Empty group messages after RSA key renewal

[ChristianeMussmacher]

Bug only reproducible on iOS clients

Situation

• Android or iOS client opens group (admin)
• adds Android and iOS clients to group
• members join group
• iOS client changes RSA key
• from this moment on, iOS client is not able to receive messages in group chat (empty messages)

--> We tested the same scenario with iOS only and Android only groups. After changing the RSA key on either an admin device or an member client, no problems were observed.

--> Bug is only triggered when both, Android and iOS clients, are members a group where iOS group member renews RSA key pair.

Closely related to: #813

[pavel-mayer]

Problem is caused by a bug in the Android client. In the getEncryptedGroupKeys() there was no check for the requested id of the public key, so an Android client might generate encrypted group keys for an old public key, but the server would assume they are using the new key.

[pavel-mayer]

The cause was actually a bug in the Android client fixed here hoccer/hoccer-talk-spike@1624a798ea24a0a92870e8ca60fde6e47caa5bb6

It also needs a fix on the server that should also fix other group key problems: hoccer/hoccer-talk-spike@722eb6b84c45c48f37e6226b888ca52517a2bde8