5G AS安全激活与DRB密钥派生信令拆解

sequenceDiagram
    participant UE
    participant gNB
    participant AMF

    Note over UE,AMF: 阶段一：NAS安全已建立（前置条件）

    AMF->>gNB: NGAP Initial Context Setup Request<br/>含 SecurityKey(KgNB) +<br/>securityAlgorithm +<br/>UESecurityCapability
    Note right of gNB: gNB从KgNB派生<br/>KRRCint/KRRCenc/<br/>KUPint/KUPenc

    Note over UE,gNB: 阶段二：AS安全激活（SRB安全）

    gNB->>UE: RRC SecurityModeCommand<br/>[SRB1/DL-DCCH]<br/>cipheringAlgorithm=NEA2<br/>integrityProtAlgorithm=NIA2
    Note left of UE: 消息仅完整性保护<br/>不加密（加密尚未激活）

    UE->>UE: 使用KRRCint验证MAC-I<br/>使用KDF派生<br/>KRRCint/KRRCenc/KUPint/KUPenc

    UE->>gNB: RRC SecurityModeComplete<br/>[SRB1/UL-DCCH]
    Note right of gNB: 该消息已同时启用<br/>完整性保护+加密

    Note over UE,gNB: 阶段三：DRB安全激活

    gNB->>UE: RRC Reconfiguration<br/>含 radioBearerConfig<br/>建立DRB（含安全策略）
    UE->>gNB: RRC Reconfiguration Complete

    Note over UE,gNB: DRB用户面数据<br/>加密+完整性保护激活

graph TD
    K["K (USIM/ARF永久密钥)"] -->|认证向量导出| CK_IK["CK ‖ IK<br/>(认证响应)"]
    CK_IK -->|KDF: FC=0x6A| KAMF["KAMF<br/>(Access and Mobility Key)"]
    KAMF -->|KDF: FC=0x69| KgNB["KgNB<br/>(gNB密钥)"]
    KgNB -->|KDF: FC=0x70| KRRCint["KRRCint<br/>(RRC完整性密钥)"]
    KgNB -->|KDF: FC=0x71| KRRCenc["KRRCenc<br/>(RRC加密密钥)"]
    KgNB -->|KDF: FC=0x72| KUPint["KUPint<br/>(UP完整性密钥)"]
    KgNB -->|KDF: FC=0x73| KUPenc["KUPenc<br/>(UP加密密钥)"]

    style K fill:#1a1a2e,color:#fff
    style KAMF fill:#16213e,color:#fff
    style KgNB fill:#0f3460,color:#fff
    style KRRCint fill:#e94560,color:#fff
    style KRRCenc fill:#e94560,color:#fff
    style KUPint fill:#533483,color:#fff
    style KUPenc fill:#533483,color:#fff

KDF Input = FC(0x6E) || P0(ACCESS_TYPE) || L0 || P0(architectural separation)

Key = HMAC-SHA-256(KgNB, FC || Algorithm_Type_Distinguisher || Algorithm_Identity || 0x00 || ...)

SecurityModeCommand ::= SEQUENCE {
    rrc-TransactionIdentifier    RRC-TransactionIdentifier,
    criticalExtensions           CHOICE {
        securityModeCommand      SecurityModeCommand-IEs,
        criticalExtensionsFuture SEQUENCE {}
    }
}

SecurityModeCommand-IEs ::= SEQUENCE {
    securityConfigSMC            SecurityConfigSMC,
    lateNonCriticalExtension     OCTET STRING    OPTIONAL,
    nonCriticalExtension         SEQUENCE {}     OPTIONAL
}

SecurityConfigSMC ::= SEQUENCE {
    securityAlgorithmConfig      SecurityAlgorithmConfig
}

SecurityAlgorithmConfig ::= SEQUENCE {
    cipheringAlgorithm           CipheringAlgorithm,
    integrityProtAlgorithm       IntegrityProtAlgorithm   OPTIONAL
}

graph LR
    subgraph "PDCP发送端（gNB/UE）"
        A[SDU数据] --> B{是否加密?}
        B -->|是| C[加密模块<br/>算法:NEA1/2/3<br/>密钥:KRRCenc或KUPenc]
        B -->|否| D[透传]
        C --> E{是否完整性保护?}
        D --> E
        E -->|是| F[计算MAC-I<br/>算法:NIA1/2/3<br/>密钥:KRRCint或KUPint]
        E -->|否| G[添加PDCP头]
        F --> G
        G --> H[PDCP PDU]
    end

CIPHERTEXT = NEA_x(KEY, COUNT, BEARER, DIRECTION, LENGTH, PLAINTEXT)

MAC-I = NIA_x(KEY, COUNT, BEARER, DIRECTION, LENGTH, MESSAGE)

graph TD
    A["SecurityModeCommand<br/>（gNB→UE）"] --> B["✅ 完整性保护（NIA2）<br/>使用KRRCint计算MAC-I"]
    A --> C["❌ 不加密<br/>因为加密尚未激活"]

    D["SecurityModeComplete<br/>（UE→gNB）"] --> E["✅ 完整性保护（NIA2）"]
    D --> F["✅ 加密（NEA2）<br/>第一条加密消息"]

    style C fill:#ff6b6b,color:#fff
    style F fill:#51cf66,color:#fff

sequenceDiagram
    participant UE
    participant gNB

    Note over UE,gNB: SRB安全已激活

    gNB->>UE: RRC Reconfiguration<br/>radioBearerConfig:<br/>- drb-ToAddModList<br/>  - DRB-Identity: 1<br/>  - pdcp-Config:<br/>    - securityConfig:<br/>      - integrityProtection: enabled<br/>      - ciphering: enabled<br/>    -headerCompression: notUsed

    UE->>UE: 为DRB-1建立PDCP实体<br/>配置KUPenc + KUPint<br/>初始化COUNT=0

    UE->>gNB: RRC Reconfiguration Complete<br/>（SRB加密+完整性保护）

    Note over UE,gNB: DRB-1用户面数据<br/>加密 + 完整性保护生效

stateDiagram-v2
    [*] --> RRC_IDLE: 开机/脱网
    RRC_IDLE --> RRC_INACTIVE: 网络配置
    RRC_IDLE --> RRC_CONNECTED_SENDING: RRCSetupComplete<br/>(NAS RegRequest)<br/>安全=无

    state RRC_CONNECTED_SENDING {
        [*] --> NAS_Security_Active: NAS SMC完成
        NAS_Security_Active --> AS_Security_Activating: 收到SecurityModeCommand
        AS_Security_Activating --> AS_Security_Active: SecurityModeComplete<br/>SRB加密+完整性生效
        AS_Security_Active --> DRB_Security_Active: RRC Reconfig<br/>DRB安全激活
    }

    DRB_Security_Active --> RRC_CONNECTED_SECURE: 全部安全激活

    RRC_CONNECTED_SECURE --> Handover_Key_Update: 切换时<br/>KgNB→Ktarget_gNB
    Handover_Key_Update --> RRC_CONNECTED_SECURE: 新KgNB派生完成

Ktarget_gNB = KDF(KgNB, 0x12 || PCI || ARFCN-DL || 0x00 || ...)

sequenceDiagram
    participant UE
    participant Source_gNB
    participant Target_gNB

    Source_gNB->>Target_gNB: Xn HANDOVER REQUEST<br/>含SecurityContext<br/>(KgNB*, nextChainingCount)
    Target_gNB->>Target_gNB: 从KgNB*派生<br/>新AS密钥集
    Target_gNB->>UE: RRC Reconfiguration<br/>withReconfiguration-withSync<br/>含新PCI/ARFCN
    UE->>UE: 垂直派生Ktarget_gNB<br/>派生新AS密钥
    UE->>Target_gNB: RRC Reconfiguration Complete<br/>（新密钥加密）