5G SEPP跨域安全与N32握手信令拆解

graph TD
    subgraph VPLMN["拜访PLMN (VPLMN)"]
        VNF[NF: AMF/SMF/AUSF]
        VSEPP[cSEPP<br/>SEPP]
    end
    subgraph IPX["IPX网络"]
        IPX1[IPX Provider A]
        IPX2[IPX Provider B]
    end
    subgraph HPLMN["归属PLMN (HPLMN)"]
        HSEPP[pSEPP<br/>SEPP]
        HNF[NF: UDM/AUSF/UDR]
    end

    VNF -->|N32f 明文SBI| VSEPP
    VSEPP -->|N32-c 握手<br/>TLS + 证书认证| IPX1
    IPX1 --> IPX2
    IPX2 -->|N32-c 握手| HSEPP
    HSEPP -->|解密后明文SBI| HNF

    style VSEPP fill:#e74c3c,color:#fff
    style HSEPP fill:#e74c3c,color:#fff
    style IPX1 fill:#f39c12,color:#fff
    style IPX2 fill:#f39c12,color:#fff

sequenceDiagram
    participant cSEPP as cSEPP<br/>(VPLMN)
    participant IPX as IPX中间节点
    participant pSEPP as pSEPP<br/>(HPLMN)

    Note over cSEPP,pSEPP: Phase 1: TLS连接建立
    cSEPP->>IPX: TLS ClientHello
    IPX->>pSEPP: TLS ClientHello (转发)
    pSEPP->>IPX: TLS ServerHello + Certificate
    IPX->>cSEPP: TLS ServerHello + Certificate (转发)
    cSEPP->>IPX: TLS Certificate + Finished
    IPX->>pSEPP: TLS Certificate + Finished (转发)
    pSEPP->>IPX: TLS Finished
    IPX->>cSEPP: TLS Finished (转发)
    Note over cSEPP,pSEPP: TLS隧道建立完成

    Note over cSEPP,pSEPP: Phase 2: N32-c能力协商
    cSEPP->>pSEPP: POST /n32c-handshake/exchange-capability<br/>SecurityCapability negotiate

    pSEPP->>cSEPP: 200 OK + SecCapabilityResult<br/>接受/拒绝/修改

    alt 协商成功
        Note over cSEPP,pSEPP: 安全上下文建立完成<br/>派生N32f保护密钥
    else 协商失败
        pSEPP->>cSEPP: 200 OK + SecCapabilityResult(rejected)
        Note over cSEPP,pSEPP: 无法建立N32f会话
    end

{
  "securityCapability": {
    "protectionPolicyIds": ["N32FullProtectionProfile", "N32ClearTextProtectionProfile"],
    "supportedSecInfo": ["IECIES", "AES_GCM"],
    "plmnId": {
      "mcc": "460",
      "mnc": "00"
    },
    "targetPlmnId": {
      "mcc": "001",
      "mnc": "01"
    },
    "n32fPurpose": ["roaming", "interconnect"]
  },
  "retransmitIndicator": false
}

{
  "securityCapabilityResult": {
    "selectedProtectionPolicyId": "N32FullProtectionProfile",
    "supportedSecInfo": ["IECIES", "AES_GCM"],
    "plmnId": {
      "mcc": "001",
      "mnc": "01"
    },
    "negotiationResult": "successful"
  }
}

stateDiagram-v2
    [*] --> Idle: SEPP启动
    Idle --> TLSEstablishing: 发起/接收TLS连接
    TLSEstablishing --> TLSActive: TLS握手成功
    TLSEstablishing --> Idle: TLS握手失败
    TLSActive --> Negotiating: 发送exchange-capability
    Negotiating --> Active: 协商成功<br/>密钥派生完成
    Negotiating --> TLSActive: 协商被拒绝<br/>可重试
    Active --> Active: N32f消息处理
    Active --> Renegotiating: 密钥更新触发
    Renegotiating --> Active: 重协商成功
    Active --> Idle: 连接断开/超时

sequenceDiagram
    participant NF_c as 源NF<br/>(VPLMN AMF)
    participant cSEPP as cSEPP
    participant IPX as IPX节点
    participant pSEPP as pSEPP
    participant NF_p as 目标NF<br/>(HPLMN UDM)

    NF_c->>cSEPP: HTTP/2 POST<br/>/nudm-sdm/v1/{supi}/am-data<br/>{明文JSON: SUPI, S-NSSAI...}

    Note over cSEPP: 1. 识别敏感数据类型<br/>2. SUPI → IECIES加密<br/>3. 整体JSON → JWE封装

    cSEPP->>IPX: HTTP/2 POST (N32f)<br/>/n32f/v1/relay<br/>JWE Protected Payload

    Note over IPX: IPX只能看到HTTP头<br/>和JSON外层路由字段<br/>无法解密消息体

    IPX->>pSEPP: 转发N32f消息

    Note over pSEPP: 1. JWE解密<br/>2. IECIES解密敏感字段<br/>3. 恢复原始明文JSON

    pSEPP->>NF_p: HTTP/2 POST<br/>/nudm-sdm/v1/{supi}/am-data<br/>{明文JSON}

    NF_p->>pSEPP: 200 OK + 订阅数据
    pSEPP->>cSEPP: 200 OK (N32f保护)
    cSEPP->>NF_c: 200 OK + 订阅数据(明文)

1. 生成临时ECC密钥对 (ephemeral public key, ephemeral private key)
2. ECDH共享密钥 = ephemeral_private_key × pSEPP_public_key
3. KDF(共享密钥) → DEK (Data Encryption Key) + MAC Key
4. Ciphertext = AES-GCM-Encrypt(DEK, plaintext_field_value)
5. 输出: {ephemeral_public_key || Ciphertext || MAC_Tag}

{
  "protected": "eyJhbGciOiJFQ0RILUVTK0EyNTZLVyIsImVuYyI6IkEyNTZHQ00iLCJlcGsiOiJ...base64url...",
  "unprotected": {
    "kid": "sepp-hplmn-key-2026-001",
    "iat": 1748304000
  },
  "recipients": [
    {
      "encrypted_key": "base64url_encoded_wrapped_CEK"
    }
  ],
  "iv": "base64url_encoded_initialization_vector",
  "ciphertext": "base64url_encrypted_payload_containing_IECIES_encrypted_fields",
  "tag": "base64url_authentication_tag"
}

{
  "payload": "base64url_original_json_payload_in_cleartext",
  "protected": "eyJhbGciOiJFUzI1NiJ9",
  "signature": "base64url_ECDSA_signature"
}

graph TB
    subgraph Direct["直连模式 (Direct)"]
        cSEPP1[cSEPP] -->|单一N32-c + N32-f| pSEPP1[pSEPP]
    end

    subgraph Hubbing["Hubbing模式"]
        cSEPP2[cSEPP] -->|N32-c/TLS| IPXA[IPX-A SEPP Proxy]
        IPXA -->|TLS hop-by-hop| IPXB[IPX-B SEPP Proxy]
        IPXB -->|N32-c/TLS| pSEPP2[pSEPP]

        cSEPP2 -.->|端到端N32f保护| pSEPP2
    end

    style cSEPP1 fill:#3498db,color:#fff
    style pSEPP1 fill:#e74c3c,color:#fff
    style cSEPP2 fill:#3498db,color:#fff
    style pSEPP2 fill:#e74c3c,color:#fff
    style IPXA fill:#f39c12,color:#fff
    style IPXB fill:#f39c12,color:#fff

TLS ClientHello → IPX-A (SNI: ipx-a.example.com)
IPX-A扮演TLS终结点，解密消息后以新TLS连接转发到下一跳
每跳都需要独立的证书验证和N32-c能力协商

graph TD
    TLS_TLS["TLS Master Secret<br/>(TLS 1.3握手产出)"]

    TLS_TLS --> N32_SK["N32 Security Context Key<br/>KDF(TLS Master Secret, 'N32-SC')"]

    N32_SK --> CEK["Content Encryption Key (CEK)<br/>用于JWE消息加密<br/>KDF(N32_SK, 'CEK')"]
    N32_SK --> CIK["Content Integrity Key (CIK)<br/>用于JWS签名<br/>KDF(N32_SK, 'CIK')"]
    N32_SK --> DEK["Data Encryption Key (DEK)<br/>用于IECIES字段加密<br/>KDF(N32_SK, 'DEK')"]

    CEK --> JWE_MSG["JWE加密的N32f消息体"]
    CIK --> JWS_SIG["JWS签名/验证"]
    DEK --> IECIES_ENC["敏感数据类型字段加密<br/>SUPI/GPSI/AV等"]

    style TLS_TLS fill:#2c3e50,color:#fff
    style N32_SK fill:#e74c3c,color:#fff
    style CEK fill:#3498db,color:#fff
    style CIK fill:#3498db,color:#fff
    style DEK fill:#3498db,color:#fff

故障现象：cSEPP日志显示 "N32-c negotiation rejected"
排查步骤：
1. 检查TLS证书是否过期：openssl x509 -in sepp_cert.pem -noout -dates
2. 验证PLMN-ID匹配：证书MCC/MNC vs 消息中的plmnId
3. 检查保护策略交集：两端是否至少有一个共同支持的保护策略
4. 确认CA信任链：pSEPP是否信任签发cSEPP证书的CA

故障现象：pSEPP返回 HTTP 401 + "Decryption failed"
排查步骤：
1. 检查密钥标识符(kid)：是否使用了已过期的密钥
2. 验证时间同步：两端SEPP时钟偏差应 < 30秒
3. 检查IECIES公钥：pSEPP的公钥是否正确配置在cSEPP侧
4. 抓包对比：消息体是否在传输中被截断或修改

故障现象：N32f消息发送后无响应
排查步骤：
1. 检查逐跳TLS状态：每段连接是否都保持Active
2. IPX路由策略：消息中的目标PLMN-ID是否在IPX路由表中
3. MTU问题：JWE封装后消息体增大可能导致分片失败
4. 超时配置：HTTP/2连接超时 vs 信令响应超时不匹配