Unable to decrypt: Unknown encryption algorithm \"undefined\".

[michel-zimmer]

Describe the bug
Decrypted event source:

{
  "type": "m.room.message",
  "content": {
    "msgtype": "m.bad.encrypted",
    "body": "** Unable to decrypt: Unknown encryption algorithm \"undefined\". **"
  }
}

Original event source:

{
  "content": {},
  "origin_server_ts": 1593761398124,
  "room_id": "!…:matrix.org",
  "sender": "@michel-zimmer:matrix.org",
  "type": "m.room.encrypted",
  "unsigned": {
    "redacted_by": "$6M1oJkRMLfn3k1ALmnLbbWq2vemoO6cCLomowO4BkbM",
    "redacted_because": {
      "content": {},
      "origin_server_ts": 1593764558519,
      "redacts": "$VmprAKEQ9RA_NloLleUbkCKPpf8FWVf5yly2gD_mW58",
      "room_id": "!…:matrix.org",
      "sender": "@michel-zimmer:matrix.org",
      "type": "m.room.redaction",
      "unsigned": {
        "age": 22680076733
      },
      "event_id": "$6M1oJkRMLfn3k1ALmnLbbWq2vemoO6cCLomowO4BkbM",
      "user_id": "@michel-zimmer:matrix.org",
      "age": 22680076733
    },
    "age": 22683237128
  },
  "event_id": "$VmprAKEQ9RA_NloLleUbkCKPpf8FWVf5yly2gD_mW58",
  "user_id": "@michel-zimmer:matrix.org",
  "age": 22683237128,
  "redacted_because": {
    "content": {},
    "origin_server_ts": 1593764558519,
    "redacts": "$VmprAKEQ9RA_NloLleUbkCKPpf8FWVf5yly2gD_mW58",
    "room_id": "!…:matrix.org",
    "sender": "@michel-zimmer:matrix.org",
    "type": "m.room.redaction",
    "unsigned": {
      "age": 22680076733
    },
    "event_id": "$6M1oJkRMLfn3k1ALmnLbbWq2vemoO6cCLomowO4BkbM",
    "user_id": "@michel-zimmer:matrix.org",
    "age": 22680076733
  }
}

2nd example for an original event source:

{
  "content": {},
  "origin_server_ts": 1616336830758,
  "sender": "@michel-zimmer:matrix.org",
  "type": "m.room.encrypted",
  "unsigned": {
    "redacted_by": "$yx-Gt9Y4eKBTsEt_s87u9ZLtfs2KGDiUrCjShwFAJOs",
    "redacted_because": {
      "content": {},
      "origin_server_ts": 1616336841906,
      "redacts": "$P3bt9HvxeaeVOsb6IFH9l2_4IlwhnjofG1YZ51aOcRk",
      "sender": "@michel-zimmer:matrix.org",
      "type": "m.room.redaction",
      "unsigned": {
        "age": 108987682
      },
      "event_id": "$yx-Gt9Y4eKBTsEt_s87u9ZLtfs2KGDiUrCjShwFAJOs",
      "room_id": "!rlKcMUJZikkJJYnDCx:matrix.org"
    },
    "age": 108998830
  },
  "event_id": "$P3bt9HvxeaeVOsb6IFH9l2_4IlwhnjofG1YZ51aOcRk",
  "room_id": "!rlKcMUJZikkJJYnDCx:matrix.org"
}

Also see element-hq/element-web#3744

To Reproduce
I'm not sure what the steps to reproduce are, but I've had problems setting up a new client (not a problem of the client - it was my fault) and in the end I had to re-link everything. (I'm not sure, if this is relevant.) And then I've connected the Electron App Desktop client again and those messages appeared.

Expected behavior
I would expect not to see the misleading error message from the screenshot below. There are no keys to be retrieved from another device if I'm not mistaken. Additionally I'm quite sure that a Message deleted message should be displayed instead.

Screenshots

Desktop (please complete the following information):

• OS: irrelevant
• Browser [e.g. chrome, safari]: Firefox, but I guess that's irrelevant - Electron Desktop is affected, too
• Version [e.g. 22]: Element version: 1.7.23

Additional context
It looks very similar to this old issue: element-hq/element-web#3744

[real-alexei]

We should probably check if redacted event should be decrypted? 14cd37e#r49837960