What are the meanings of SEC_PARAM_MITM and SEC_PARAM_LESC

[JoonDong]

I learned that paring is consist of 3 phase (Exchange of Pairing Information, Authentication, Key distrbution) and TK, STK are generated at the phase of Authentication and reamainder of keys are generated at the phase of Key distrbution when connection is encrypted by using the STK.

So deos enabling SEC_PARAM_MITM(Man In The Middle protection) means that SoftDevice generate TK, STK?

So deos enabling SEC_PARAM_LESC means that SoftDevice distribute LTK, IRK, CSRK?

But it is possible to piar when SEC_PARAM_MITM=0, SEC_PARAM_LESC=1, although TK, STK is not generated so connection is not encrypted by using the STK.

Am I misunderstanding?

[Petter Myhre]

First you have to separate between LE legacy pairing and LE Secure Connections(LESC) pairing.

If both devices support LESC, LESC will be used, otherwise LE legacy pairing will be used. If you set SEC_PARAM_LESC to 1 you will say to the peer that you support LESC. In LESC you dont' have TK, STK++, see the Vol 3, Part H, Section 2.3 in the Bluetooth Core 5 specification.

In LE legacy pairing the IO capabilites, Out of Band (OOB) authentication data availability, and MITM protection requirement determine the generation method of a temporary key (TK). The TK is then used to generate the short term key (STK).

There are three possible TK generation methods:

• Just Works
• Passkey Entry
• OOB

If both devices have OOB set it will be used. If at least one of the devices has MITM set, Passkey Entry entry will be used (if possible), if not Just Works will be used.

So if you set SEC_PARAM_MITM to 1 you will say to the peer that you require MITM protection.

To distribute keys you have to bond, and you will only bond if both devices want to bond. By setting SEC_PARAM_BOND to 1 you will say to the peer that you want to bond.

[JoonDong]

What happens if I set to 1 both SEC_PARAM_MITM and SEC_PARAM_OOB?

What do both devices use for generating TK?

Is this violation of BLE spec?

[Petter Myhre]

If both devices have OOB set it will be used, independent of what MITMs are. With OOB one device creates a 128-bit random number, provides it to the SoftDevice, and transfers it over to the other device, with for examples NFC. On the other side the device provides the 128-bit random number to the SoftDevice. The 128-bit random number is the TK.

It is not a violation.

[JoonDong]

And if MITM is 1 and PassKey Entry method is selected as IO capacitiblies combination in LE Security Connection, one of Just Works, PassKey Entry, OOB, and Numeric Comparison(added to legacy pairing) is used to generate LTK?

And does LTK in LE Security Connection replace STK(connection following pairing)'s role in addition to subsequent connection after pairing?

And does LTK in legacy pairing is only used in subsequent connection after pairing?

[Petter Myhre]

And if MITM is 1 and PassKey Entry method is selected as IO capacitiblies combination in LE Security Connection, one of Just Works, PassKey Entry, OOB, and Numeric Comparison(added to legacy pairing) is used to generate LTK?

Please see Vol 3, Part H, Section 2.3.5.1 in the Bluetooth Core 5 specification.

And does LTK in LE Security Connection replace STK(connection following pairing)'s role in addition to subsequent connection after pairing?

Yes

And does LTK in legacy pairing is only used in subsequent connection after pairing?

Yes