TDesktop session util

This repo contains utility to parse and generate tdata folder for Telegram Desktop application.

This tool is very unfinished, only basic stuff works like extracting and writing MTProto auth_keys

Reading session

const fs = new NodeFsAccess('/path/to/tdata')
const session = new TdataSession(fs, () => new TdNodeCryptoProvider())

const { localKey } = await session.readKeyData()
const mtp = await session.readMtpAuthorization(localKey)

Writing session

const fs = new NodeFsAccess('/path/to/tdata')
const session = new TdataSession(fs, () => new TdNodeCryptoProvider())

const localKey = await session2.writeKeyData({
    count: 1,
    order: [0],
    active: 0,
})
await session2.writeMtpAuthorization(mtp, localKey)

How it works

Data name

TDesktop allows for multiple "data names", which is basically a prefix for data storage. Default one is data, but you can choose any using -key CLI parameter.

Local key

TDesktop uses something called "local key" to encrypt most of the files. The local key itself is stored in key_datas, where data is the default data name. That file can be passcode-protected, in which case you will need a correct passcode to decrypt it.

Encryption

Without going too deep into details, encryption used is the same as the one used in MTProto v1 for message encryption (see Telegram docs for details).

There, instead of auth_key a local key is used, and instead of msg_key lower 16 bytes of sha1 of the contents are used.

Before encrypting (and computing sha1), content size is prepended, and the result is padded.

File naming

To name different files, TDesktop uses 8 lower bytes of md5 hash, with half-bytes switched (i.e. 0xFE => 0xEF).

So, for example:

const filename = 'data'
const md5 = crypto.md5(filename) // 8d777f385d3dfec8815d20f7496026dc 
const md5Lower = md5.slice(0, 8) // 8d777f385d3dfec8
const result = swap8(md5Lower).toUpperHex() // D877F783D5D3EF8C

D877F783D5D3EF8C is the folder that is most likely present in your tdata folder, which is derived simply from data and is used for your first account data.

Multi accounts

For second and third accounts, TDesktop appends #2 and #3 to the base data name respectively.

MTProto auth keys

Auth keys are stored in a file named same as account data folder but with s appended. So, for the first account that would be D877F783D5D3EF8Cs.