layer_crypto/
obfuscated.rs

1//! AES-256-CTR cipher for MTProto transport obfuscation.
2//!
3//! Telegram's obfuscated transport encrypts the entire byte stream with two
4//! separate AES-256-CTR instances (TX and RX) whose keys are derived from a
5//! random 64-byte init header sent at connection start.
6//!
7//! Key derivation from the 64-byte `init` buffer:
8//! ```text
9//! TX key = init[8..40]           TX IV = init[40..56]
10//! RX key = reverse(init)[8..40]  RX IV = reverse(init)[40..56]
11//! ```
12
13#[allow(deprecated)]
14use aes::cipher::{KeyIvInit, StreamCipher, generic_array::GenericArray};
15
16/// AES-256-CTR stream cipher pair for MTProto obfuscated transport.
17pub struct ObfuscatedCipher {
18    #[allow(deprecated)]
19    rx: ctr::Ctr128BE<aes::Aes256>,
20    #[allow(deprecated)]
21    tx: ctr::Ctr128BE<aes::Aes256>,
22}
23
24impl ObfuscatedCipher {
25    /// Build cipher state from the 64-byte random init buffer.
26    #[allow(deprecated)]
27    pub fn new(init: &[u8; 64]) -> Self {
28        let rev: Vec<u8> = init.iter().copied().rev().collect();
29        Self {
30            rx: ctr::Ctr128BE::<aes::Aes256>::new(
31                GenericArray::from_slice(&rev[8..40]),
32                GenericArray::from_slice(&rev[40..56]),
33            ),
34            tx: ctr::Ctr128BE::<aes::Aes256>::new(
35                GenericArray::from_slice(&init[8..40]),
36                GenericArray::from_slice(&init[40..56]),
37            ),
38        }
39    }
40
41    /// Build cipher from explicit key/IV pairs (used when MTProxy secret
42    /// mixing has already been applied externally via SHA-256).
43    #[allow(deprecated)]
44    pub fn from_keys(
45        tx_key: &[u8; 32],
46        tx_iv: &[u8; 16],
47        rx_key: &[u8; 32],
48        rx_iv: &[u8; 16],
49    ) -> Self {
50        Self {
51            tx: ctr::Ctr128BE::<aes::Aes256>::new(
52                GenericArray::from_slice(tx_key),
53                GenericArray::from_slice(tx_iv),
54            ),
55            rx: ctr::Ctr128BE::<aes::Aes256>::new(
56                GenericArray::from_slice(rx_key),
57                GenericArray::from_slice(rx_iv),
58            ),
59        }
60    }
61
62    /// Encrypt outgoing bytes in-place (TX direction).
63    pub fn encrypt(&mut self, buf: &mut [u8]) {
64        self.tx.apply_keystream(buf);
65    }
66
67    /// Decrypt incoming bytes in-place (RX direction).
68    pub fn decrypt(&mut self, buf: &mut [u8]) {
69        self.rx.apply_keystream(buf);
70    }
71}
layer_crypto/obfuscated.rs

layer_crypto/
obfuscated.rs
