-_ не могут встречаться в base64. Может, это разделитель?
Keys for end-to-end encrypted secret chats are generated by a new instance of DH key exchange, so they are known only to the parties involved and not to the server. To establish the identities of these parties and to ensure that no MitM is in place, it is recommended to compare identicons, generated from hashes of the DH secret chat keys (key visualizations).