You can find information regarding the status of the encryption signature in the info of end-to-end-encrypted chats. This information is visible in any client (web- and desktop-client, iOS and Android).
The signature status provides additional security information and is intended to inform the user about a potential attack on the end-to-end encryption of the chat. The signature is created using the private key of the creator or inviter of the respective chat. The participating user verifies the signature and displays the result in the app. Complex man-in-the-middle attacks on the end-to-end encryption are intended to become visible through this.
Status for the signature
Signature valid (green):
The signature is available and valid. The chat access key was sent by the inviting user and it has a valid signature for their private key.
Signature unavailable (yellow):
The chat access key has been sent without a signature. This can happen when an older version is used by the inviting user. Please contact the user through other means to verify this.
Signatur not valid (red):
The chat access key was sent with a signature that does not match the users current private key. This can happen if the private key of the user has changed, e.g. by resetting the encryption password after the chat access key was sent. To verify that the chat has been initiated by the user it is recommended to contact the user outside of the application.