Telegram confirmed the behaviour we observed and addressed this issue in version 7.8.1 for Android, 7.8.3 for iOS and 2.8.8 for Telegram Desktop. However, cryptographic protocols are expected to rule out even such attacks. This attack is mostly of theoretical interest. In particular, Telegram encrypts acknowledgement messages, i.e. messages that encode that a previous message was indeed received, but the way it handles the re-sending of unacknowledged messages leaks whether such an acknowledgement was sent and received. Telegram confirmed the behaviour we observed and addressed this issue in version 7.8.1 for Android, 7.8.3 for iOS and 2.8.8 for Telegram Desktop.Īn attacker can detect which of two special messages was encrypted by a client or a server under some special conditions. This allows, for example, to alter the order of “pizza” and “crime” in the sequence of messages: “I say yes to”, “all the pizzas”, “I say no to”, “all the crimes”.

We disclosed the following vulnerabilities to the Telegram development team on 16 April 2021 and agreed with them on a disclosure on 16 July 2021:Īn attacker on the network can reorder messages coming from a client to the server. We thus focused our efforts on analysing whether Telegram’s MTProto offers comparable privacy to surfing the web with HTTPS. While Telegram is often referred to as an “encrypted messenger”, this level of protection is the only protection offered by default: MTProto-based end-to-end encryption, which would protect communication from Telegram employees or anyone breaking into Telegram’s servers, is only optional and not available for group chats. We made several suggestions to the Telegram developers that enable providing formal assurances that rule out a large class of cryptographic attacks, similarly to other, more established, cryptographic protocols.īy default, Telegram uses its bespoke MTProto protocol to secure communication between clients and its servers as a replacement for the industry-standard Transport Layer Security (TLS) protocol. As a result of our analysis, we found several cryptographic weaknesses in the protocol, from technically trivial and easy to exploit to more advanced and of theoretical interest.įor most users, the immediate risk is low, but these vulnerabilities highlight that Telegram fell short of the cryptographic guarantees enjoyed by other widely deployed cryptographic protocols such as TLS. We performed a detailed security analysis of the encryption offered by the popular Telegram messaging platform.

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