crackle cracks BLE Encryption (AKA Bluetooth Smart).

crackle exploits a flaw in the BLE pairing process that allows an attacker to guess or very quickly brute force the TK (Temporary Key). With the TK and other data collected from the pairing process, the STK (Short Term Key) and later the LTK (Long Term Key) can be collected.

With the STK and LTK, all communications between the master and the slave can be decrypted.

crackle was written by Mike Ryan [email protected] See web site for more info: http://lacklustre.net/projects/crackle/

Table of Contents

• Modes of Operation
  • Crack TK
  • Decrypt with LTK
• Running crackle
• Sample Files
• See Also
• Thanks

Modes of Operation

crackle has two major modes of operation: Crack TK and Decrypt with LTK.

Crack TK

In Crack TK mode, crackle brute forces the TK used during a BLE pairing event. crackle exploits the fact that the TK in Just Works(tm) and 6-digit PIN is a value in the range [0,999999] padded to 128 bits. If the required pairing frames are present, the brute force process takes less than one second on modern CPUs. The method employed is to compute all possible confirm values and to compare them with the existing ones of the pairing exchange. Otherwise, with the -s option, it fallbacks on another technique that requires less information but takes much more time. It simply generates every possible STK/sessionKey and tries to decrypt encrypted datas flow.

After the TK has been cracked, crackle goes on to derive the remaining keys used to encrypt further communication and uses these to decrypt the encrypted L2CAP data. If the LTK is exchanged (typically the first order of business after encrypted communication is established), crackle outputs this value. The LTK can be used to decrypt any future communicaitons between the master and slave.

Decrypt with LTK

In Decrypt with LTK mode, crackle uses a user-supplied LTK to decrypt communications between a master and slave. This mode is identical to the decryption portion of Crack TK mode.

Running Crackle

Crack TK Mode

In Crack TK mode, crackle requires a PCAP file that contains a BLE pairing event. The best way to generate such a file is to use an Ubertooth to capture a pairing event between a master and a slave.

To check if your PCAP file contains all the necessary packets, run crackle with the -i option:

crackle -i <file.pcap>

If you have all packets, the program should produce output similar to this:

Warning: No output file specified. Won't decrypt any packets.
TK found: 412741
Specify an output file with -o to decrypt packets!

To decrypt all packets, add the -o option:

crackle -i <file.pcap> -o <output.pcap>

The output file will contain decrypted versions of all the encrypted packets from the original PCAP, as well as all the unencrypted packets. Note that CRCs are not recalculated, so the CRCs of decrypted packets will be incorrect.

Decrypt with LTK

In Decrypt with LTK mode, crackle requires a PCAP file that contains at a minimum LL_ENC_REQ and LL_ENC_RSP packets and the LTK used to encrypt the communications.

The format for LTK is a 128 bit hexadecimal number with no spaces or separators, most-significant octet to least-significant octet. Example:

-l 81b06facd90fe7a6e9bbd9cee59736a7

To check if your PCAP file contains all the necessary packets, run crackle with -i and -l:

crackle -i <file.pcap> -l <ltk>

If you have all the packets, the program should produce output similar to this:

Warning: No output file specified. Won't decrypt any packets.
Specify an output file with -o to decrypt packets!

To decrypt all packets, add the -o option:

crackle -i <file.pcap> -o <out.pcap> -l <ltk>

The output file will be produced similarly to the output file described above.

Sample Files

Grab some sample files for cracking with crackle. Refer to the README inside the tarball for more information:

https://lacklustre.net/bluetooth/crackle-sample.tgz

See Also

• Ubertooth: http://ubertooth.sourceforge.net/
• libbtbb: http://libbtbb.sourceforge.net/
• #ubertooth on irc.freenode.net

Thanks

Major thanks go to Mike Ossmann and Dominic Spill from the Ubertooth project. None of this would be possible without them.

Big time thanks go to Mike Kershaw/dragorn of Kismet for help creating and working with PCAP files.

Thanks go to the rest of #ubertooth on irc.freenode.net.