(The question is at the end)
CONTEXT: It's an intership assignement at a -small- network operator, I'm to understand e.v.e.r.y single step of the security procedures the USIM is implicated in. The 3GPP is my friend but still, I didn't find the answer to this particular question.
In order to use the LTE network, the User Equipement (UE) starts an attachement request destined to the MME and declares its network capabilties (the key generating algorithms it can use).
Later on, the HSS/AuC uses chooses an algorithm compatible with the UE capabilities to produce the Authentication Vector (AV) and sends it to the UE through the MME.
When the UE receives the AV, it sends some of its parameters to the USIM (the software part of the commonly know "SIM card") which uses the algorithm the HSS chose in order to start the mutual authentication (UE <-> Network), a process that requires that both the UE and the HSS/AuC use the same key generating algorithm during the attachement procedure (it's mutual after all)
QUESTION: How does the USIM know which key generating algorithm the HSS/AuC chose during the attachement procedure?
I'm opend for suggestions/discussions.
Peace.
EDIT: Found it! check the answer below.
I found it, the answer is the AMF field.
After the HSS/AuC chooses one of the algorithm the UE presented it creates Authentication Vectors (AVs).
An AV contains the AUTN which is composed of -among other things- the AMF field (16 bits). The last 7 least significant bits of AMF can be used to keep track of the encryption algorithms the HSS/AuC used
Ref: 3GPP TS 33.102 Annexe F.2