This is a specific question about implementing group messaging via the Signal Protocol.
After device 1 has sent a message encrypted with a sender key distribution message (SKDM) to many recipient devices, can each of the recipient devices use the same SKDM (that was generated by device 1) to encrypt its outgoing messages, or does it have to generate its own SKDM?
About a year ago, I asked this question on the Signal Community Forum here: https://whispersystems.discoursehosting.net/t/can-devices-share-sender-key-distribution-messages/1648, but nobody answered it.
No, each sending device ("sender") must create its own SKDM. Ie, it cannot use an SKDM that was generated by another device. Hence, "sender" is the first word of SKDM.