Telegram Desktop passcode
#1
Hello everyone!

Diving into the Telegram Desktop source code I was able to write a program that decrypts and parses the local storage.
One option the client provides is to set a passcode - any string of any length - that is optionally used to encrypt a part of said local storage.

Testing whether the passcode is valid is a multi-step process (shortened for clarity):
1. Read the salt (32 bytes), encrypted data and sha1 of decrypted data from a file.
2. Compute a PKCS5_PBKDF2_HMAC_SHA1 on the UTF8(passcode), using the salt, 4000 iterations, keysize of 256 bytes
3. Use a Telegram-specific KDF to get the AesKey and AesIV (Relatively cheap - bunch of memcpy and 4x sha1)
4. Perform an AES-IGE-DECRYPT on the encrypted data using the derived key and IV from step 3.
5. Compare sha1 of decrypted data with the sha1 read in step 1. If they match - passcode is correct.

Using OpenSSL I am able to test around 100 passcodes/s on a Ryzen 1700. That is a sad number.
The obvious time-hog in this process is the 4000 iterations on the PKCS5_PBKDF2_HMAC_SHA1.
Judging from the benchmarks (that use 1000 iterations as far as I'm aware) I estimate that I could achieve a few MH/s using a GPU powered approach.

Since the process is probably too complex to be added as a whole is it possible to use hashcat (or other programs that I am unaware of) to compute the PKCS5_PBKDF2_HMAC_SHA1 keys with the mentioned parameters and pipe generated passcode-key pairs into another program that then validates the keys?

Thank you in advance,
Miha Zupan


Messages In This Thread
Telegram Desktop passcode - by MihaZupan - 06-21-2018, 01:05 PM
RE: Telegram Desktop passcode - by atom - 06-21-2018, 02:00 PM
RE: Telegram Desktop passcode - by MihaZupan - 06-21-2018, 02:11 PM
RE: Telegram Desktop passcode - by atom - 06-22-2018, 09:48 AM