Telegram is a popular secure messaging service with third biggest user base as of 2021. In this paper, we analyze the security of Telegram’s end-to-end encryption (E2EE) protocol in presence of mass-surveillance. Specifically, we show >that Telegram’s E2EE protocol is susceptible to fairly efficient algorithm substitution attacks. While official Telegram clients should be protected against this type of attack due their open-source nature and reproducible builds, this could potentially lead to a very efficient state sponsored surveillance of private communications over Telegram, either on individuals through a targeted attack or massively through some compromised third-party clients. We provide an efficient algorithm substitution attack against MTProto2.0 — the underlying authenticated encryption scheme — that recovers significant amount of encryption key material with a very high probability with few queries and fairly low latency. This could potentially lead to a very efficient state sponsored surveillance of private communications over Telegram, either through a targeted attack or a compromised third-party app. Our attack exploits MTProto2.0’s degree of freedom in choosing the random padding length and padding value. Accordingly, we strongly recommend that Telegram should revise MTProto2.0’s padding methodology. In particular, we show that a minor change in the padding description of MTProto2.0 makes it subversion-resistant in most of the practical scenarios. As a side-effect, we generalize the underlying mode of operation in MTProto2.0, as MTProto-G, and show that this generalization is a multi-user secure deterministic authenticated encryption scheme.

In this work, we propose a construction of 2-round tweakable substitutionpermutation networks using a single secret S-box. This construction is based on non-linear permutation layers using independent round keys, and achieves security beyond the birthday bound in the random permutation model. When instantiated with an n-bit block cipher with ωn-bit keys, the resulting tweakable block cipher, dubbed CTET+, can be viewed as a tweakable enciphering scheme that encrypts ωκ-bit messages for any integer ω ≥ 2 using 5n + κ-bit keys and n-bit tweaks, providing 2n/3-bit security.Compared to the 2-round non-linear SPN analyzed in [CDK+18], we both minimize it by requiring a single permutation, and weaken the requirements on the middle linear layer, allowing better performance. As a result, CTET+ becomes the first tweakable enciphering scheme that provides beyond-birthday-bound security using a single permutation, while its efficiency is still comparable to existing schemes including AES-XTS, EME, XCB and TET. Furthermore, we propose a new tweakable enciphering scheme, dubbed AES6-CTET+, which is an actual instantiation of CTET+ using a reduced round AES block cipher as the underlying secret S-box. Extensivecryptanalysis of this algorithm allows us to claim 127 bits of security.Such tweakable enciphering schemes with huge block sizes become desirable in the context of disk encryption, since processing a whole sector as a single block significantly worsens the granularity for attackers when compared to, for example, AES-XTS, which treats every 16-byte block on the disk independently. Besides, as a huge amount of data is being stored and encrypted at rest under many different keys in clouds, beyond-birthday-bound security will most likely become necessary in the short term.