What is the LENGTH parameter in EEA2?

Question

In the EEA2 option for encryption in LTE the AES-128 algorithm is mechanized. There, the nonce used in the CTR Mode is a 128-bit value. The first 64-bits are covered by a COUNTER, a BEARER, a DIRECTION, and a LENGTH parameter. LENGTH parameter is supposed to control the output of the KEYSTREAM length (not changing the bits themselves).

However, I was not able to find anywhere a useful description of how this parameter actually works. Available documentation gives detailed descriptions of the COUNTER, BEARER, and DIRECTION parameters (these amount to 38-bits) and the rest 26-bits are set to 0 (the remaining 64-bits are set to 0 and then incremented).

Does anyone know what this LENGTH parameter does and how it works? It seems to me it has no particular reason to be mentioned...

Answer 1

It is simply a parameter for indicating the length of the keystream in bits. Internally, EEA2 will call AES the appropriate number of times to produce that much output, truncating the last block as needed. It is not mixed into AES inputs or the key, so like you said it has no effect on the output bits themselves, beyond choosing how many are taken.