AES CCM encryption with Android decryption failing

[Maz Shar]

I have encrypted 12 bytes on the nrf51 and sent this + 4 byte MIC across to an Android device.

However, when I try to decrypt on Android using a Cipher with "AES/CCM/NoPadding" transformation and "BC" provider, I get a BadPaddingException which states that the "mac check failed".

I am using the same 8 byte initialisation vector on both ends.

I have got this working with ECB, however, as documented, I cannot decrypt ECB on nrf51 which is what I would like to do - android device responding with encrypted string.

Has anyone got this successfully working? Is CCM (with nrf51 compatibility) supported in Android?

Also, we found that the java Cipher has a MAC/MIC size of 8 rather than 4 bytes like nrf51 is generating. Is there any way to change this? I believe this is causing the issue.

[bjorn-spockeli]

AES-CCM is the encryption used by Bluetooth LE so there should not be any compatibility issues with the nRF51's AES-CCM module and Android Bluetooth stack.

As far as I know there is no way to change the size of the MIC from 4 to 8 bytes.

-Bjørn

[Maz Shar]

Could you explain how this is possible if on the nrf51 it sends 4 bytes MIC and Android expects 8? How can Android successfully decrypt?

[Jay]

Maz did you find a workaround for this on the Android side? Thanks

[Maz Shar]

@Jay Unfortunately not. I ended up using ECB instead and found an open source ECB software library to decrypt

[bjorn-spockeli]

@mazshar : I not an Android developer, but it could be that the Android BLE stack does not use the Cipher class to encrypt the link, but rather an internal crypto library.