5. ATT level security and digital signing
Closely related to security are the concepts of authentication, authorization, and the access rights of resources. Attribute permissions are a combination of access permissions, encryption permissions, authentication permissions and authorization permissions.
Authentication is the process which guarantees that a message has originated from a trusted party. Authorization is a confirmation by the user to continue with the procedure. Authentication does not necessarily provide authorization.
In BLE, the following access permissions are possible:
Readable
Writeable
Readable and writable
The following encryption permissions are possible:
Encryption required
No encryption required
The following authentication permissions are possible:
Authentication Required
No Authentication Required
The following authorization permissions are possible:
Authorization Required
No Authorization Required
Encryption, authentication, and authorization permissions can have different possibilities; for example, a specific attribute could require a particular kind of authentication or a certain minimum encryption key length. An attribute can have several combinations of permissions that apply; for example, a specific attribute could allow any of the following:
Read if encrypted (authentication not required)
Write if authenticated and encrypted
Read or write if authenticated and authorized (irrespective of encryption)
5.1. Authentication and data signing
Digital communications are ubiquitous and in the case of wireless transmissions there must be a robust way of determining if the messages we receive have originated from the expected end or if they have been altered. Just like a paper document, a digital document or message can be signed with a digital signature.
A digital signature is a mathematical scheme which provides extremely high probability that the received message is original. The benefits that digital signatures provide are:
Authentication: When the message is received and the signature is verified, we can confirm the originator of the message.
Message integrity: A digital signature assures us that the received message has not been altered in any way.
Non-repudiation: This means that the party that has signed the message cannot deny at a later time that they have signed it.
In the Bluetooth Low Energy specification, authentication is provided either by enabling encryption or at the ATT layer with the Connection Data Signing procedure.
5.1.1. Connection Data Signing procedure
This procedure is invoked when encryption is not required and the Connection Signature Resolving Key is available (LE security mode 2). A device generates a new CSRK for each set of peer device(s) to which it sends signed data in connections. The inputs to the signing function are the Data PDU, the CSRK, and the SignCounter value, which is a counter incremented every time a message is signed. The resulting signature is composed of the counter value and the Message Authentication Code, as can be seen in the figure below.
Every Attribute operation indicates in its opcode if the message is signed or not by means of a bit called Authentication Signature Flag. If the Authentication Signature Flag of the Attribute opcode is set to one, the Authentication Signature value will be appended to the end. Only the Write command may include an Authentication Signature. Authentication signatures are not sent when the link is encrypted, since an encrypted link already includes authentication data on every packet.
The receiving device is protected against a replay attack by comparing the received SignCounter with previously received SignCounter from the same peer device. If the SignCounter was previously used then the receiving device will ignore the Data PDU.
Figure 14 Generic format of signed data
5.1.2. Data signing on an encrypted link
When pairing has been completed, most of the encryption operations are performed at the Link Layer, and the Host, which runs on more complex hardware like a CPU, doesn’t have to be involved. In this manner the power consumption of the device could be reduced significantly.
When an encrypted Link Layer connection has been setup, authentication data are included in every Data Channel PDU. You can see below the format of a Data Channel PDU.
Figure 15 Data channel PDU
The MIC field stands for Message Integrity Check and is in fact a Message Authentication Code (named like this to avoid confusion with Media Access Controller). The MIC field must be included in an encrypted Link Layer connection, with a Data Channel PDU with a non-zero length Payload. If the PDU has a zero-length Payload or the connection is unencrypted, then it must be omitted.