Flutter 自签名证书

前言
Flutter项目中服务器使用了自签名证书，如果直接使用https请求或者wss请求的话会报证书签名错误。

HandshakeException: Handshake error in client (OS Error:
I/flutter (28959): │ 💡 CERTIFICATE_VERIFY_FAILED: unable to get local issuer certificate(handshake.cc:359))

或者

CERTIFICATE_VERIFY_FAILED: self signed certificate in certificate chain(handshake.cc:354))
信任自签名证书
1.忽略证书校验
你可以通过以下步骤忽略证书验证。从而接受自签名证书

首先创建一个class 重写HttpOverrides

class MyHttpOverrides extends HttpOverrides {@overrideHttpClient createHttpClient(SecurityContext context) {return super.createHttpClient(context)..badCertificateCallback = (X509Certificate cert, String host, int port) {//add your certificate verification logic herereturn true;};}
}

然后在main 方法将MyHttpOverrides 设置成全局的HttpOverrides。

HttpOverrides.global = new DevHttpOverrides();
当系统检测证书有问题时，会通过回调badCertificateCallback让我们有机会接受或拒绝这个错误的证书，当返回true时，表示我们接受这个证书，会继续完成通讯请求，当返回false时，表示我们不接受错误的证书，https通讯将失败。

当然你也可以在badCertificateCallback添加自己的证书校验逻辑。强烈不建议在正式环境将badCertificateCallback直接返回true，因为这样会接受所有的证书，相当于SSL加密层形同虚设，很容易造成中间人攻击。

2.使用本地CA根证书验证服务器证书
我们可以将服务器用来生成自签名证书的CA证书放在本地，设置为受信任的证书，然后用这个证书校验服务器证书。代码如下：

class MyHttpOverrides extends HttpOverrides {@overrideHttpClient createHttpClient(SecurityContext context) {//加载本地根证书ByteData data = await rootBundle.load('assets/data/ca-cert.pem');final SecurityContext clientContext = SecurityContext()//使用此方法设置受信任根证书..setTrustedCertificatesBytes(data.buffer.asUint8List())return super.createHttpClient(context)..badCertificateCallback = (X509Certificate cert, String host, int port) {//add your certificate verification logic herereturn false;};}
}

这种方式能极大提高安全性。

常见问题
CERTIFICATE_VERIFY_FAILED: Hostname mismatch

Flutter ssl默认会检测证书 subject 中的CN字段和请求url 中的host 是否一致。如果不一致会报这个错误。

如果请求的url时https://baidu.com/tanslate , 那么服务器的证书CN字段必须包含Baidu.com。

很可惜目前基于flutter3.0 没有提供相应的API 去忽略hostname 检测，我们能做的就是在badCertificateCallback 添加自己的校验逻辑，如下：

  ..badCertificateCallback = (X509Certificate cert, String host, int port) {if(host=="baidu.com")return true;else return false;};

2.本地CA根证书验证服务器证书时，当服务器证书的签名不对时，会回调到badCertificateCallback, 而其他证书错误也会回调到badCertificateCallback, 如果要如何区分不同的证书错误原因，则需要手动去校验证书的签名等信息。

class MyHttpOverrides extends HttpOverrides {MyHttpOverrides() {}@overrideHttpClient createHttpClient(SecurityContext? context) {ByteData data = await rootBundle.load('assets/data/ca-cert.pem');final SecurityContext clientContext = SecurityContext()..setTrustedCertificatesBytes(caCertificate);return super.createHttpClient(clientContext)..badCertificateCallback = (X509Certificate cert, String host, int port) //根据根证书校验服务器证书bool isverify = verifyCertificate(ca_cert, cert.pem);if (!isverify){//签名错误return false;}if (currentIp != host) {return false;} else {return true;}};}
}

其中verifyCertificate 方法如下，用到了x509这个package。传送门

import 'dart:convert';
import 'dart:typed_data';
import 'package:x509b/x509.dart' as x509;
import 'package:asn1lib/asn1lib.dart';import 'flutter_log.dart';bool verifyCertificate(String caCert, String serverCert) {// var strX1PublicKeyInfo = "-----BEGIN CERTIFICATE-----\nSOME PUBLIC KEY\n-----END CERTIFICATE-----";// var strX2Certificate = "-----BEGIN CERTIFICATE-----\nSOME CERTIFICATE\n-----END CERTIFICATE-----";var x1PublicKey = (x509.parsePem(caCert).single as x509.X509Certificate).publicKey as x509.RsaPublicKey;var x2Certificate = x509.parsePem(serverCert).single as x509.X509Certificate;var x2CertificateDER = decodePEM(serverCert);var asn1Parser = ASN1Parser(x2CertificateDER);var seq = asn1Parser.nextObject() as ASN1Sequence;var tbsSequence = seq.elements[0] as ASN1Sequence;var signature = x509.Signature(Uint8List.fromList(x2Certificate.signatureValue!));var verifier = x1PublicKey.createVerifier(x509.algorithms.signing.rsa.sha256);return verifier.verify(tbsSequence.encodedBytes, signature);
}Uint8List decodePEM(String pem) {var startsWith = ['-----BEGIN PUBLIC KEY-----','-----BEGIN PRIVATE KEY-----','-----BEGIN CERTIFICATE-----',];var endsWith = ['-----END PUBLIC KEY-----','-----END PRIVATE KEY-----','-----END CERTIFICATE-----'];pem=pem.trim();//HACKfor (var s in startsWith) {if (pem.startsWith(s)) pem = pem.substring(s.length);}for (var s in endsWith) {if (pem.trim().endsWith(s)) {Log().i("certificate:substring");pem = pem.trim().split(s)[0];}}//Dart base64 decoder does not support line breakspem = pem.replaceAll('\n', '');pem = pem.replaceAll('\r', '');return Uint8List.fromList(base64.decode(pem));
}