如何为iOS 11.0及以上版本（含iOS13以下系统）实现证书透明度校验以规避ATS被绕过的问题

Great question—this is a common pain point when building secure iOS SDKs that need to enforce Certificate Transparency (CT) across older iOS versions. The system's built-in CT check via kSecTrustCertificateTransparency is only available on iOS 13+, so we need alternative approaches for iOS 11-12. Let's break down the practical solutions:

1. Manual SCT Parsing and Validation #

Since iOS 11-12 don’t expose CT results directly via Security framework APIs, you’ll need to manually extract and validate Signed Certificate Timestamps (SCTs) from the certificate chain:

Steps to Implement: #

• Capture the SecTrust object: In your URLSessionDelegate’s urlSession(_:didReceive:completionHandler:) method, grab the SecTrust instance provided for the connection.
• Extract SCT extensions: For each certificate in the chain (use SecTrustCopyCertificateChain(_:)), look for the CT extension with OID 1.3.6.1.4.1.11129.2.4.2. This extension contains one or more SCTs.
• Parse ASN.1-encoded SCTs: SCTs are encoded in ASN.1, so you’ll need to decode them using APIs like SecASN1Decode or a lightweight ASN.1 parser. Each SCT includes:
  • The timestamp when the certificate was logged
  • The ID of the CT log that issued the SCT
  • The digital signature of the log
• Validate SCT signatures: Verify that the SCT’s signature is valid using the CT log’s public key. You’ll need to maintain a list of trusted CT logs (e.g., the same list used by major browsers like Chrome).
• Enforce minimum valid SCTs: Ensure at least one valid SCT exists from a trusted, active CT log.

Example Snippet (Extracting SCT Extension): #

func extractSCTs(from certificate: SecCertificate) -> [Data]? {
    guard let extOID = SecOID(rawValue: "1.3.6.1.4.1.11129.2.4.2"),
          let extData = SecCertificateCopyExtensionValue(certificate, extOID) as? Data else {
        return nil
    }
    // Parse ASN.1 data to extract individual SCTs (custom implementation needed)
    return parseASN1EncodedSCTs(extData)
}

Note: Parsing ASN.1 and validating signatures can be verbose—you’ll likely need to wrap Security framework calls to handle edge cases reliably.

2. Use a Mature Third-Party Library #

For most projects, using a well-maintained library is the most efficient way to avoid reinventing the wheel. TrustKit is a popular choice that supports iOS 11+ and handles CT validation automatically across versions:

How to Use TrustKit: #

• Add TrustKit to your project: Use CocoaPods or Swift Package Manager to include the library.
• Initialize TrustKit at app launch: Configure it to enforce CT checks, either using the default trusted log list or specifying your own:

  import TrustKit
  
  func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
      let trustKitConfig = [
          kTSKSwizzleNetworkDelegates: false, // Disable swizzling if using your own delegate
          kTSKPinnedDomains: [
              "your-target-domain.com": [
                  kTSKEnforceCertificateTransparency: true,
                  // Optional: specify custom trusted CT logs
                  kTSKCTLogList: ["https://ct.googleapis.com/logs/argon2023/"]
              ]
          ]
      ] as [String: Any]
      TrustKit.initSharedInstance(withConfiguration: trustKitConfig)
      return true
  }
  

• Integrate with your URLSession: Make your URLSessionDelegate conform to TSKURLSessionDelegate or use TrustKit’s delegate wrapper. The library will handle SCT parsing/validation for iOS 11-12, and fall back to system-native CT checks on iOS 13+.

3. Hybrid Compatibility Strategy #

To get the best of both worlds—system-native accuracy on iOS 13+ and reliable fallback on older versions—implement a conditional approach:

• For iOS 13+: Check the kSecTrustCertificateTransparency key in SecTrustCopyResult(_:) as you’re already doing.
• For iOS 11-12: Use either the manual SCT validation method or TrustKit to enforce CT checks.

Key Considerations for All Approaches: #

• Never rely solely on ATS: As you noted, ATS can be bypassed via user-installed profiles, so always perform CT validation directly in your URLSessionDelegate logic.
• Protect your validation logic: To prevent tampering (e.g., on jailbroken devices or via reverse engineering), consider obfuscating your CT validation code or embedding it in a compiled static library.
• Keep trusted CT logs updated: CT logs are periodically added or retired, so ensure your list (whether manual or via a library) stays current.

内容的提问来源于stack exchange，提问作者hrybrn