SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/tech-e2ee-metadata

What can a messaging provider sometimes learn despite encrypted content?

It may still learn metadata, such as which devices receive one message.

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  1. Every plaintext word
  2. All private signing keys
  3. Which devices share a message ✔

Why: End-to-end encryption protects message content, but routing and notification systems still need operational information. Timing, recipient devices, and traffic patterns can reveal relationships without revealing plaintext.

Huh, didn't know that: Transport encryption can hide some metadata from outside network observers, but not necessarily from the service.

The dad joke
The message wore a blindfold, but its delivery label was still visible.
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References

Answer: Which devices share a message. End-to-end encryption protects message content, but routing and notification systems still need operational information. Timing, recipient devices, and traffic patterns can reveal relationships without revealing plaintext.

  1. RFC 9750, Messaging Layer Security Architecture www.rfc-editor.org/rfc/rfc9750.html
    no technical mechanism in MLS prevents them from determining which devices are recipients of the same message
    Checked 2026-10-10.

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