Abstract We study the use of symmetric cryptography in the MTProto 2.0 protocol, Telegram’s equivalent of the TLS protocol. We give positive and negative results. On the one hand, we formally and in detail specify a slight variant of Telegram’s “record protocol” and prove that it achieves security in a suitable bidirectional secure channel model, albeit under unstudied assumptions; this model itself advances the state of the art for secure channels. On the other hand, we first motivate our slight deviation from MTProto as deployed by giving two attacks on the original protocol specification: one of practical, one of theoretical interest. Then, we give two attacks on the implementation, which are outside of our formal model: one targeting the client, one targeting the server. The client-side attack enables plaintext recovery by exploiting timing side channels, of varying strength, in three official Telegram clients. On its own this attack is thwarted by the secrecy of header fields that are established by Telegram’s key exchange protocol. We thus chain this attack with an attack against the implementation of the key exchange protocol on Telegram’s servers. This final attack breaks the authentication properties of Telegram’s key exchange, allowing a MitM attack. More mundanely, it also reduces the cost of the client-side plaintext-recovery attack. In totality, our results provide the first comprehensive study of MTProto’s use of symmetric cryptography, as well as highlight weaknesses in its key exchange.

We study the use of symmetric cryptography in the MTProto 2.0 protocol, Telegram’s equivalent of the TLS record protocol. We give positive and negative results. On the one hand, we formally and in detail model a slight variant of Telegram’s “record protocol” and prove that it achieves security in a suitable bidirectional secure channel model, albeit under unstudied assumptions; this model itself advances the state-of-the-art for secure channels. On the other hand, we first motivate our modelling deviation from MTProto as deployed by giving two attacks – one of practical, one of theoretical interest – against MTProto without our modifications. We then also give a third attack exploiting timing side channels, of varying strength, in three official Telegram clients. On its own this attack is thwarted by the secrecy of salt and id fields that are established by Telegram’s key exchange protocol. To recover these, we chain the third attack with a fourth one against the implementation of the key exchange protocol on Telegram’s servers. In totality, our results provide the first comprehensive study of MTProto’s use of symmetric cryptography.