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There is something I don't understand about whatsapp's privacy policy.

Your Messages. We do not retain your messages in the ordinary course of providing our Services to you. Once your messages (including your chats, photos, videos, voice messages, files, and share location information) are delivered, they are deleted from our servers. Your messages are stored on your own device. If a message cannot be delivered immediately (for example, if you are offline), we may keep it on our servers for up to 30 days as we try to deliver it. If a message is still undelivered after 30 days, we delete it. To improve performance and deliver media messages more efficiently, such as when many people are sharing a popular photo or video, we may retain that content on our servers for a longer period of time. We also offer end-to-end encryption for our Services, which is on by default, when you and the people with whom you message use a version of our app released after April 2, 2016. End-to-end encryption means that your messages are encrypted to protect against us and third parties from reading them.

From whatsapp privacy policy.

If the media messages are e2e encrypted, what is the point of storing popular messages on the server for a longer period?

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2 Answers 2

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Media attachments are shared and forwarded more frequently to multiple recepients than messages. If retention period is small, the attachment has to be reuploaded by the sender.

Sender encrypts media elements with AES-CBC-256 key and uploads it to the server. The sender encrypts the key and the URL and sends it to the recepient. If the sender and recepients want to forward the attachment, they only have to reencrypt the key and URL for the receiving party. This saves server space, time and bandwidth when sharing media multiple times. Media elements remain end-to-end encrypted at the server.

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  • That is what I thought too, but apparently that is not what is happening. According to The whitepaper Or if you believe it is, can you reference the whitepaper for me?
    – yukashima huksay
    Jan 13, 2021 at 15:55
  • Whitepaper > Transmitting Media and Other Attachments
    – defalt
    Jan 13, 2021 at 16:21
  • So each media file is encrypted only once, with one key, for each time that a user uploads it (And not when it is forwarded)?
    – yukashima huksay
    Jan 15, 2021 at 7:15
  • Yes. The key never changes. Only the key and URL are forwarded using e2ee.
    – defalt
    Jan 15, 2021 at 8:17
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It explains what it means. It's not storing messages it is storing media.

Since transmitting media can take a lot of bandwidth, it is better and easier for everyone if there can be a copy that is cached somewhere for lowered traffic needs and faster access. If a lot of people in the group and accessing, downloading, and playing that media over a long period of time, it's a waste of resources to have to re-download and re-cache that media. So it is saying that it will keep a copy of that media for potentially a longer period of time until it is not being accessed so much.

The concept you are looking to understand is: "caching".

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  • Can you elaborate please? Are you suggesting that media messages in groups are stored unencrypted in whatsapp servers? Are you also suggesting that this caching is specific to group chats and not for when a messages is forwarded many times in private chats? What I don't understand is the benefit of caching in e2e connections.
    – yukashima huksay
    Jan 13, 2021 at 14:36
  • Why are you assuming that they are unencrypted? Why would they need to be unencrypted? Cached data can be encrypted.
    – schroeder ♦
    Jan 13, 2021 at 14:43
  • From my understanding, when I send you a media, it should be encrypted with your publicly, so only you will be able to view it. And if you want to forward that to the third person, you should encrypt it with their public key again. And upload it again. in this case, for each time a media is forwarded. A new encrypted blub should be stored in the blob storage. And that blob will be downloaded only once, by you. So there is no point in caching it.
    – yukashima huksay
    Jan 13, 2021 at 14:46
  • " And that blob will be downloaded only once, by you." -- that's a big assumption and that assumption has nothing to do with encryption
    – schroeder ♦
    Jan 13, 2021 at 14:47

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