My Pebble smartwatch, like many (most?) other smart watches, uses Bluetooth Low Energy (Bluetooth Smart, BLE, BTLE, etc) to communicate with my smart phone.

There's some security in the link, including authentication, data encryption, and signing, however last summer researchers suggested an attack that appears to compromise the encryption.

I have email sender names and subjects, phone numbers and names, and other data sent to my watch over this link, further this link allows me to query my smart phone and to some degree control it. Most worrying, I typically act on the information it provides, so if incorrect information was sent, such as an emergency text from a loved one, it can be a way to control me to some degree.

The impact of this particular attack isn't immediately clear to me, can someone help break it down for me?

I'd like to know if there are any simple steps I can take that will prevent this attack. If not, I may need to discontinue use of it until security is improved.

Are there other attacks I should be aware of for Bluetooth Smart?

2 Answers 2


The attack you mention isn't really all that scary or surprising. It doesn't actually attack the encryption used by the devices, but rather the pairing process. When you pair a bluetooth device, an encryption key has to be exchanged. This is done by deriving an AES key from a PIN number. They effectively try all pin numbers against a recorded pairing session in order to recover the AES key that is used long term.

If you establish the pairing for your device securely (in an environment free from attackers), then this attack will be useless, as once the encryption key is exchanged during pairing, it is used for future updates and the key exchange doesn't have to be repeated.

I would rate this vulnerability as very low chance of issue. It could have been avoided pretty trivially by having client and server each choose half of the key for the exchange, but it isn't a huge deal.

  • If someone else pairs once in an attacker's presence, then the attacker can spy on the communications thereafter, correct? So if I were an attacker, I could give a watch as a gift to someone, then when pairing I will always have access to their data stream if I'm nearby, or have a receiver nearby, correct?
    – Adam Davis
    Commented Jan 31, 2014 at 15:11
  • @AdamDavis - if you can monitor the pairing process, then you can obtain the key for communication between the devices that were paired. If a bluetooth device is paired with more than one device, it will only communicate with one at a time and the keys used are different. Commented Jan 31, 2014 at 15:13

Yes, there are other attacks you should be wary of.

There are three types of authentication with Bluetooth "Smart" (aka Bluetooth Low Energy):

  1. Out of Band - This is what you need. Somehow exchange a 128-bit key between the two devices. The main example is to use NFC between the Bluetooth device and a smartphone. If any private information is being exchanged, ensure this pairing mechanism was used.
  2. "Just Works" - Zero protection. The encryption key is the numeral '0'.
  3. Passkey Entry - Six digit PIN, or a value between 0 and 999,999, which is in turn used for encryption. Note that while it is a 128-bit key, the remaining digits are left-padded with '0'.

Source: Bluetooth: With Low Energy comes Low Security.

On the other hand, this theoretical security mechanism increases the security of Bluetooth "Smart" with Merkle's Puzzle.

If you want to delve a little further. this relatively recent whitepaper discuses bypassing passkey authentication.

Here's a quip straight from the bluetooth.org website: The overall goal of keeping the cost ... to a minimum was used in making compromises on security capabilities in Bluetooth Smart (low energy) technology.

A little later, this little tidbit appears: "Just Works and Passkey Entry do not provide any passive eavesdropping protection​."

  • 1
    I'm not sure Merkle's Puzzle article article is completely serious. It proposes saturating the BLE network with puzzle data broadcasts for 1.6 hours before key exchange can happen. I guess in some special scenario this might still be useful/usable.
    – domen
    Commented May 11, 2015 at 16:04

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