’t have.
I haven’t actually synced a new phone to Signal, does everything just carry over? I assumed you needed to transfer your account from phone to phone, not just link a new device.
Yes
Okay, but can’t it be an optional feature? I’d like it if a new device could download message history from an old device by having both online at the same time.
Optional how so? It’s a rotating key. Unless you have all of those keys to export into your computer, then you’ll be stuck with the current synced key.
You can still push old message history from your main device to your other devices, you can re-encrypt
No shit. I’ll need to look into this. Thanks for learning me up.
Matrix does it this way
I don’t see why the current key can’t encrypt old messages and send those. I admit I might be missing something obvious though. Maybe something like not wanting to accidentally leak old messages? As in it’s less attack surface or something?
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Unabilifiedness
Thanks
What does this mean? I use my phone and computer, and they sync up in real-time without any issues.
It means that if you have chats on one device and install Signal on another one, the chats don’t transfer to it. After you link new device, new chats do sync perfectly fine.
After they dropped SMS support and called that a feature, now I can’t wait for their hottest new bug!