https://devzone.nordicsemi.com/f/nordic-q-a/3290/a-question-about-cccdwhen I add my own service and add characteristic, a odd phenomenon confuse me. When I set cccd_md.write_perm: BLE_GAP_CONN_SEC_MODE_SET_OPEN(&amp;cccd_md.write_perm); My ble device can&#39;t be bond with iphone4s But when I set it like this: BLE_GAP_CONN_SEC_MODE_SET_ENC_NO_MITMen-USTelligent Community 13Wed, 30 Jul 2014 07:30:55 GMThttps://devzone.nordicsemi.com/thread/12002?ContentTypeID=1Wed, 30 Jul 2014 07:30:55 GMT137ad170-7792-4731-bb38-c0d22fbe4515:b653c346-1b56-492e-8a72-514899dde096wlgrd<p>I was pretty close though ;) Thanks user9384</p><div style="clear:both;"></div>https://devzone.nordicsemi.com/thread/12001?ContentTypeID=1Wed, 30 Jul 2014 07:28:59 GMT137ad170-7792-4731-bb38-c0d22fbe4515:b97ff39f-f5af-4b03-8c3b-733a7c0bc0eeKnut Eldhuset<p>SET_OPEN means that the descriptor can be read on an unencrypted link. ENC_NO_MITM means that the descriptor can be read only on an encrypted link, with or without MITM protection.</p><div style="clear:both;"></div>https://devzone.nordicsemi.com/thread/12000?ContentTypeID=1Wed, 30 Jul 2014 07:14:10 GMT137ad170-7792-4731-bb38-c0d22fbe4515:a0911a73-b169-4147-a8cb-a5bb214b5056wlgrd<p>So everything works fine with iOS, connection in Android works but it cannot write to the CCCD? I guess there is something wrong with your Android bonding procedure.</p> <p>I <em>think</em> that <strong>SET_OPEN</strong> means no protection at all and all data is visible on air for everyone to see. <strong>ENC_NO_MITM</strong> means that the data exchange is encryptet based on a generated Short Term Key (STK) but no passkey (keyboard, OOB like NFC) is used, so transactions are still vulnerable to Man In The Middle attacks. In other words, if someone is sniffing the packet transactions during connection, they can easily read out the key exchange and decrypt your data.</p> <p><em>Setting this open for everyone to edit, in case I say something terribly wrong</em></p><div style="clear:both;"></div>