BlueMod+B20/BT2.1
User Guide
Release r03 www.stollmann.de Page 13 of 38
SSP defines the following association models based on the Input/Output (I/O)
capabilities of the two devices:
Just Works:
This method is used when at least one of the devices does not have display
capability of six digits and also is not capable of entering six decimal digits using a
keyboard or any other means (no I/O).
This method does not provide MITM protection.
Compared to the legacy pairing with a fixed PIN, the security level provided by this
method is much higher.
Numeric Comparison:
If both devices have a display and both sides can accept a “Yes/No” user input, they
may use Numeric Comparison. This method displays a six digit numeric code on
each device. The user shall compare the numbers to ensure they are identical. If the
comparison succeeds, the user(s) shall confirm pairing on the device(s) that can
accept an input.
This method provides MITM protection, assuming the user confirms on both devices
and actually performs the comparison properly.
Passkey Entry:
This method may be used between a device with a display and a device with
numeric keypad entry (such as a keyboard), or two devices with numeric keypad
entry.
In the first case, the display is used to show a six digit numeric code to the user,
who then enters the code on the keypad.
In the second case, the user of each device enters the same six digit numeric code.
Both cases provide MITM protection.
Legacy Pairing: (Bluetooth 2.0 compatible pairing mechanism)
In this case both devices needs to enter the same PIN code of minimum four digits.
The BlueMod+B20 uses the PIN code saved in the local register S318
(compare: BlueMod+B20/BT2.1 AT Command Reference).
Possible combinations of I/O capabilities and the possibility of MITM protection are
listed in the table below. For each case of the “MITM protection” an example of the
serial messages between the BlueMod+B20 and the DTE are listed.
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