# 陳毅 - 讀書會 - 21/07/21

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- [x] 閱讀 Netmanias-06-LTE Security II-NAS and AS Security
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## Intro
### NAS Security

The purpose of NAS security is to securely deliver NAS signaling messages between a UE and
an MME in the control plane.
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The NAS security keys are derived from $K_{ASME}$ and
new keys are generated every time EPS AKA is performed.
- $K_{NASenc}$ : 加密
- $K_{NASint}$ : 完整性
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### AS Security

The purpose of AS security is to securely deliver:
1. RRC messages between a UE and an eNB in
the control plane
2. IP packets in the user plane

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The AS security keys are derived from $K_{eNB}$ and new keys are generated every time a new radio link is established (that is, when RRC state
moves from idle to connected).
- $K_{RRCint}$ : RRC message 與 SRB (Signaling Radio Bearer) 的完整性保護
- $K_{RRCenc}$ : RRC message 與 SRB 的加密
- $K_{UPenc}$ : User-Plane 的資料加密
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## NAS Security

### NAS Security Setup

需特別補充的步驟:
2. [MME] Deriving NAS security keys
3. [MME] Generating NAS-MAC for integrity protection
> NAS-MAC: Message
Authentication Code for NAS for Integrity

7. [UE] Verifying the integrity of the Security Mode Command message

### Delivering a Security Mode Complete message

8. [UE] Encrypting the message using the selected encryption algorithm (EEA1)

9. [UE] Generating NAS-MAC for integrity protection

### After NAS Security Setup


- When NAS messages are being sent, they are encrypted first and then integrity protected before being sent.
- When received, however, the NAS messages are integrity verified first and then decrypted.


## AS Security

### AS Security Setup


需特別補充的步驟:

5. [eNB] Generating MAC-I for integrity protection


需特別補充的步驟:
- 無

### Delivering a Security Mode Complete message


### After AS Security Setup
- When RRC messages are being sent, they are integrity protected first and then encrypted before being sent.
- When received, however, RRC messages are decrypted first and then integrity verified.
- User packets are encrypted but not integrity protected. The user packets encrypted by a sender using the encryption key (KUPenc) are decrypted by the receiver using the same encryption key (KUPenc) to get the original user packets.


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**我的觀察:slightly_smiling_face:**
- RRC 只能由 UE 發起，所以在圖中僅有單向傳輸，與 NAS Security 有些微差異。
- User Data 不做完整性檢查的原因，我認為是基於成本考量。
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## Security Context

Data relating to security that has been set in the EPS entities during these procedures is called an EPS security context, which can be either a NAS security context or an As security context.


A partial native EPS NAS security context is transformed into a full native after the SMC procedure is completed.

### 資料的關係表